Allison Automatic Transmission Checking Codes

Mechanics Hub - October 27, 2013
The Allison MD 3000 electronic transmission is a regular component in a lot of fleets. We have been using them for many years and there have been some faults with them including wiring issues and clutch failures but for the past few years they have been bullet proof.

I think one factor is the synthetic transmission oil we have been using for the last 5 years. The price was hard to take at first but now looking at the non stop reliability we have been experiencing I am a big supporter of the synthetic brands.

The synthetic oils can take more heat and provide better lubricity which is the main contributor to the great results we have been seeing. For the last couple of years we have ordered the smaller 2000 series Allison transmissions that have proven themselves as well.

Pictured above is the Transmission electronic control unit (ECU) which is called the WTEC 3. This is on a pusher bus underneath the driver's compartment. It's not the best place to locate a module but it's sealed quite well.

We use the Pro Link to check for codes or you can use the shift keypad. With Allison DOC  in today's models you need the software and laptop to get in and check out problems.

This will get you started on the troubleshooting process. With these particular models the wiring has given us a lot of trouble with resistance being too high. The wiring has to be coaxial with interference resistant covering over the twisted pair series of wires.

There is an output speed sensor and engine speed sensor that are exposed to the environment on the outside of the trans so they are both vulnerable. If the output speed sensor develops an open circuit the transmission will be locked in first gear until the problem is repaired. Just an example of what might occur on these models.