Wife has a 79 automatic........When she tries to use her cruise control the car surges approx 8-10 mph.......It will be set at 55 and car surges from about 50 up to about 60........Other than that car runs fine....Suggestions on where to start looking?????

 

Yep, this one's easy to diagnose, not so easy to fix.

You have vacuum leak(s) in your vacuum system somewhere, the vacuum that drives the C Control is tapped from the same source as your headlights and heater controls, if a leak is present then you get unstable vacuum at all points, including the C Control diaphragm, in fact, as a simple test try this, get on to a freeway or open road, get car to 35 MPh and engage CC, put up with surging and I bet that over time the surging will reduce, maybe even eventually stopping. This is because the vacuum will eventually settle down to a constant state.

how do your headlight operate? Are they Ok too, what about your air vents? Do they operate perfectly? I am guessing they dont operate perfectly, they may still operate but irratically or slowly.

good luck finding your leak but be aware that the vacuum system isnt as scary as some on this forum will have you believe, you just need to learn and understand it, it all makes sense eventually. I may be a sick puppy but I actually like the vacuum system on later C3's, its challenging but fun.

Cheers, Dennis.

 

If the speedometer needle also jumps, here's another possibility. On my car the problem was the rotating disk in the transducer had somehow moved a bit on its shaft and disk was rubbing slightly on the sensor plate. When I inserted the square tang of a small round file into the socket where the speedometer cable from the transmission fits and spun the file, I could feel and hear the rubbing. With nothing to lose by trying, the fix was to very carefully tap on the shaft with a small hammer while holding the disk in an attempt to reposition the disk toward the rear so it would not rub. After a few taps, I would reassemble the unit and check whether it was still rubbing. I was surprised - it worked! And it's still working just fine even after much use since the fix. Speedometer needle and cruise speed hold are both rock steady.

Good luck!
Jerry

 

There is a good discussion on how to fix this issue on this forum. Here is the link.

http://forums.corvetteforum.com/c3-t...ain-speed.html