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I guess sucking throught the hose and checking the diaphragm moving... or not

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So you don't make a wrong diagnosis, you can't test the diaphragm by putting vacuum on the tube. There is an internal vacuum release valve that prevents vacuum from building up until, 1. The electrical circuit is activated, and 2. The input rotational speed of the speedo cable is about 25 mph.

You could, conceivably, hook up the electricals, and rig some way to turn the speedo input, while applying vacuum. Just be careful that you don't rotate it too fast or you will grenade the insides. I've never needed to do this on the few that I have repaired, but I wouldn't recommend using a drill motor.

I have the factory speed control manual, which has pictures and gives a thorough description of operations, but it is a 21Meg zip file.
I will try and find the link from which I downloaded it and post it later.

Robert

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