Vehicle Speedo and Tacho (stepper motors)

[iceblue]

Not really sure if I should post in this sub-forum, but...

I have an instrument cluster from a car which no longer displays the vehicle and engine speed on the gauges. When you start it up the speedo starts oscillating and the tacho does not move at all. When driving, the speedo continues to oscillate and the tacho stays on zero.

If I open up the cluster and turn the needles so that they are no longer on the zero position (i.e. speedo shows e.g. 120km/h and tacho 4000rpm) and then power it up, the dials return to the zero position but oscillate while doing so. Once they reach zero they stay there (but with the speedo needle still oscillating).

Both dials are driven by stepper motors (each motor has 4 contacts). The stepper motors are controlled by a microprocessor.

So my question, does it sound like the stepper motors are the problem or could it be something else? It is strange that both would fail simultaneously. It has worked intermittently a few times over the past few weeks, but it is always either both gauges working or both not.

 

[MikeMl]

Sounds like the two stepper motors share some of the wires going back to the ECU, and one of the common wires has lost continuity, effectively putting one or more windings of the two stepping motors in series. I'd be looking for continuity from each motor connection back to the ECU, likely goes through multiple connectors through the various wiring harnesses.

Do you have a wiring diagram of the car?

 

[iceblue]

The problem is isolated to the actual instrument cluster (fortunately). I have fitted a different cluster into the car and that cluster works fine. (have also bench tested the faulty cluster and it behaves the same as in the car).

So then I should be looking for a faulty connection somewhere between the stepper motors and the onboard microprocessor? There is a darlington transistor array on the PCB as well, so I would assume the motors are controlled via the darlingtons. Could one failed darlington possibly cause this?

 

[MikeMl]

The problem is isolated to the actual instrument cluster (fortunately). I have fitted a different cluster into the car and that cluster works fine. (have also bench tested the faulty cluster and it behaves the same as in the car).

So then I should be looking for a faulty connection somewhere between the stepper motors and the onboard microprocessor? There is a darlington transistor array on the PCB as well, so I would assume the motors are controlled via the darlingtons. Could one failed darlington possibly cause this?

I missunderstood; I thought the micro was remote to the cluster.

Yes to your question, if the darlingtons are somehow shared between the two gauges, then one failure could effect both.

 

[iceblue]

After some more testing I discovered the motors are connected directly to the microprocessor, no transistors between them. I swapped the motors and got the exact same results, so seems like something is wrong with the micro. So fixing it seems out of the question.