Saturday, February 23, 2008

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.


John said...

learn more about electrical troubleshooting tips at

John Whelan said...

Instead of just placing a link you could add to this post with your knowledge.

busdriver4 said...

I was wanting to know if you could at all possiable help me with this problem that we have with a Thomas built bus? I have been driving a Thomas bus for 3 years now and would not change for nothing (it is my caddy) however I do have a problem with the heating for some reason when winter comes I have the worst heater ever made..I have talked to my bus mechanic about this problem and he just reassures me that all Thomas buses are like this! We have a total of 4 2001 models and there is 1 out of these models that heat up..The windows do not defrost so we have to turn the fans toward the windshield and passenger door to unfog it and all they do is blow out cold air that is if you get air out of any of them! I have heard from lots of mechanics in which are not diesel that the filter is probably plugged up but our mechanic reassures us that it is not..I have also heard from other people that it could be the thermostat is bad but our mechanic reassured us that Thomas bus has no thermostat for it is not a semi..Could you PLEASE help me with this matter and if you tell me that my mechanic does not know what he is talking about I will agree with you..I just want answers on why this is happening to ease my mind for it gets very cold and I do have 64 kids on the bus that complain it is cold..Thank you for your time...

John Whelan said...

The biggest problem with the Thomas heating system is the length of the bus and the area it has to heat up.

Does the engine temperature rise in a reasonable amount of time? Compare this time to the other bus that's heating up OK.

You're saying there's no air pressure out of the front heaters. If the filters are OK check the ducting and overall performance of the heater motors.

I've had to seal around the body structure that surrounds the front heater motors with silicone. Just look around the upper panels facing the cab.
If the engine temperature is slow to come up then of course check the engine thermostat.
Check the coolant pump (inline with the 1" heater hoses feeding the inside heaters). Make sure it working.
A common problem is the engine fan sticks on. If you accelerate and hear a major thrust of air (bus stopped). Go back and see if the fan is going full tilt while the engine is still cold. It should not lock on until 200 degrees.

There is a coolant sensor (one terminal) at the thermo hsg. that usually fails.
If your engine does not have shutters in front of the radiator.

(To help with engine warm-up)

Place a winterfront (piece of plywood) on the mesh door opposite the charge air cooler/radiator. DO NOT strap it directly to the core but on the outside of the mesh access door (approx. 6" clearance)

I hope this helps....