Sunday, March 05, 2017

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!!  

Thursday, November 03, 2016

Mercedes MBE 900 Diesel Particulate Filter Cleaning

The emission controlled diesel engine causes a lot of soot and ash build up through out the intake and exhaust system. The sticky GOO that invades literally every square inch of the engine's components has to be removed by way of passive or parked regenerations or physically cleaning off the build up before it's too late.

The video shows us using a household cleaner to soak the diesel particulate filter from a Mercedes MBE 900 diesel engine. This filter was condemned by one of the local shops we took it to for cleaning. They baked it at extreme temperatures to remove as much residue as possible. In this case the test for flow was too low for them to give it a pass.

So our options were to buy a cleaned DPF from the Mercedes dealer for $800.00 or experiment on our own with 'Mr Clean'. In the past we've used it to flush out cooling systems when there's oil contamination or for removing soot from small emission parts but this was something we wanted to try since there was nothing to lose. Mr Clean is an excellent degreaser so our plan was to let the DPF soak for 2 days and thoroughly steam clean it off.

I'm happy to report that since reinstalling this DPF there has been no fault codes for close to three months. The typical 2631 code (turbo boost performance) has not reared it's ugly head for some time now. Since then we've done 2 more buses with the same success. How long until the DPF starts to get plugged up again is hard to say.

They're supposed to be serviced or replaced around 150,000 km according to the Mercedes estimate but that's up for discussion. The plastic drum we used to soak the filter was a used DEF container. One gallon of Mr. Clean is all it takes to get a good strong solution. The soot and ash collected on the filter is liquified so when you use the steam cleaner it comes out looking really good.

Another thing we do is clean contaminated sensors and reuse them instead of spending 50-60 dollars each. Replacement parts for emission controlled diesel engines are expensive so any shortcuts or experiments that work is a bonus :)

Friday, May 06, 2016

Park Brake Not Releasing C2 School Bus

Park Brake Not Releasing C2 School Bus from John Whelan on Vimeo.
The park brake on a Thomas C2 school bus has an interlock that requires the service brake to be applied before the park brake releases. Most of the issues we had was the stop lamp module and park brake module located at the air management unit depending on the year of the bus the bank of modules are at the very back in front of the differential housing or behind the transmission.

There was a recall years ago regarding the stop lamp switch located on the right side of the AMU looking towards the back end. The problem was the internal switch wasn't contacting properly so Thomas revised the design and there have been very few problems since then. If you go into  accessfreightliner you can bring up the AMU layout for the VIN you're working on.

The park brake light on the dash is a warning to the driver not to drive away with the park brake on, When a driver pulls out the park brake valve the modules sense this and through the J1939 messaging system air pressure changes trigger a signal that is sent back to the dash illuminating the park brake light. When you come to a stop and shut down the engine if the park brake valve is not activated this triggers an alarm until the park brakes are applied.

Unless there's an air leak most of the trouble with AMU modules is the switch contacts. The first check is to see if the brake lights are working if not the stop lamp switch in the module can be tested by jumping the signal to the bulk head module bypassing the internal switch in the module. If the brake lights work there is a problem internally with the stop lamp module.

The same test can be done with the park brake module. In the video the two terminals that sends signals to the BHM are jumped and if the park brake indicator light comes on then there is an internal problem in the module itself. There are only two wires to this module so it's a straight forward test. Most of our troubles have been with these 2 modules.

Also you can not rule out opens and resistance in the wiring harnesses. This is another possibility that you might have to deal with. If you have a Freightliner dealer in your city you need to get a user and  password to the accessfreightliner website. Here you will find the schematics and troubleshooting required to fix an electrical problem. Without this information it's very difficult to blindly find an electrical fault especially with multiplexing that's here to stay for a long time.

Wednesday, March 02, 2016

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.

Wednesday, January 20, 2016

Mercedes MBE 900 Parked Regen

Using Mercedes Software To Perform a Regen from John Whelan on Vimeo.

In 2007 everything changed for diesel engines. The newly integrated emission systems were now the most important cog in the wheel. They had to meet certain parameters according to the onboard computer programming and if they did not an engine code would appear and stare you in the face until the fault was repaired. At first it was frustrating since we had not worked extensively on these emission controlled MBE 900 models.

The Mercedes dealer become regular aquaintances and saw our buses many times over the 5 year warranty period. Engine codes appeared then went away and popped back up again a couple of days later and so on. Once warranty expired we knew it was time to get into these engines more than ever. The online resources called DDSCN was very helpful along with the Detroit Diesel engine software we had loaded up on our laptop.

There was a lot to absorb since everything was monitored by the MCM (motor control module). Turbo boost, EGR and diesel particulate filter cleanliness determined how well these engines were going to run. Once there was a restriction in the DPF a regular code 'dpf out of range high' would pop up until we did a parked regen or repaired a sensor or plugged up sensor air line. Without the diagnostic software we could not do a thing to get these engines back in shape.

The DPF section in the software allows you to watch all of the temperatures in the DPF and DOC. The differential pressure indicated if there was a restriction (3 psi max). The video goes into a parked regen where the temperatures go as high as 1,000 degrees cooking out the DPF. This half hour process does the job and most times it makes a huge difference in driveability.

Another option to troubleshooting was an over the road regeneration where you could monitor the regen process while on a road test and record the data. The Detroit Diesel software is very good for narrowing down a problem. Once there is a code the software can troubleshoot the code step by step which really made life easier for the techs.

