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.

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-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.

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

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.

Cat 3116 Engine Installation-Thomas Buses

This is a Cat 3116 Engine installation into a Thomas Pusher. This process is much easier with the transmission installed [MD 3000 series]

The transmission jack supports the trans. and the hydraulic crane hooks on nicely to the front engine lifting eye. With some assistance we rolled the whole assembly into the backend with ease. You'll notice the rear subframe and bumper have been removed. This is one great advantage of owning a Thomas pusher although we don't want to be doing this particular operation too often...

Thomas Bus Heater Blower Motor Replacement

There are 4 Front heater blower motors on the Thomas School Bus Saf-T-Liner Model. With Winter on it's way this is an important maintenance area, defrosting of the windshield has to be accomplished quickly.
The heater blower motors also serve as defrost motors with a quick change up on the controls. Each motor has 4 wires for high/low speed and 2 grounds.
The direction can be reversed using the ground wires. One is grounded directly while the other is connected internally back to the motor, reversing these 2 will change direction.

This Thomas Pusher Bus is a 1992 model, removing the middle and left side panels will allow access to all 4 heater blower motors.
The tandem motor in the middle looks after main defrost and heat while each motor on the ends provide floor heat to the driver and defrost/heat to both ends of the cab.
The main failures on these motors are dry bushings causing noise and seizure and blower cage noise.

Allison Automatic Transmission Wiring & Troubleshooting

The Allison Transmission Control Unit on the 1999 Saf-T-Liner is located under the floor directly below the driver. There are 3 plug-ins which is an easy way to tell this is a W-TEC 3 system [World Transmission Electronic Unit 3rd generation]. The Transmission vehicle speed signal enters here and exits at two different plugs, one for the speedometer and the other to the Cummins ISC ECU.



We will trace the exiting wire from the ECU that provides our Vehicle Speed Signal since that is the code and the speedometer is operating normally. Since we have speedo operation that gives us an obvious clue that the incoming signal to the Trans ECU is OK.[resistance should be checked regardless]
The wire we want to trace goes from the Trans ECU to the VIM [Vehicle Interface Module]
Removing the dash back cover makes this check MUCH easier.


Remove the cover from the VIM








The VIM is an interface between the Trans ECU and the chasis wiring. Various operations are accomplished through this module like the exhaust brake, back up lights, neutral start, high idle and of course our vehicle speed signal runs through here to the Engine ECU.
[Click the Pic to Enlarge]
Now we can check the continuity and see if we have a circuit. After checking this out there is no connection between the Trans. ECU and the VIM.
I'll run a new wire to repair the open circuit, an easy fix. It's nice to get a gimme once in awhile!

In the VIM you'll notice a series of relays and 2 fuses. If you experience a dead keypad and there is battery voltage to the VIM then check these fuses. One is main power & the other is ignition power.



The other vehicle speed signal wire [tan wire # 157] comes from the trans ECU directly to the speedo. The speed signal to the VIM is the same number [tan wire #157] since it comes from a different plug.




This is where it all begins from the output shaft housing of the transmission. The output sensor is a single coil unit that sends an AC voltage to the Trans ECU.
There is a toothed wheel rotating inside the housing that creates the signal to the sensor.
The Allison 3000 Automatic Transmission is a very reliable unit when maintained properly. The wiring problems come and go but are easy to trace and repair with the right tools and wiring diagrams.

Basic Electricity How It Works

Basic Electricity applies to any type of machinery running on a 12 Volt Direct Current system. Once you see how it works it's much easier to understand, instead of making it complicated I will explain the basics in layman's terms. This is the chasis electrical panel on a 1995 Thomas Saf-T-Liner[pusher]. note: Click on the Photos to Enlarge
It looks a lot more complicated than it really is. Any 12 Volt electrical system has a source of power from the batteries, in this case we run on 3 top post 900 CCA [cold cranking amp rated] batteries.

That's a lot of juice available to supply voltage to the body and chasis circuits, but there are environmental and mechanical factors that will cause a severe voltage drop if not detected.

Voltage is like a river, the heavier the flow the stronger the current. If there's a dam down river this will cause resistance and restrict the flow. If the source [the river] is cut off so is the flow.

Any resistance between the battery and the load will cause a voltage drop, this can be caused by a short circuit, open circuit or just plain resistance from corrosion. A good rule of thumb when troubleshooting a 12 volt circuit is to:

Step 1...Check the battery -Sufficient Voltage?
Step 2...Check the load i.e. A Light or Relay, is it working?
Step 3...Check the connections
Step 4...Check the wiring

Very basic steps but most electrical problems are found using these steps, check the simple things first.

Say for instance you have 13.5 volts at the battery and 0.0 volts at the load. This means you have an obvious Open Circuit problem [a rubbed through wire possibly]

Using a digital ohmmeter you can ground one end of the circuit and using the two ohmmeter leads at the other end connect one lead to the faulty wire and the other end to ground. What you're doing here is checking for Continuity [the ohmmeter has it's own battery voltage which runs through the faulty wire].

By the way, the multi-meter you use for newer equipment must be Digital to safeguard the electronic systems that are much more common in new trucks and buses. If the circuit breaker is popping on and off then there is obviously a Short Circuit which can be hunted down using a Short Finder [a very slick tool to have which saves a lot of time and grief]

You hook up the auto-reset breaker then slide the meter along the wiring until it finds the exact spot where the short is [the indicator needle stops moving] along the frame or body.
Clean the connections and check the load i.e. marker light or tail light for poor grounds & connections.
That's a Quick Overview on Troubleshooting a Basic 12 Volt Electrical Circuit.

Thomas Pusher School Bus - Removing Radiator

This Thomas School Bus had a leaking radiator that required removal and repair. All the body panels must be removed to gain access to the radiator. The air charge cooler must be removed as well, it cools the turbocharged intake air to create more density/volume.




Fortunately this radiator core was repairable being a 1999 model, This is the best way when running a fleet of 70 School buses. The Thomas Pushers have transversely mounted radiators since they have rear drive engines and transmissions.