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.

Allison Automatic Transmission Troubleshooting and Repair

I'm working on an Allison MD 3000 Automatic Transmission with a wiring problem. The trouble started with a fault code on the engine side of a 1999 Thomas Saf-T-Liner equipped with a Cummins ISC Electronic Diesel Engine.[Click Pic To Enlarge]

The Cummins fault code was "241" meaning a loss of the Vehicle Speed signal to the Engine ECU, the amber warning light flashes on the dash alerting the driver. Without a vehicle speed signal the engine ECU [electronic control unit] does not know if the bus is moving or stationary so the exhaust brake and high idle will not work. There was also a drivability problem with loss of power which was related, since the fuel system also needs to know how fast the bus is travelling to regulate fuel pressure.

Normally the bus has to be moving over 5 mph and running over 1000 rpm for the exhaust brake to work and has to be parked for the high idle to come on.

Since the output speed signal is supplied from the transmission output speed sensor we have to start there and check the sensor for resistance and the wiring for continuity.
Here I'm checking the two wires between the sensor and the Transmission ECU for continuity with a Digital Multimeter. The output sensor at the trans should read around 300 ohms.The wire is a twisted pair coaxial wire which helps avoid interference, it checked OK so on to the next step.





















This is the connector at the Trans Ouput Speed Sensor which has been jumped across for the continuity check I performed above. you can see the twisted wire here, no other wiring can be used since we are working with a very sensitive ECU.
On my next post I will continue with this diagnosis and show you the conclusion.

A typical road test in a Thomas School Bus, this unit has a Cummins ISB Model [electronic] Diesel Engine and when roadtesting it's always best to find a good grade to find any faults with performance.
Sometimes the transmission has to be considered if it's shifting poorly. If you're climbing a hill and the trans. upshifts too soon you'll start lugging the engine which feels like an engine power problem.

Cat 3116 Diesel Engine Installation into a Thomas School Bus

Thomas School Bus Diesel Engine Installation: This engine was rebuilt in our shop because of a wayward exhaust valve seat that didn't want to stay in one piece. So consequently everything from the head down had to be rebuilt.

Step one:
Tow in dead School Bus.
We will utilize our shop made tow bar that has been around for a couple of decades, but still does the job.

It may still make it to the "Smithsonian Institute"



The Bus made it into the shop and is now waiting for some Diesel Power to give it life once again.







Mounting "Allison New World" Transmission to engine flywheel housing and flexplate [much easier on the floor!]







Engine and transmission are together now [the transmission is supported by the trans. jack]
The entire assembly will be installed together.






The bumper and frame support were removed previously which makes the installation job much easier!
The complete engine and trans. assembly slid into place nicely.






The engine is resting on the front and rear engine mounts. Now comes the fussy part connecting wiring, hoses and accessories.







Tow bar installer obviously having a good time!








Once the bus to be towed is aired up it's easily manuevered into the shop with two operators.








Having lots of space to work is always a bonus! I will post more pictures as the job progresses.
The final picture will be smoke coming out the tailpipe....

That's basically all there is to it when installing a diesel engine/transmission into a Thomas School Bus.

Watch This Project On Video Put Together With Still Pictures

Allison 3000 Series New World Automatic Transmission-troubleshooting a fault code

We recently had a no shift problem on one of our 2002 Thomas HDX pusher buses.

The code read on the pro-link was 45-12 which is an open circuit to the A solenoid. The A solenoid controls shifting into neutral which explains the no shift problem.

Since solenoids A, D, and J feed off the same voltage source we are probably looking at a low side or ground side wiring problem.


Disconnect the Deutsch bulkhead connector at the transmission and the T-connector or Blue connector at the ECU located under the removable cover on the right side of the dash.

Locate A and G circuits[trans. side] which is marked on the connector face. Run a jumper[Kent Moore #J39197] and check the corresponding wires at the ECU end, they will be T-1 and T-4. Using a digital multimeter [do not use analog meter!] check for opens, shorts and excessive resistance.

Check continuity and resistance to the A solenoid in the transmission itself. The control module[bottom side of trans.]houses the solenoids and hydraulic controls.

The result is resistance in the ECU to Bulkhead connector. So we will run a new shielded twisted pair of wires for the A solenoid circuit.



Replace wire ends for terminals T-1 and T-4 using Kent Moore J34182 Crimping tool. Once lock is remove from T-Connector The wires easily slide out of plastic locking tooth.







Lock newly installed wires into T-Connector at ECU and install
back into ECU with red plastic lock.








The Deutsch Bulkhead Connector unlocks and locks easily from transmission end when making repairs.









Use [Kent Moore J-38582] to remove wire ends from bulkhead connector.








Crimp new ends on replacement wiring using [Kent Moore J-38125-7] crimping tool.








Push in new wire end into bulkhead connector using [Kent Moore J-41194] insertion tool.

The code went away after this repair and the bus went back into operation. We have had a lot of success with wire replacement to fix transmission code problems since the wiring is exposed to the elements and tend to get damaged from the plastic convoluted loom that is used on most School Bus applications.

If the wiring would have checked out OK them we would have had to drop the Transmission Control Module and most probably would have had to replace the A Solenoid.

The tooling noted in this repair is required to do a proper job. Of course the cost of repair is diminished greatly when you can perform your own troubleshooting and repair. The WTEC 2 or WTEC 3 troubleshooting manual is essential with this kind of work.
The WTEC 2 ECU has two connectors while the WTEC 3 ECU has three connectors for easy identification of both systems.

WTEC=World Transmission Electronic Controls.