The filter cartridge in the DPF is replaceable which we had to do a few times because the regen process was not working. It's similar to an air filter that you clean out and stick back in the cannister. Eventually it's not going to clean up properly over time due to the compounded accumulation of debris. Soot and ash are a never ending problem throughout an emission controlled diesel engine and the MBE had lots of that with sensors plugging up and EGR valves sticking.

They are definitely not the perfect engine when combined with emissions but working on them and gaining experience helped our shop know what steps to take whenever a fault occurred.

Saturday, November 28, 2015

Air Brake Job Using Machining Tools

Machining School Bus Brake Shoes from John Whelan on Vimeo.
The lathe in the video makes a complete cut on the brake shoe allowing 100% contact. This is necessary when a chatter or brake pulsation is occurring with brakes applied. We used to machine all of our brake shoes on all brake jobs but found it to be time consuming. Now we replace the drum and shoes plus hardware.

There have been some cases where the contact is not favorable and machining was necessary. Our buses use all sizes of brake shoes from 6 inch on the steering axle to 8.5 inches on the drive axle. All air brakes come standard with automatic slack adjusters. I talked to a sales rep from Canadian Brake and he says there are still customers who purchase manual slack adjusters. I haven't seen one of those for years.

Since we run a fleet of school buses it's policy to replace the auto slacks every brake job. There is only one drive axle so 200.00 for a couple of slacks is nothing. Spring brake pots get replace every 5 years. Our choice is to use MGM piggy back pots because of their superior quality. For an extra 50.00 that's great insurance.

A few months ago I attended a trade show in town and talked to the Haldex slack rep and he informed me to not grease them too often. This will disable the mechanism with the hydraulic effects of the grease. We have auto grease systems installed on our S cam tubes and slack adjusters. That has caused grease leakage on to the brake shoes from over greasing which is picked up by the government vehicle inspectors.

If it's bad enough the wheel has to be removed and shoes cleaned up. This is a procedure we want to avoid so now we advise the auto grease system installers to avoid hooking up to those areas including the steering box bearing. That's another spot the DOT makes notes on during their fleet inspection thinking it's a seal leak.

Getting back to the brake lathe it does wonders on pesky brake problems that the operator experiences. There was one time when we just kept machining down the shoe material until there was a total 100% contact to fix a pulsation. The drums had to be machined as well. There was a lot of material removed to accomplish this but the front brakes last twice as long as the drive axle brake so it was worth the labor to do so.

Sunday, October 04, 2015

Cat 3116 Tune Up Tools and Procedures

CAT 3116 Diesel Engine Tune Up from John Whelan on Vimeo.

Cat 3116 Injector Adjustment

The video shows how the Cat Tuneup Tool Kit works on a 3116 diesel engine. When the injector is installed I find it's easier to place the dial contact on the injector by feel and then having a look to make sure it's seated properly. In the Cat tune up kit you need the dial indicator, contact point, timing gauge block and magnetic base.

On top of the valve cover is a data plate that has the factory timing spec. This is the reading that you need to stick to when the timing tool is all set up. First of all you need to preset the dial indicator on the timing gauge block and set it exactly to 62.00 mm.

Rotate the engine so cylinder #1 is on the compression stroke. Both valves will be closed and cylinder #6 will be on the exhaust stroke. Injectors 3 5 6 can be set in this position along with 1 2 4 intake and 1 3 5 exhaust valves. (Intake .015 in exhaust .025 in) The injector spec on the valve cover can vary plus or minus .020 mm.

Rotate the engine 360 degrees and adjust the remaining valves and injectors. The Cat tool kit comes with a manual and gives you a detailed step by step procedure for these adjustments. It's a fussy operation but once everything is set these engines run very nicely. Keep the valve adjustment on a regular schedule because of a weakness in the exhaust valve stems that blow apart if there running at high temperature and rpms.

I hope the video gives you a better idea what's involved with injector set up on the 3116. The entire tune up kit is pricey and the last time I checked it was up around $4000.00 Canadian. We had to purchase it however that was in the mid 1990s and we paid much less than that. You can not get away with setting up these engines (except valve adjustment) without the kit.

Thursday, September 24, 2015

Cummins ISC Diesel Engine Turbocharger Failure

Cummins ISC Turbocharger Replacement from John Whelan on Vimeo.

This failure occurred without any notice. In the video you can see the nut that loosened off and backed right off of the turbo shaft. Once this happened it caused seizure and a large amount of oil started pumping through the intake system. The turbo bearings are lubricated with pressure engine oil so there's a big mess when this type of failure happens.

The oil did help in a way flowing into the charge air cooler and leaking out a crack in the cooler core. So replacing the charge air cooler was added to the list. Once the turbocharger was replaced with a re-manufactured unit along with the charge air cooler this engine was back in business. It's important to flush out the air piping in case debris got trapped. The steam cleaner does wonders for this type of task.

Preventing this failure would be hard to do since normal preventive maintenance does not involve removing air intake and boost piping for inspection during a service. This would take too much time so the only other clue would be poor boost performance or turbocharger noise audible to the mechanic or driver.

I hope you find this video informative as food for thought. A failure of this kind will happen again undoubtedly somewhere...somehow. It falls under the "how the hell did that happen" category. If you have a comment please state it below. I like to get feedback form readers who have gone through something similar.