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

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.

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.

Allison Transmission Problems-Hard Shifting

Allison Transmission Problems vary and one typical complaint is hard shifting. I recently looked at a 1999 Thomas Saf-t-liner [pusher] with an Allison MD 3000 series transmission. It had a hard shift into 2nd gear and a hard downshift as well. No codes came up so it was time to check the mechanical side and do a clutch pressure test. The test has to be turned on "enable clutch test" with the "ProLink" so each shifting range can be turned on with the unit running in the shop.

The pressures are compared to main convertor pressure and sits around 270 psi on this test. The individual clutch pressures must be within 5 psi. If there is a big difference that tells us the clutch itself is worn or there is a hydraulic problem. This test revealed no problem in this area and one conclusion could be a clutch that has to be physically inspected for imperfections. There is always 2 clutches applied to get a forward gear in this case it's C1 and C4 for 2nd gear.

The next step is a big one, whether to disassemble the trans and inspect for problems or live with the hard shift. I tried to "fast adapt" the shift parameters with the "ProLink" which did not change anything. This is one of the allison transmission problems you might encounter, if we get into this project any deeper I'll keep you posted.

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.

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.

Thomas Buses - Allison Transmission Wiring Modification

Thomas Buses are driven by an Allison MD 3000 Series Automatic transmission. This 1995 Saf-t-liner model had no power to the operators trans shift pad. The shift pad is also the Trans ECU [electronic control unit] so as I have said before always check the source , in this case the batteries.

There was a power feed open between the batteries and the rear control box at the engine compartment. I was able to limp the bus home by jumping battery power to the terminal board inside the control box which goes to directly to the operators cab supplying battery power to the shift pad/ECU.

This is obviously how I determined the open from the battery to the control box/terminal board [standard wiring on all Saf-t-liners]. The code for this fault is 35-00 [push both arrows on the shift pad at the same time with key on to retrieve code {under normal circumstances}

The control box at the engine compartment houses the engine intake heater components, lights etc. as well as the main battery power to the front electrical control box below the driver's side window.





My plan is to totally bypass the trans battery supply circuit from the battery to the ECU. Here you can see I've already cut the wires [larger #8 guage pos. and neg. wiring come from the battery while the smaller #10 guage pos. and neg. wires are running up front to the ECU]




This is the terminal board I mentioned earlier, factory wiring with spade connectors that connect the battery feed wiring to the trans. ECU wiring.

The first picture [top] shows the #8 pos. and neg. wires hooked up and running directly to the trans ECU. This will eliminate the extra #10 wires shown in the rear control box.


In the drivers cab the rear cover has been removed for easy access to the back of the trans. ECU. The upper plug can be removed and inspected for battery power. Both pos. and neg. #18 wires [2 each] run into this plug directly from the battery. The #8 wire is downsized under the dash to accommodate the smaller wiring into the ECU plug-in.



The 12V source from the battery is shown here with no connections in between. With a Digital Multimeter you can check for a battery voltage and a good ground.






So now you know when you have a shift pad with no reading [key on] the first thing to check is the battery voltage.

I hope this has helped you become more familiar with Thomas Buses equipped with Allison MD 3000 Series Automatic Transmissions.

Thomas Buses-How To Replace A Signal Switch

Thomas Buses have a signal switch that is very easy to replace. Most of the time the switch does not cancel properly which is a pain for drivers who have other things to look after and don't want to be constantly reaching for the the signal switch to manually turn it off after every corner.
The Thomas School Bus Saf-t-liners are similar with many similar components, the signal switch design did not change for many years.

Remove the upper and lower covers on the steering column. The upper cover is easier to remove when the steering column is tilted all the way down.






Use an allen wrench to remove the 2 signal switch screws holding it to the column.

Here is a top view of the upper screw being removed. As you can see the access to the signal switch is very easy.







Unplug the 2 wiring harnesses to the signal switch, they only fit one way.

Important Note:

One thing that you should watch for is to make sure the wiring running down the steering coluumn has enough slack. I've had problems where the signals do not work or short out on the column because of not having enough extra wiring to accommodate the tilt steering column movement .

The signal arm is held in the switch with a catch and can be removed by pulling on it aggressively until it releases from the switch.

Reverse this procedure and now you know how to replace the signal switch on Thomas Buses.

Cummins Engine Repair- ISC Engine Code 434

Cummins Engine Repair not only involves mechanical but electronic repairs as well. The ISC model has an ECM that sends and receives messages that controls every performance aspect of this engine. The main source of power is the battery that depends on wiring that feeds the Cummins ECM sufficient voltage to run the engine systems.
Pictured is the laptop shot of an active code #434 a power interruption that derated the engine to a crawl. Derating protects the engine from sustaining high speeds and rpms which would cause more severe damage. This ISC Cummins Engine is in a 1999 Thomas School Bus Saf-T-Liner. With an active code you can retrieve it with the diagnostics switch [on the dash] & ignition switch turned on. The code will beep each number with a pause, in this case 4 beeps 3 beeps 4 beeps with a pause in between each #.

A closer look [click on photo to enlarge] shows numerous fuses on the positive feed wire to the Engine ECM [electronic control module]. Our code information is telling me there is a power problem so "the first thing to check is the Source."

Using a digital ohmmeter check the fuses for continuity and clean all the connections.

Note: Before this maintenance step I could not get a reading on my laptop, there was a communication error between the engine and the "Cummins Insite" software.
After cleaning all the connections and testing the batteries the communication problem was erased and the code disappeared.

