MaxxForce DT Diesel Engine Injector Miss in a School Bus

An injector miss isn't too hard to detect since the engine code that showed up on the dash indicated a missing injector. This involved replacing the injector and a couple of seals on pressure sensors. The oil manifold is heavy and takes some work to move it on and off the top end of the engine. Once the injector is replaced and the hold down is torqued (22 ft lbs) the manifold sits on top supplying the high pressure oil from the HPOP (high pressure oil pump).

This high pressure system can get up to 4500 psi so if there are any weak spots and a leak develops the injectors stop firing properly. One test that really works well is to remove the main oil feed hose from the high pressure manifold and apply shop air to the manifold seeing if there are any leaks. If so there may be a failed seal someh where in the system.

If there are no leaks then there's a possibility of a bad IPR (injection pressure regulator) that screws into the HPOP controlling the pump output or the pump it self is not putting out pressure. Is this common? unfortunately yes we have changed out a HPOP and they are not cheap! $1700.00 while an injector runs around $500.00 which is Canadian funds. Yes it can a big owwie on the pocket book with these diesels. So I wanted to suggest something to anyone who runs MaxxForce engines.

They are bad for premature engine wear so the easiest solution is to shorten engine oil changes. We are now changing engine oil at 6000 mile (10,000 km). It's a cheap alternative compared to a premature inframe. Our fleet has had to rebuild 4 of our 2010 - 2011 DT engines due to no compression / high blow by situations. This really drives up the costs replacing pistons, liners, rings bearings, cylinder rebuild etc.

The labor on top of all this adds to the cost and down time. Another thing that would be worth going for is engine oil samples so engine conditions can be analyzed well before obvious problems occur. The results from engine oil testing covers everything from metals, fuel or coolant contamination. Well worth the cost! That's all I have to say about this subject. Leave a comments or questions below.

Mercedes MBE Diesel Engine Pusher School Bus

The cold start on this bus was perfect without any hold ups. It was plugged in and the batteries are obviously in good shape. Of course with most diesel engines the white smoke is normal with incomplete combustion going on. Heat from compression is low in this case so it takes a while for normal combustion temperatures to occur. The rule of thumb with diesel engines in my book is to put the engine to work as soon as you get normal oil pressure or at least make sure the high idle mode is on. Idling a diesel in cold weather is not a good practice because of the build up of carbon and related combustion byproducts that happen with cold starts.

These MBE 900 diesels are good solid engines except for the emissions which was adapted to the original block in 2007. The blocks can not be swapped from non emissions to these 2007 models because of added modules that were engineered to meet emission standards. I know this from experience we have a 2008 MBE that has a cracked block and no one can find me a bare block for it. There are plenty of non emission blocks around but nothing newer. The long block is available from the dealer but at a huge cost and a core charge that we would not get back due to the crack in the block.

When the warranty was up with these engines we were in for a lot of challenges. The engine codes were running away on us with daily occurrences with the EGR valves, DPF diesel particulate filter, EOS electrostatic oil separator along with boost and charge air codes that did not let up. Two of these pushers were sports run buses and almost every trip they would radio back and tell me the engine was coding. One saving grace was the ignition key could be cycled on and off which would reset the code until it came back again. But this would stress me out regardless being helpless with the bus so far away. One bus was 4 hours out of town and I spent hours trying to line up a service call to do a parked regen or else the bus was dead in the water.

Eventually these 2 bad boys were taken off of the sports run and we started using newer Cummins ISC diesel engines. The pressure was off and if you have read any of my other posts Cummins is my choice for a diesel engine medium duty ISB and ISC models. Yes they don't last forever there are quirks and failures but an extremely smaller percentage compared to other diesels on the road. Preventive maintenance had proven to extend down time and the extra cost is money well spent.

School Bus Automatic Transmission Control Module

School Bus Allison Automatic Transmission Control Unit from John Whelan on Vimeo.

