Peter,

I read your reply with interest. My first question (if you had posted for help) would have been "how do you know the sender is OK". The usual test, and the one given in the manual I believe, is a resistance check from the plug at the back of the speedo unit.

But that will look fine on an intermittent with both common problems, sender falling apart and flex wiring insulation crumble back at the diff., unless you were fortunate enough to make your readings after a failure and before it fixed itself. Even that I suspect just by the nature of those wires back there; just getting in the car readjusts them.

Then again, the plug itself has been reported here as a source of intermittents. The contact spring tension goes or it wears the PC board it mates with on the speedometer.

Another suspect if you've ever had the circuit board out of your cluster for a tach upgrade or compensator board problem: the flex circuit fuse. You may have noticed while tracing the red/blue power it arrives otherwise unfused. I wondered about that design until thinking, perhaps, Volvo didn't want to make disabling the odometer as easy as lifting a fuse in the panel. Anyway, the intermittent in the flex fuse comes from not tightening the screws, or possibly from overtightening, fracturing the copper trace.

Here is a link to click with some of my notes on the internals in hope it may help you or others.
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Art Benstein near Baltimore