Cruise Control Surging

[Big Chip]

My 79 C35 is having CC issues and I am hoping someone can steer me onto the right track. It is the version with resume.

When I set the CC it will speed up a couple miles per hour so. I am not sure if that is the way they work but I have a vague recollection of my dad's 79 suburban doing the same thing. After the speed levels off it will speed up and slow down but not in any particular cycle. It seems to be random, if that makes sense? It will sometimes speed up 4-5 MPH and stay there. When you hit a big bump it will disengage. I am thinking it is probably the transducer but I haven't tested anything yet. Hopefully someone has had a similar issue and can tell me if I am on the right track.

 

[flying2275]

If it is speeding up or slowing down the orifice tube (see red arrow in picture) on the transducer can be adjusted but it sounds like you have something else going on as well. If it slows down adjust the orifice tube out, if it speeds up adjust it in. Each 1/3rd turn changes approx 1 mph.

Its probably the transducer but I would first check for vacuum leaks, there are many used in this system. Don't forget the one that runs to the brake pedal switch. Also check for any binding in the throttle linkage.

If you don't find anything wrong with them check the servo. Disconnect the vacuum line that goes into the servo, push the servo diaphragm in and plug the vacuum fitting the back with your finger and release pressure on the diaphragm. If the diaphragm doesn't stay compressed while plugging it, its leaking and needs to be replaced.

If that's fine I'd change the transducer.

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[flying2275]

Also you could with the key off, disconnect the connectors at the transducer and hook it up so only the "engage" wire is connected to the "hold" terminal. Turn the key on but DO NOT start the engine. Move the slider to the resume position and release it. You should hear the transducer make a noise. That's a good thing. Go and smack the transducer (simulating hitting a bump) and see if it clicks again, if it does something internally is bad.

 

[bucket]

Wow! That's the best info I've seen about the old cruise setups!

 

[Big Chip]

Thank you very much!! I will give that stuff a try tomorrow and see what I come up with. I really appreciate the information.

 

[flying2275]

No problem. Its actually a fairly simple system when you get into the schematics. I recently went thru the process of installing factory cruise in my 1979 from mostly junk yard parts so I've become well acquainted with the system. If you end up replacing the transducer I wouldn't recommend messing with a used one. I pulled 4 out of junk yards and all were bad. Ended up buying a new one.

Most of that info I posted is from the Cruisemaster GM service training manual.