School buses these days come with Allison automatic transmissions. In my opinion there's no other choice for running gear. Allison has got it nailed with engineering design and software. We just purchased the newest Allison software and it does a nice job of laying everything out with troubleshooting and tutorial videos all in one. Recently one of our Navistar conventional buses was on a field trip and called in with the transmission not wanting to shift.

I went out and for some reason after cycling the key a couple of times the bus was limped back to the shop. The basic checks are transmission codes and wiring connections, bad wiring from the TCU to the transmission. The code P0614 was a torque data fault which is a communication problem between the TCU and ECU (engine module). There is no schematic to follow it's all about how much torque the transmission will allow from the engine.

This feature was put into place to protect the transmission from damage if for some reason i.e. the engine delivers too much torque due to over fueling. After checking all of the pin connections and the wiring the J1939 resistance was checked and we got a reading of 57 ohms which is normal. If you're not familiar with J1939 it is the network of communication between all of the vehicle control modules.The engine, transmission, ABS brakes, body control module etc.... so if there is an issue with this system the messages will not connect properly and strange things start to happen.

The green and yellow twisted pair of wires goes to each module so a high speed signal will occur when the operator wants to shift into a certain gear or signals the ABS module that braking is taking place and so on. The modules read it's sensors and adjusts to accommodate any changes such as driving up a hill or coasting on the highway. When there is a bad wire in this twisted pair the messages will be all mixed up and something in the operating system will fail followed by a fault code on the dash alerting the driver.

A great advantage we have running a fleet is there are buses with the exact same specs. which gives us the ability to swap modules to cancel it out during troubleshooting steps. In the case of the P0614 code when we changed the TCU with a good working unit from another bus the code disappeared. This saves the guessing game and the rules are when you order an electronic part and use it you own it. The price for these modules are well over a thousand dollars so be sure before you order one. They have to be specially programmed according to the part number of the original module and the specs related to the vehicle serial number so I'm sure returning it would be a hardship.

Thanks for reading this post I hope it helped out!!  

Hard Start DT466E

DT 466 ENGINE OIL PUMP from John Whelan on Vimeo.
The regular engine oil pump supplies the HEUI high pressure pump so since the supply wasn't meeting the demand we had a hard start situation. The engine oil pressure regulator appeared to be scored so we attributed the lower oil pressure to that defect.

The oil pump drive and gears had looked alright but who knows what a little wear will do. It could be thousands of an inch causing problems. So after replacing the pump and regulator and using 0W40 oil for the cold start season starting improved drastically. Once we pulled the engine hood off of this bus which was a conventional international 4300 getting at the parts was a breeze.

There was no laying on our backs except for removing the oil pan. One characteristic with all of our DT466E diesel engines was they ran like crap when they were cold. It took some time for the oil pressure in the high pressure side to regulate itself and provide the oil pressure to the injectors. A lot of injector problems were with the injector o-rings that separate the oil from the fuel. They would deteriorate and create an engine miss.

The last of these buses got sold to a church group who took them down to Guatemala. They shipped them by boat the second time around because of an incident while travelling through Mexico. Those road blocks cost a lot of money and you have to watch your back at all times. I talked to a Mechanic who just left to go down there and so some repairs. I hope he has all of the tools and parts he needs. The technology down there is not up to speed so if something breaks it gets fixed in what ever way possible.

The DT466E being electronic needs diagnostic software to analyze faults so I hope the 2 engines stationed down there last for a long time and don't give them grief. I donated a few old parts sitting on the shelf for the air dryer. The mechanic said the air compressor isn't building up very fast. The air dryer most likely needs servicing since it takes out all of the \moisture and oil from the system. Leave it too long and a restriction will build up slowly but surely.

School Bus Pusher Radiator Removal Shortcut

Changing the radiator or removing it for repair at the local radiator repair shop is a big operation on the pusher school bus. The charge air cooler, oil cooler and radiator are sandwiched on top of each other making it hard to get the radiator out of the chassis.

