billy mild

Thoughts? It sounds like the Brake/Clutch switches will have an issue over bumps, but the cruise will cut out all together. My issue is different.

Cliff Harris

Read through these two links:

http://www.superchevy.com/how-to/vem...ontrol-repair/
http://www.superchevy.com/how-to/82001/

The latest issue of Vette magazine (June 2015) has an "technically speaking" article on troubleshooting the cruise control.

billy mild

After going through the vacuum portion of the cruise control system, it appears the diaphragm in my Cruise Servo is bad. It will not hold vacuum when depressed and both holes covered up.

So my question is can I still get a brand new or do I need to get a good used one?

crowz

When I replaced the cruise servo on my 86 coupe I bought one for a trans am.

The vette servo listed for $350 or something stupid like that. The trans am one was $125.

Same servo it just didn't have the vette tax on it

billy mild

New Cardone Cruise Servo and it acts like the one in the car. I have not installed it yet but I tried to see if it held down after being pressed and it didn't. according to the article above it should.



Watch the video.

crowz

Its supposed to work just like its doing. The bleed off is internal.

One line is for vacuum.

One is for the brake pedal release.

The vacuum line should hold vacuum but you will need tool to check that like a mightvac etc. It wont hold the diaphragm in like your trying to do.

The diaphragm is your compressing is open to air when the cruise isn't engaged. Otherwise your throttle wouldn't return to idle when the cruise turned off.

billy mild

My issue is pulsating cruise. It will be steady for a while, then start to get on the gas, let off, open up throttle, shut off. It will repeat that cycle while the cruise is on.

Does that sound more like an electrical issue or servo? I have gone through these troubleshooting steps and honestly everything seems like it would be fine.

antfarmer2

How does the hose look to your brake? reread look at the connection on the pod

crowz

That can be a bad servo but it can also be a vacuum leak somewhere.

billy mild

I have a mityvac device and have pulled vacuum on both of the lines and the hold without issue. I would think that eliminates the whole vacuum leak issue. Also I have tightened up all the vacuum lines under my plenum on the ZR-1. I have 0 vacuum leaks.

crowz

No, you verified theres nothing wrong with the servo. Not if theres a weak vacuum source from the engine side.


Basically the motor sends vacuum to the servo all the time as long as its running.

If there is a leak in the line to the servo from the motor or a leak somewhere else lowering the vacuum of the motor to feed the servo that will cause issues.

Next there the brake bypass switch. This is a two fold setup. It uses an electrical switch to shut the cruise off when you tap the brake or clutch pedal if its a manual. But the brake switch also has a vacuum port setup. This is a safety feature to bleed vacuum off in the event of the electrical part not working. If there is a leak in the line feeding that part or the valve itself at the pedal you will get the problem your having.

So to sum it up, motor side, brake pedal side or vacuum lines in general is what you need to trace.

billy mild

So you are saying I need to put a vacuum gauge on the supply line from the engine? I have not done that yet.

I have pulled vacuum using a mityvac on both the lines going to the servo. Meaning I removed them from the servo plugged the mityvac into the line that goes to the engine and the other line that goes to the brake switch. I pulled vacuum and it held without issue.

I will get a vacuum gauge and test the supply line with the engine running.

crowz

Id start with the brake pedal line first since it works and then goes bonkers. With the pedal not pushed you should be able to pull vacuum on that line where it meets the servo and have it hold vacuum. Remember dont go crazy running up the vacuum which might damage the pedal switch.

gerardvg

Hi

Yes the vacuum test is the way to go, I know the electrical brake switches work or they do not. The vacuum hoses tend to crack with age and heat. You should have one vacuum hose connected to the brake switch, unplug it and see if it holds vacuum. You can do that at the hose to the cruise servo in engine bay. Should hold vacuum brake off.

The other vacuum hose goes to the engine vacuum port I think.
The early C4 has cruise control connected to the instrument cluster, is your speedo display smooth and accurate? If the gear in the trans is damaged and shows erratic speed that could be your problem.???

The speed sender can easily be removed without unbolting the exhaust. I know that because I had to replace the one ring seal recently

Unbolt the hold down assembly and unplug the electrical connector. Check to make sure the wiring has nor en damaged and wiggle the speed sender out. The gear cannot fall into the trans so just fish it out with a small screwdriver, you can get new speed senders on ebay cheap.

Good luck

These cars are hard to drive slow without a working cruise control.

Cliff Harris

You already have a vacuum gauge in your MityVac.

kg4fku

Found this article Click here

The pictures in the article give a step by step procedure on how to troubleshoot and fix the CC assembly.

Sounds to me that either the CC servos are bad or the VSS circuit is bad.

Was the battery replaced just before the issue with the CC was noticed? I have heard that sometimes a C4 unknowledgeable shop will attempt to remove the CC servo in order to replace the battery. This usually results in the servo becoming damaged.

billy mild

I put a vacuum gauge on the vacuum supply hose for the servo. I started the engine and it pulled 20 hg of vacuum. It is solid, didn't move at all. So that checks out okay.

Next I pulled vacuum on the dump hose that goes to the Brake Switch. I pulled about 20 hg of vacuum then let it sit. It didn't leak any vacuum at all. I then pressed the brake pedal and it released all the vacuum. So that checks out okay.

I did notice that my car has this speedo healer device. Could this cause an issue?

I may just replace the servo and see if that fixes my issue.

3 pedals

The cruise in my 85 acts just like yours. At 55 MPH the cruise isn't usable, at 75 MPH it is better but not right. Over the past 10 years I have tested and retested everything, even pulling the instrument cluster out. Everything checks perfect. When I first got the car, I swapped in a TKO transmission and used an Abbott box to calibrate the speedometer. I have questioned if the Abbott box was the problem but never saw any other complaints.

kg4fku

I would say it is just as suspect as any other part on the car. I would contact the manufacturer and find out if there is a history of this problem.

http://www.abbott-tach.com/Welcome.html

If there is, I would look into a different fix to your issue(s).

crowz

It could be the speed converter or the servo. What happening is what I went thru a long time ago. Mine was the servo two times and once it was the brake switch.

But what happens is the speed gets outside what the cruise can handle so it shuts off. As mentioned that could be it sensing the speed changing radically for a split second or the servo not responding the way it should.