This worked on a 1992 Volvo 740

Since no one could give me a difinitive answer as to whether this could be done, I just decided to go for it and see what happened.

I have a dead speedo and odomter. I couldn't figure out how to tell if it was the sender, the wiring to the speedo, or the speedo itself.

I have a 15 year old RadioShack analog VOM. I switched it to 10 volts alternating current range. I had a friend hold the leads on S+ and S- (those are the markings where the wires go into the speedometer. (The speedo was not in the car).

I drove up to approximately 15mph, and as I did, the needle smoothly moved up towards 1 volt. As I slowed down, the voltage dropped smoothly as well. As far as I can tell, this means that the pulse generator in the rear end is working fine, as is the wiring that comes from it. If I had known this beforehand, it would have saved me major headaches. Now I can just concentrate on getting the speedometer fixed.

Aside: does anyone know why the A/C compressor will not work when the instrument cluster is not in the dash? I hope I didn't fry anything in the A/C by trying to turn it on without the instruments in place.

With the instrument cluster out, the alternator is not charging. The AC compressor is prevented from engaging when the voltage is low.
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john

Wow! Neat. I'm glad you told me that before I ran the car down to nothing. No wonder the wipers were moving so slow.

The alternator field is energized through the idiot light circuitry.
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john

i guess this means the speedo IS dead. good job, chuck.

OK: to the FAQ!