The objective in this video is to show you a short cut that all mechanics love to do saving time and some frustration. The way we do it is to support the upper components and loosen off the radiator mounts. The frame supports have to be loosened as well to allow space so the the rad core can be removed freely.

The cooling fan which is a Parker system is braced to the main frame around the radiator so it needs to be chained up out of the way. In most cases if the radiator is original and has a leak the chances of repairing it is remote I'm sorry to report.

This rad core is over 12 years old and has gone through a lot of weather and road debris kicking up into and around the core area on a daily basis. There are cores out there but we had to get one from Thomas buses. I hope this video helps you bus mechanics out there.

I would appreciate it if you shared or commented on this post. It's great to get some feedback from others who have had similar problems and found a faster way to get them done. Time is a precious commodity in a fleet garage and with that said I'll continue to share videos on repairs that will help out fellow mechanics.

Watch the video below on what our shop did to remove the radiator on a pusher school bus:


Radiator Repair Thomas Pusher School Bus from John Whelan on Vimeo.

A Common School Bus No Start Problem

Thomas School Bus Vandalock

This a common no start problem with all school buses and will continue to be since Vandalocks are standard equipment. The Vandalock as the name states locks out the start circuit and locks the access entries to the inside of the bus. The Thomas pusher in the video does not have any lock out for the entrance doors just the side door and rear emergency exit window.

However there's a cheap and easy solution which is to purchase a metal bike lock (u shaped) and install a couple pieces of angle iron to the entrance door leafs. We only do this with our sports run buses which do a lot of out of town trips. Just recently one of our sports run pusher buses were in Vancouver and the batteries got ripped off.

This is the first time this has ever happened to us perhaps because of the high return for lead at the scrap yard or someone with a truck needed a good set of batteries? Who knows but it's the sign of the times so a lock will be going on both of our sports run battery doors. The service call cost $800.00 in total so a couple of $20.00 hinge locks is a solid investment.

Getting back to the Vandalock system...when we talk to our drivers over the radio we can usually run them through the steps to check all of the exits to see if the latches are either locked or unlocked. One of the students could have very well kicked the latch closed since it's in the perfect spot on the side exit door as seen on the video.

The buzzer will stay on with the ignition key on with no engine crank until the latches are disabled. Vandalock systems are great to have as long as everybody is trained properly.

International School Bus Vandalock 

 

The International school bus vandalock is the same principle as Thomas buses but designed totally different. The conventional models have the entrance door and the rear door set up to lock out access and the engine start system. The video demonstrates how it works but the there is a minor flaw with the lever that activates the micro switch above the entrance door.

This micro switch has to be activated for the bus to start. Every year we have no start problems because of the lever dropping down while the bus is enroute. A rough road condition will cause this and the lever will drop down deactivating the switch causing an engine no start ... alarms will stay on while the ignition is turned on. One solution is to tighten the lever fastener to increase resistance but ever so slightly so the activating rod has enough push to move the lever.

School Bus Photos 1997 Thomas Pusher

Thomas School Bus Photos from John Whelan on Vimeo.
Oh the memories....this 1997 Thomas school bus pusher is long gone but the photos tell it all. Back then our fleet had 13 Cat 3116 diesel engines. From new they were nice and shiny without any problems until suddenly several of them experienced an intermittent transmission shift episode.

We searched high and low for the fault and scratched our heads and even got the dealer involved thinking ...warranty?? Well what it ended up being was a corroded battery supply wire from the battery to the shift module and transmission control unit.

There was a butt splice half way down the frame which wasn't sealed with heat shrink or soldered at all. The crimped connector was blue with corrosion and caused the battery feed to cut in and out or it would totally cut out and our bus was dead in the water.

 The transmission shop would not give us warranty because the harness was OEM from the factory. So there was some discussion on who pays but it all came out fair at the end. The wiring was all hard wired (no multiplexing) and searching for problems in wiring took a lot of time.

If I had a second chance at this mishap the first thing I would check is "the source"! There was obviously a huge voltage drop between the battery and the TCU. I already mentioned that the ground and battery feed are fed directly to the TCU using #10 wiring and then they break down to #18 wires running into the interface module and TCU.

