I'm working on an Allison MD 3000 Automatic Transmission with a wiring problem or so I thought. The transmission output speed signal is the source signal to the Engine ECU for road speed, cruise control and exhaust brake operation.
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 ON ANY IMAGE TO ENLARGE)
NOTE: To check for transmission codes without the Pro Link Scan Tool press the 2 arrow keys simultaneously with key on. You will get a 4 digit code i.e Code # 35 00 is a battery interruption to the Trans ECU module.
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.
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.
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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.
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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 access much easier.
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.
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 check these fuses. One is main power & the other is ignition power to the VIM.
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.
Thanks for checking out this article!
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