The speedo has never worked since I bought the car (an '81). I just discovered that the problem is with the lower cable that goes from the transmission to the cruise control unit. The tip of the plastic cable end that goes into the cruise control unit is twisted off, so it does not engage the unit. I am going to order a new cable, but before I install it, I want to figure out whether it twisted off because it was installed improperly or because the the cruise control unit is faulty. I removed the unit to test it. I inserted a screwdriver into the hole where the cable from the transmission enters. If I turn it clockwise, it seems to tighten up as if some pressure or vacuum is building up. If I turn it counter clockwise, it keeps turning, but doesn't sound too good. It sounds like metal to metal wear, with a lack of lubricant. I am guessing this is the direction it turns when in the car, but not sure. I am also guessing I need to replace the unit or take it apart and try to clean and grease all the inner workings. Ideas?

Actually, you are correct. The cruise unit is called the transducer. It is a common cause of the lower cable failure. If you check around for someone who does speedometer repair, they will likely also repair the transducer. It is the same unit for many GMs cars, so a good used one may not be too hard to come by.

Ken Styer

you can get a rebuilt one from http://tld-corvette.com/index.htm for $67

Chesh, I put some pages together when I got my cruise system working again, if you have a look at them at http://www.nottingham.ac.uk/~eaztl1/Cruise_control/Cruise_control_recon.htm, you might find something of use. Possibly your transducer's had it, but you never know, you might be able to revive it..
Good luck

When I spun the input shaft of the transducer, it made some friction-like noises that sounded metal on metal, so I gave up and bought a rebuilt one. Thanks for your help everyone!

excellent article by Cleo!!

 I took my 77 transducer apart. It was loaded with carbon depoist. Cleaned it with some carb cleaner lubed everything up and works fine. I guess the carbon build up came from the vacuum source.