Step one is check for voltage drop... hindsight is easy after the fact but the time we spent along with the dealer on this issue was excessive. The problem always gets fixed it's a matter of how many hours it takes. In the video you can see the dash and interior layout.

The fit and finish was very good on these models. We've had Cummins power as well in these buses .... official model >>>> ER Saf T Liner. They handled nicely and gave us a lot of miles without trouble. However the Cat 3116 engines did have valve problems and required a few rebuilds. Normally our buses stick around our fleet for 12 to 15 years depending on the mileage.

If there is premature rust sometimes we retire them early considering the cost it would take to repair properly. Thanks for hanging out...please leave a comment and/or share this post if you liked it. Until next time take'er cool :)

The School Transportation News Vlog

Editor's Note: The STN Vlog covers recent news concerning the school bus industry. The topics discussed are interesting and informative. I've placed the home page link below for STN. The mechanic information I get off their site is priceless with new technology on green energy and the latest mechanical engineering plus research and development. Below are more details on what will be covered in the Vlog below.

*School Transportation News Editor Ryan Gray goes into detail on NTSB's recent release of its investigation report on the March 12 crash of a motor coach in New York City.

*GK Design's fresh concept for a next-generation school bus.

*He also discusses the showing of the documentary La Camioneta at the Central American Resource Center where STN spoke with Arturo Zepeda, Community Organizer, and Juan Carlos Lopez, Immigrant Day Laborer.

*Lastly he talks about the viral video of New York school bus monitor Karen Klein, who was verbally assaulted by a group of students on the bus.



If you want to know more about school transportation news go to http://stnonline.com and check out their site. You will find a lot of extremely useful info on the latest technology and innovations along with special reports on what's going on in the school bus transportation industry.

Mechanic School Bus Pusher Model Overview

Mechanic Notes: working on school bus pushers.
The Bus Mechanic has a different approach compared to working on a pusher school bus since the drive  train is reversed at the back end. The controls are hard wired to a major junction box in the rear engine compartment. They have a single drive axle (1998 model pictured here) with a stubby driveshaft which eliminates a 30 foot driveline. That's one advantage to this design along with a very low noise level for the operator.

Since all the weight is on the rear axle another obvious advantage is the great traction and handling, especially in the winter. The multiple leaf spring pack provides a solid ride but without the hard knocks of a forward chassis model. Having one drive axle compared to a Class 8 tandem makes life easier when it comes to axle work. With just one axle the brake shoes are a special size at 8 5/8 inch wide with 16 1/2 inch brake drums.







The rear engine design has it's pros and cons. The front end is a real treat to work on as you can see here. however you  need to lay over the top of the engine for valve sets or head removal. The transverse fan is common in ER models which use a hydraulic drive fan motor but in this MVP Thomas pusher with a CAT 3126 the fan drive is belt driven.

As a Mechanic you will have new and exciting challenges in whatever piece of equipment you work on and in time you will always find easier ways to work around problems and get to know the symptoms / solutions as time goes on.

Mechanic Tips-School Bus Fleet Preventive Maintenance

Every Fleet Mechanic will have there own ideas on how to set up the best PM program for their equipment which will vary for different locations. With different climates, road conditions and power trains the procedures will never be exactly the same. Our shop likes to condense the annual PM since the regular checks are carried out every 5000 Kilometers (3000 Miles). Oil changes are done every 15,000 Km with an oil sample to be analyzed. Fuel filters are replaced at every oil change.

