Spdometer Failure Diagnostics

[MontFenn]

I bought an OBD device to plug into my E46. I then bought an adapter cable for my E30, but I can't get it to work, I assume that is because the device is OBD II and the E30 has an earlier version. I was hoping it might tell me something about my speedometer problem. (I don't get much out of it on the E46 either!)
The speedometer is only intermittently working. It shows a reading sometimes and then drops down to zero, and spends more time on zero than showing a reading. Is there a good way to try to determine if it is the sensor or the instrument cluster that has failed?

[Kelso840]

Sounds like an intermittent connection at the gauge.

[Panici]

Check & clean the ground connections on the back of the cluster. Could be loose/corroded.

[MontFenn]

Yes. It was due to bad connections in the cluster. Resoldering them has fixed the problem.

The auto electrician I took the car to also installed the reconditioned compressor that I had bought some time ago, so I now have working air-con and know how fast I am going.