I have owned several BMWs with ZF automatic transmissions, and much as I like these transmissions, they don’t last forever — especially if the previous owner neglected to service them. Whenever I buy a used BMW, I like to immediately service the timing belt (if any), and the automatic transmission.  As to replacing the timing belt, if that hasn’t snapped yet, then it’s as if all past sins of maintenance neglect for that part have been forgiven and things are 100% OK again. As to the transmission, not so. Even the best transmission service in the world won’t fix problems caused by past neglect.

On several of my cars with such transmissions, they were ostensibly in good health but eventually failed, and in my experience, the failure has always been gradual. I assume it’s due to an internal component mechanically disintegrating or wearing out, and the debris or sludge in the bottom of the transmission oil pan tends to support that hypothesis.

By contrast, if an electronically controlled transmission fails suddenly, without any prior warning such as slippage or juddering, I would suspect an electrical problem. That’s exactly what happened to at least one of our Audi A6 project cars.  So, that’s where I’m looking next.

On the ZF 5HP-24A, the bad components that can cause electrical problems are:

• Transmission control module (TCM)
• Cable from the TCM to the transmission
• Cable from there into and inside the transmission
• Solenoids

I have already had three different TCMs in our project car, and none of them have fixed the problem. It’s possible that they are all bad, but I’m going to shelve that hypothesis for a while, and focus on the alternatives.

The TCM has a built-in output test for the solenoids. If any or all of these fail, we can’t as yet rule out a problematic cable, either, but isolating the problem beyond that point becomes a lot more viable. So, that’s the next test.