The Allison 2000 and 3000 Transmissions  are all running on TES-295 Synthetic oil which has drastically cut down on repairs. Right now we change the oil every 3 years, the newer buses have prognostics which tells us with a dash light indicator when the filter and oil needs to be changed. Air filters are replaced every year regardless of the mileage. Steering Filters are serviced along with changing the oil every 2 years. Valve Sets are carried out according to the Manufacturer i.e Cummins every 5000 Hours or 240,000 Km (Electronic Models). With the introduction to emission controls in 2007 we now have to perform manual regens if required and replace the DPF (diesel particulate filter) every 150,000 Km (90,000 Miles)

Air Dryers every 2 years with a new unloader kit and dessicant filter. Dirt is a factor on school buses so we service interior heater filters and the entire lighting system (Non-LED lights). We check the starting and charging systems including the battery pack. If necessary we will run tests on the batteries and alternator. years ago alternators and starters were removed for rebuild but the time and parts did not add up as an economic strategy so we decided to give these components a 5 year life span before determining if they would need replacement. The cost has come down so much on electrical reman units, it's feasible to go this route with the added bonus of a warranty period.
 
Spring brake chambers are replaced every 5 years regardless of mileage and condition. The Mercedes MBE 900 requires a coolant and thermostat change every 2 years, the other diesel engines in our fleet do not have a time or mileage limit but we're working on that to prevent antifreeze deterioration and thermostat failures. Our tires tread depth is measured every Summer and pressures checked during each service.

Common sense plays a role with any preventive maintenance program and we found the best way to set one up is to sit all the Mechanics down in one room and go through the fleet  figuring out the best program to cover all the bases. Breakdowns will quickly eat up your labor time and parts budget like wildfire so keeping on top of maintenance with a feasible program will pay you back 10 times over.                                                                     

STN Going Social With Mechanic & School Bus Industry News

School Transportion News has a new social network that covers everything imaginable in the school bus transportation industry. I especially enjoy the TechNet section which covers school bus repairs and new technology. This is a discussion group that anyone can join and participate in with questions and answers as well as comments.

With changing technology it's essential to keep in contact with other professionals in the field and share ideas and answers to challenges encountered on a regular basis.

Here's the link if you want to check it out.  I highly recommend this site for up to date knowledge and a free ticket to rub elbows with an experienced group of school bus mechanics willing to share their knowledge.
http://www.stnonline.com/social

Delco 38MT Starter Planetary Gear System

Planetary Gear Systems have been around for awhile. A very common use for them are automatic transmissions. The gear ratios can be altered by changing which gear is driving and which gear is being driven. There is always one of these 3 gears that has to be fixed.
This Delco 38 MT starter pictured has been operating in a 2007 C2 School Bus. You can see the rust forming on the terminals. This is common since starters are placed low in the engine and close to the elements. We generally remove the starters on all our units every 2 years and give them a check over.

We lubricate the bearings/gears and check brushes plus wiring.
On this model the solenoid and relay switch(fastened to the side) are non-serviceable so they have to be replaced if there is any doubt on their mechanical condition.





Starters have come a long way and now most are using a gear reduction with more torque delivered for cranking over high compression diesel engines. The Delco 38MT on our Mercedes MBE 900 Diesel engines does the job even with it's light weight. These starters pack a punch and are light weight with mostly aluminum castings.

The planetary gear system in this starter has the typical sun gear (driven by the starter motor) the ring gear is fixed and part of the of the starter body, while the planet gears are part of the actual armature that delivers the starting drive to the engine ring gear.

This set up is extremely durable since there are 3 planetary gears to absorb the load and speed required. I've always been impressed by what a starter can do. It takes a lot just to crank over a diesel engine by hand and when you look at this medium sized starter it's really extraordinary especially when you consider the extra load required for cold starts.

The planetary gear system on this starter has a gear reduction of 3 1/2 : 1 ratio which slows down the output of the drive gear (3 1/2 revolutions of the starter motor to 1 revolution of the output drive gear). What this accomplishes is a high torque output that enables the smaller starter motor to punch out the turning force required to crank over the engine.

Here's a great explanation on planetary gears at "How Stuff Works"

This mechanic information will give you a basic understanding on new starter models that are in use today.

HOMEPAGE

Used School Bus For Sale

Choosing a used bus for sale can be a difficult process if you don't know where to look. Ebay is by far the best resource for finding any kind of bus including used school buses. To locate a used school bus for sale follow the link below and you will be taken to Ebay, in my opinion the safest place to shop online.

