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.

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 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.