There is a variety of 7.5 and 10 amp fuses inline with battery to ECU supply power. I traced the wiring back along the frame as well to check out any other possible problems. The rule of thumb in this situation is to check connections and wiring closest to the environment and work your way up the line.

Using a Snap-On Micro Vat Battery Tester I concluded that all the batteries were in good condition. This tester does a very quick assessment of battery condition and also tests starting and charging systems quickly.

The Conclusion to this problem was simple, servicing the battery connections which had enough resistance to cause the #434 code to occur. It makes sense since the ECM reads Voltage at a very sensitive level.

After an extensive road test the code did not come back and this experience was a good one telling me [and you the reader] to always check the simple things first!

Now we're ready for the next challenge with Cummins Engine Repair.

Recommended: Diesel Engine Repair Manuals

Cat 3116 Engine-Repairing Common Engine Oil Leak

The Cat 3116 Engine has a common oil leak that is very easy to fix. The first step is to remove the steering pump which is driven off the air compressor. The next step is to remove the rear housing from the air compressor, it's held on with 4 nuts on studs, there is a square o-ring that seals the rear housing to the main compressor body. The Compressor is a "Bendix 850 Model" which is a 2 stage [2 cylinder] unit.



The Cat 3116 Engine oil pressure guage and air compressor oil feed lines run off the housing pictured here bolted to the engine block which is connected to the main oil pressure gallery. There is an o-ring seal between this housing and the engine block.





This 2nd block is the air compressor rear support block that has an o-ring seal as well. This block also serves as an oil drain for the 3126 model which has a oil pump above that supplies high pressure oil to the HEUI system [Hydraulic Electronic Unit Injectors].




Pictured here is the oil pressure manifold housing removed. The heat is absorbed by these 2 o-rings and they become very brittle causing an oil leak. Diesel Engines run from the "heat of Compression" so you can bet the engine block is smokin' hot! when operating temperatures are met.





The manifold housing o-ring has been replaced and is secured with 2 M8 Metric bolts. These 2 areas are a common oil leak in both the Cat 3116 Engine & 3126 Diesel Engine .






Recommended: Diesel Engine Repair Manuals

Haldex Brakes-Self Adjusting Slack Adjusters

Haldex Brakes self adjusting slack adjusters are installed at the factory on all Thomas Buses equipped with air brake systems. This allows much better control over brake adjustment especially for a large school bus fleet operation. At the same time there are operating and maintenance procedures that must be followed to get the best performance out of Haldex Self Adjusting Slack Adjusters.

Here Are Some Important Maintenance Steps To Follow:

• Visual and operational checks must be made at every scheduled PM check
• Check adjusters and anchor brackets for visual damage
• Automatic Adjusters should not be operated as manual adjusters
• Standard chasis grease may be used to lubricate Haldex Slack adjusters
• Lubricate the Standard 409-10 model every 50,000 miles or 3 mths
• Lubricate Reduced Maintenance 409-10 once a year
• No-Lube 429-10 adjusters are sealed and do not require lubrication
• DO NOT use Moly-disulfide grease as this will effect the operation of the slack adjuster
• Replace the slack adjuster if the TORQUE is less than 13 ft lbs when backing off the adjuster. You will hear the ratcheting sound while doing this.
• Haldex Slack Adjusters should be checked for operation between 90-100 p.s.i. in the air brake system reservoir. Anything over 100 p.s.i. starts to measure deflection in the foundation brakes and does not give a true reading
• To self adjust Haldex Brakes the operator has to make several full brake applications [spring brakes released] preferably with brakes hot and over 90 p.s.i in the air brake reservoir.
  
  Here is a very good video explaining proper Brake Adjustment on an air brake system. The self adjusting slack adjuster in this video is not a Haldex but the exact same procedure applies. In my experience with a properly adjusted Haldex slack adjuster the push rod travel averages at 1.5 inches of travel. Well below the Department Of Transport maximum allowed brake adjustment specs.
  
  Self adjusting slack adjusters are now standard equipment on all Thomas Buses and are a real asset for large fleets who must keep up with Department Of Transport Standards especially with air brakes and proper adjustment. I hope this post has informed you on the proper maintenance of Haldex Brakes.

Charging System Test - Heavy Duty 12 Volt System

A charging system test is a very basic operation with the right tools. Pictured is a Leece Neville 160 Amp Alternator which works very hard to keep the three 900 CCA [Cold Cranking Amps] bus batteries charged. Thomas School buses have a large demand with extra lights, electric motors and the engine air intake heater. The diesel engine in this case is a Cummins ISC Electronically controlled unit.



You're probably wondering what the heck is that device hooked up to the batteries? That's my tool of choice, a 'SUN' VAT 33 [Volt Amp Tester]. This model has been around for decades and still very reliable, our shop does have a digital tester but I like this one for doing a quick check on the charging system. The reason I'm testing the charging system is because of an unusually low voltmeter & ammeter reading.




The main leads are hooked up to the positive and negative battery terminals while the amps pickup is clamped around the positive cable to read alternator ouput back into the batteries. The engine must be running at high idle [approx. 1200 rpm]

The black control knob I'm turning puts a load on the batteries which should kick in the alternator to charge very close to maximum. Once I bring down the system voltage to 12V the alternator should normally put out a full charge of approx. 160 amps.

The test revealed the alternator is only putting out 80 amps maximum which will not keep up to the demands of a fully operational school bus & must be repaired or replaced.

This is a fast and easy way to do a charging system test on a 12 Volt Heavy Duty charging system.

Click here to sell your car online at eBay!