Sellers have to be up front and honest with their selling practices because Ebay is watching. Every seller has a feedback rating that tells everybody how previous transactions went with past purchaser comments and ratings.

When looking for a used school bus for sale check out the sellers feedback. Have a look at the pictures provided and send off an email if you have a question for the seller. High mileage school buses doesn't mean they can't go another 100,000 miles depending on condition and what you plan on using it for.

Most used school buses are in very good condition since they were maintained under strict guidelines for obvious reasons, carrying school children everyday. The body rust could be an issue if the school bus came from a wet mountainous region that used a lot of road chemicals.

Gas or diesel engine?
My choice is always diesel since they can really take a licking with high mileage. Since most uses after the school fleet life of a bus is light duty, the life expectancy of a diesel will outweigh a gas engine. This all depends on the condition of the engine and how it was cared for.

The drivetrain is generally an automatic transmission. Allison transmissions are the most common. They are very durable and can go for many miles after retirement if cared for and maintained properly.

I have many posts here on my blog on common diesel engines and the Allison transmission found in school buses. They information is about failures and fixes along with maintenance tips. The search box in the top left hand corner will help you find what you are looking for.

Used Bus For Sale

Mechanic Troubleshooting - Allison Transmission Wiring

Our fleet has absolutely zero manual transmissions, school bus drive trains are unless requested differently come standard with Allison MD 3000 electronic transmissions. Once they get a few years use there are electrical glitches that occur.

In this case the code pointed to the C3 pressure switch. This switch reads the clutch pack #3 oil pressure in the transmission module. It activates at around 37 psi when pressure is operating the clutch release and engagement.

If it fails, a code 32 will pop up. Most times the wiring is at fault so we will check the continuity from the ECU to the Trans. Module. (shown in the photo above)
At the front end the blue connector has to be disconnected from the ECU where the appropriate terminals have to be probed and checked for resistance. The Allison troubleshooting manual is a "must have" for this procedure. For newer units the WTEC 3 is required(world transmissions electronic controls). These manuals are available at an Allison dealer.

You can tell if the system is in the 3rd generation from the 3 connectors at the ECU. In this case the ECU is located under the driver side floorboard attached to the body(Thomas 1999 Saf-t-liner).

The resistance read 25 ohms which is excessive. As with other codes replacing wiring has become a common occurrence. We use shielded wiring which we run front to back to eliminate the defective wire. Many times if transmissions act up and shift awkwardly in wet weather there's a good chance you have a wiring issue.

How To Manually Check for Transmission Codes

Press the 2 arrows on the shift pad with key on and read the 4 digit code that comes up. Record the first 4 number code then press mode which will bring up a second code(if any) . I hope this mechanic information has helped you out.

Cummins Engine Repair - Camshaft Gear Noise

Here's an email I received from Jonah Barnes, Lead Mechanic for the School District in Sierra Vista, Arizona. He noticed a noise coming from the front end of a Cummins C Diesel Engine. It turned out to be the camshaft gear. The photos tell the story and I'm amazed how long this gear lasted. It just goes to show how durable a quality metal casting can be and how long it can take a pounding without blowing apart. This gear would have made a huge mess if it exploded inside the timing cover.


"Quote"

Hello John,
It really wasn't too noisy actually. When the compressor placed an added load on the cam gear it would actually rock the gear back and forth and "knock" against the front cover. It almost really did sound like a " bongo drum".

According to the other mechanics who have worked here longer than I the previous attempts of replacing the compressor would cause it to be quiet for about a week or so then it would go back to the intermittent drum beat. They were adament that it was the compressor again and they were just getting bad ones.

They even went as far as replacing the fan hub and alternator a few times with no luck. I used redneck diagnosis and used a piece of fuel line and placed one end in my ear and the other end I listened around until I was sure it was in the front timing cover.

