Cruise Control 1981 Camaro

25

Asked by Mar 21, 2015 at 08:42 PM about the 1981 Chevrolet Camaro Z28

Question type: Maintenance & Repair

I have fitted a New Transducer I purchased from Rick Camaros (there support has been less then helpful)
The problem is that it increases in speed same as the old unit did.
I have checked all the vacuum lines they seem to be going to the correct positions. can't find any leaks.
I can't see that is should matter I have an Edelbrock manifold and a Holley 600 carburetor.
Everything works as it should except that it increases in speed on the flat and down hills to the point I have to disengage the cruse by using the break. Uphill (slight grade not steep) it is not as bad.
My question is, have you got any ideas on what I should be looking for to rectify the problem Please, Note, I marked where the adjuster on the side of the Transducer started from and tried adjusting each way, that made no difference at all.
your help would be greatly appreciated

7 Answers

66,045

Get yourself a copy of the factory shop manual. Faxon Auto Literature in Riverside, CA (they have a website) or eBay should have one. They'll be step by step troubleshooting guides for this in the manual. Going downhill your speed will increase some because you're basically just coasting. But on a flat run your speed should stay relatively steady. Off the top of my head it sounds like the throttle may be sticking or not responding fast enough to make the minor adjustments needed, on a flat run, to maintain the set speed. Possibly the throttle return spring is at fault? HTH. - Jim

1 of 1 people found this helpful.
25

Thanks for that, I will see if I can find the manual, I know they were a pain, It is in the car so would be nice if I can get it to sort of work. What I mean by increases in speed, It increases the vacuum on the Diaphragm and pulls the Accelerator on further, eg start it at 30MPH have to turn it off at 90MPH or it will keep going up

1 of 1 people found this helpful.
66,045

I'm looking at my 1972 Cadillac Shop Manual. You're problem is covered in the diagnosis chart. While this is talking about a different vehicle I'm sure that GM was using similar systems across the board during this time period. The causes of your "steadily accelerating" problem is caused by 1) vacuum hose connected directly to servo. 2) Defective transducer. By what I'm reading there's a vacuum solenoid valve that's supposed to "dump" vacuum to decrease vehicle speed as needed or when the brake pedal is depressed. According to the diagram I'm looking at there's a second vacuum "dump valve" attached to the brake pedal which cancels the cruise control when you step on the brake. I'm guessing yours is good since the cruise will cancel when you step on the brake. I'd be looking at the transducer assembly, if you have one. You'll still want to get the Shop Manual for your car but hopefully this will point you in the right direction. HTH. - Jim

2 of 2 people found this helpful.
25

Him Jim Thanks for that, I purchased another transducer fitted it, no difference at all, Just thinking, I Have tested the vacuum lines around the system all checked ok, if the break dump valve had a slight leak when the unit is activated, do you think that would course the problem, maybe the transducer is trying to compensate for the slight leak? what is your thoughts? please

1 of 1 people found this helpful.
66,045

I'm thinking that if that were the case the car would still maintain a relatively constant speed. Because as vacuum was leaking out at one end more vacuum would come in to maintain the set speed. From what I'm reading the transducer "knows" how fast you're going based on how fast the speedometer cable is spinning. You have a magnet and cup assembly inside the transducer just like on the speedometer head on the dash. This (somehow) regulates current going to the solenoid vacuum valve inside the transducer. It appears from the picture that this valve slides up and down to regulate vacuum to the servo that's connected to the throttle. If that slider valve is leaking internally and allowing vacuum to pass on to the servo when it shouldn't be you'd have a constant increase in vehicle speed. Kind of like if you slid your hand over the hose of your vacuum cleaner. The other thing that I'm thinking is if the vacuum solenoid valve is sticking and not moving as it should. That'd cause vacuum to continually pass on to the servo when it shouldn't. Again, constantly increasing vehicle speed. Have you performed any electrical tests on the system to insure that you're getting power when and where you should? Since you replaced the transducer with no change we may be dealing with an electrical problem. Was the transducer used, rebuilt, or a new old stock (NOS) unit? HTH. -Jim

25

The transducer was a rebuilt on from Ricks Camaros, The electrical test checks out, next weekend I will fit the old one back up to see if it is the same fault, it has been a while

66,045

Sounds good. Now that we have an idea of how the system works we can hopefully figure out what ails it. If possible, see if you can "T" a vacuum gauge in line at the servo so the vacuum gauge can be safely viewed while driving. That way you can see what's going on with the transducer. Is the transducer continually sending vacuum to the servo? Is the servo sticking? This way you can see the system "in action". HTH. -Jim

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