Cruise Control Servo
Thread Starter
Advanced
Joined: Aug 2012
Posts: 59
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From: Seattle WA
Cruise Control Servo
My cruise control doesn't work and I'm trying to diagnose what is wrong. Found this which is very helpful: http://www.vetteweb.com/tech/vemp_10.../photo_11.html
I didn't have a vacuum tester yesterday so I didn't test that, just did a visual inspection of the lines. I got one today so I will test that when I get home. Anyway, I pulled the servo and tried the test in that above link. I had to remove a metal cap to get to the release valve, and it wouldn't hold pressure. I noticed if I covered the middle screw hole of the 3 that hold the servo to the bracket, then when I press the diaphragm it would hold pressure, but not if I didn't cover the screw hole. Is this normal? I was trying to go see a servo that I could test the same way, but everything I found at parts stores wasn't in stock. I was hoping that someone might have experience with this and could help me out before I spend $80 on a part that might actually be working correctly.
Another note about the servo. When I change the cruise control switch between on and off, I don't feel a clicking from the pin moving inside the servo. Does anyone else, and is it obvious? I'm getting 8 volts between pins b and d when on, 0 when off.
Thanks,
Richard
I didn't have a vacuum tester yesterday so I didn't test that, just did a visual inspection of the lines. I got one today so I will test that when I get home. Anyway, I pulled the servo and tried the test in that above link. I had to remove a metal cap to get to the release valve, and it wouldn't hold pressure. I noticed if I covered the middle screw hole of the 3 that hold the servo to the bracket, then when I press the diaphragm it would hold pressure, but not if I didn't cover the screw hole. Is this normal? I was trying to go see a servo that I could test the same way, but everything I found at parts stores wasn't in stock. I was hoping that someone might have experience with this and could help me out before I spend $80 on a part that might actually be working correctly.
Another note about the servo. When I change the cruise control switch between on and off, I don't feel a clicking from the pin moving inside the servo. Does anyone else, and is it obvious? I'm getting 8 volts between pins b and d when on, 0 when off.
Thanks,
Richard
Last edited by rjschwar; Aug 28, 2012 at 04:08 PM.
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
Some things that will defeat the cruise control are leaks in the vaccuum lines (common) and the dump valves on the brake (and clutch) pedals. If these valves are not set properly, the cruise will not engage.
Thread Starter
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Joined: Aug 2012
Posts: 59
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From: Seattle WA
I'm going to test the lines tonight and already tried the lift up on clutch and brake and then engage trick...didn't work. I was just curious about the servo as I had it removed yesterday.
Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
There is no click at the servo when turning the on-off switch back and forth. The 2 solenoids get energized when at speed and the car is in motion. They cycle on and off at speed. The solenoids vent to the atmosphere when not energized and difficult for me to know exactly what you are doing or if your test is valid or not.
The servos don’t go bad much and you starting with the complex items at square 1. One of the most common problems is the brake (and clutch) electrical switches. They either are not making contact or the adjustment plunger is set wrong. If either are the problem, then it will never engage. I would check those items first (and vacuum lines) then move on to the complex items.
Did you check the 3 hose check valve under the plastic cover on the right side of the engine? If it is cracked or broken, there might not be enough vacuum for the cruise to operate.
The servos don’t go bad much and you starting with the complex items at square 1. One of the most common problems is the brake (and clutch) electrical switches. They either are not making contact or the adjustment plunger is set wrong. If either are the problem, then it will never engage. I would check those items first (and vacuum lines) then move on to the complex items.
Did you check the 3 hose check valve under the plastic cover on the right side of the engine? If it is cracked or broken, there might not be enough vacuum for the cruise to operate.
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Joined: Aug 2012
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From: Seattle WA
I'll check that stuff tonight and report back. For the brake and clutch switches beyond pulling up on brake and clutch while driving and try to engage what is the best way to check them? Try and get under the dash and visually look at them?
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
I think you're most likely to have a problem with the vacuum lines and the vacuum check valve. Trace those wherever they go and see if anything is amiss.
It's very common for the vacuum check valve to fail -- it's an easy $4 fix.
It's very common for the vacuum check valve to fail -- it's an easy $4 fix.
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
When you checked the dump valves, did you reposition them in their brackets? The valve bodies are threaded, but are held in place without nuts. To locate them correctly, you would force them down a few "clicks" in the brackets, and then bring the pedals up to establish the final position.
Thread Starter
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Joined: Aug 2012
Posts: 59
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From: Seattle WA
I tested the vacuum of the supply, and at idle there was nothing. Pulled out the check valve and it didn't hold vacuum on any of the ports. Looks like that might be the problem. Now I just need to find a replacement one. Any ideas on where to get that? Couldn't seem to find one at any local auto parts stores.
Thanks,
Richard
Thanks,
Richard
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Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
I tested the vacuum of the supply, and at idle there was nothing. Pulled out the check valve and it didn't hold vacuum on any of the ports. Looks like that might be the problem. Now I just need to find a replacement one. Any ideas on where to get that? Couldn't seem to find one at any local auto parts stores.
Thanks,
Richard
Thanks,
Richard
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
4th Gear
Joined: Mar 2013
Posts: 4
Likes: 0
Hey guys, on my 84 automatic I have 2 switches (plungers) on my brake pedal ,1 with the vacuum line attached, the other I assume for the brake lights. There are 3 wires. Tested wires with test light and 1 is lit when brake is in rest position, other 2 are off, all 3 light when brake is pushed, 2 go back out when brake is released. Is this test correct, I cannot get my cruise to engage, servo,3 way check valve, lines, power source, all are good. Just wondering about the brake switch, any ideas??
Last edited by Zedderguy; Apr 18, 2013 at 09:40 AM.