It actually made the sound faintly using the hose when the compressor governor unloaded but it was not as pronounced as when the compressor was running. I have only been with the school district here just a few months but have been a mechanic most of my life.

I have run into a lot of different broken things but this gear being broken like this and the engine still running down the road and running routes without missing a beat (literally) is pretty amazing I think.

They said it has been doing this almost 3 years but no one really thought it was that serious. I contacted cummins here and they know of no recalls on the gear but they are available from cummins for about $350.00 US.

I am originally from Indiana and my dad was an engineer for cummins for several years and he was pretty amazed too that it was still running. I don't think it would have much longer though. I will be sure to keep you informed of other mysterious problems I encounter on my end.

The more information out there the more it can help other people who are sitting there scratching there head. We tend to get a lot of life out of a bus out here because of not having to deal with rust issues so there are several high mileage buses just waiting to keep me employed!

Thanks again for your help.

Jonah Barnes
Lead Mechanic
Sierra Vista, Arizona

"Unquote"

Thanks to Jonah for sharing!

I love to receive emails with Mechanic Information sharing experiences of real life repairs & troubleshooting. This is helpful for Mechanics & Truck Owners who always have new and interesting problems to solve.

Diesel Engine Repairing & Troubleshooting.

One of my favorite tools for diesel engine troubleshooting and repairing is a digital heat sensor.

You can compare the block temperature to the coolant gauge reading, check for an engine miss, confirm any type of heat sensor that's sending a signal.

Case in point: I had a Cummins ISC (electronic) diesel engine that was not heating up properly. So the first thing you want to do is get the story first hand from the operator.

Questions like: How long until it DOES heat up or when did it START to develop?

In this case the operator stated the problem started when the cold weather showed up. So the first thing to do is a scenario with the bus (2000 Thomas Saf-t-liner school bus). So I went for a road test and the first thing I noticed was the engine was heating up very slowly.












I used the digital heat sensor and confirmed the coolant gauge was correct and the engine was running at 170 degrees after a 1/2 hour road trip.

From experience I know that the engine fan locks on with these engine models so I tested both the coolant sensor and charge air sensor.

They have a 3 to 3.5 ohms reading. No ohms means a failed sensor (open circuit).


These sensors determine when the engine fan should come on full which is usually around 200 degrees, in this case the fan came on full right from a cold start up.

Since both sensors were good I replaced the control module which cured the problem(located below the engine fan on the frame). Parts aren't readily available for this module and rebuilding it isn't worth the downtime.












When you have a fleet of school buses there is an advantage to numerous buses having the same specs.

The troubles usually happen more than once and history will help with diesel engine troubleshooting and repairing.

Thomas Buses-How To Check & Replace A Fuel Guage Sending Unit

A fuel guage sending unit is a ground circuit for the fuel guage. The position of the sender float changes the resistance to ground by way of a resistance winding that is built into the sending unit. High or low resistance determines a full or empty fuel tank. The complaint I'm dealing with is an erratic fuel guage reading, the guage needle is bouncing all over the place every time the driver turns or stops suddenly. This Mechanic information is for anyone who knows how to turn a wrench and you'll see how easy it is to check out the sending unit.
Firstly, every fuel guage sending unit is very similar on older vehicles. On newer automotive applications the sender is built into the fuel tank electric fuel pump assy. This is an integral unit and quite expensive.
I'm working on a 2000 Thomas Bus Saf-T-Liner which is running a Cummins ISC Diesel Engine. In this case the sender is seperate and very easy to test.
The sender is located 3 seats back under the floor. The floor plate has to be removed to gain access.
The sender can be tested using a jumper wire. Disconnect the wire connector at the middle terminal of the sender [this comes from the fuel guage]. Connect the jumper to the loose wire and hook it to ground.
[click on each picture to enlarge]
Turn on the ignition switch and the guage should go right over to the full mark. What this does is tell us the gauge is OK since the wire to the sender has been grounded directly without any resistance. The sender has been bypassed and now we're sure it has to be replaced.
Remove the retaining screws and ground return wire and replace the sender the same way it came out. It's that easy!
So now you understand the circuit through a fuel guage sending unit and how easy it is to diagnose and repair. I hope this mechanic information has helped you.

