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.

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.