Wednesday, October 10, 2007

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.


Anonymous said...


I have a problem with my ISC in my motorhome. How do I make contact in order to ask questions? Not specifically an allison problem. But with your experience it may be quite simple to you. I have signed up for emails.

Anonymous said...

I am anonymous, but name is george at

mikaljains said...

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