Thomas Buses - Air Throttle Repair

During most of the 1990's before the electronic age the engine throttle was air controlled on Thomas Bus Saf-t-liner models. The high idle was a welcome addition since it was operated with the flick of a switch on the dash.
In the photo the air line can be loosened while the engine is running where there should be no air with the high idle switch turned on. The throttle linkage is moved into high idle with spring pressure[no air]. The regular foot throttle air line is attached to the underside of the cylinder. {Click Photos To Enlarge}

The air is controlled with an electrically actuated air valve that is normally open with full air pressure and no power from the switch. The air pressure moves the internal piston to the idle position compressing the internal springs. When the high idle switch is turned on, battery power closes the switch and blocks air pressure to the air throttle allowing spring pressure to kick the throttle linkage up to high idle[around 1000 rpms].



The disassembled air throttle consists of springs pistons and seals and is easily serviced. The one thing that does fail is the 1/4 inch adjustment bolt on the end cap wearing through the washer that seats on the internal spring. Thomas buses have changed since this decade and are electronically controlled with an onboard computer but if you still own a saf-t-liner pusher these systems are reliable and easy to maintain.

Diesel Engine Oil - Switching to CJ-4 For 2007

To meet 2007 EPA standards low emission engines must use CJ-4 engine oil formulated especially for diesel engines using exhaust aftertreatment systems. New EPA standards are a 50% reduction in oxides of Nitrogen and 90% reduction in particulate matter. CJ-4 is a low ash diesel engine oil that can be used in older engines but comes at a higher price. The warranty becomes invalid if regular CL-4 oil is used in '07 diesel engines but the 10 to 30% increase in price is significant when it comes to a fleet operation.

You could stock both oils but there is a risk of operators who normally add oil to there assigned unit could inadvertently grab the wrong oil and mix the two. Mind you one litre of the wrong oil would not be detrimental to the life of the 2007 diesel engines but over time it would make a difference. The possibility of an engine oil test after a failure could happen.


Our fleet operation only stocks the CJ-4 so there is no confusion and zero risk to jeopardizing the integrity of the DPF [Diesel Particulate Filter]. The DPF has a regenerating cycle when a difference in input and output exhaust gases in the DPF [muffler] are picked up by the engine ECM [electronic control module]. The DPF literally cooks out the carbon build up and it gets kicked out the exhaust pipe which creates extremely high temperatures.


EPA standards are here to stay, with vehicle emission standards being on top of the heap when it comes to environmental concerns. Our School Bus Fleet will be receiving a Hybrid International School Bus next September. This will be an interesting technology to experience and another brand new learning curve to grasp.

Thomas Buses-HDX Rear Emergency Window Modification

Thomas Bus HDX Pushers have a rear emergency exit window that has 2 ground switches. One for the alarm, warning the driver that the window is open and the other is an interlock that does not allow the bus to start when the window is latched.

The problem we were having was water seeping through the seal causing a circuit to ground tripping the alarm even when the window was fully closed. The quick fix was to grab an air line and dry off around the switch and grounds. Pictured here is the alarm ground switch[click on pic to enlarge]

This is the window locking latch that must be unlatched for the bus to start. The rivet on the outside is going to ground while the handle contacts a strip riveted on the inside of the frame.






We installed 2 intermittent switches in a currently available Thomas Part that encases the 2 switches and does the same job as the factory install.






The window must be removed to rewire the new installation. The 2 original ground switch wires are attached to the new switches in the casing and grounded at the case.






Once installed the [starting interlock] bolt style latch locks the window and the other switch is depressed [just above] to shut off the window alarm.
The latch catches on a "C" shaped flat bar that was fabricated to lock the window.




The latch bolt is in the open [run] position and the bus will start, this is a safety factor that newer Thomas Buses come with from the factory.

I will be updating this post soon with part numbers.