Hi, 87 vert. Cruise has not worked since I got it. I was working my way through a trouble shooting program and hit a snag. I have correct voltage to the module, I have correct vacume to the module. When I test the dump hose from the module it will not hold vacume. Program said that it should dump when brake is applied but did not tell me what to do if the dump hose would not hold the 15 inches of vacume. Mine bleeds right down. Does that point to the brake switch? I did a visual and the Brake switch seems to be in the right place and opens/closes as the brake is applied. Thanks for any help.

 

Replace the module.

 

From agent 86's stash:

http://members.shaw.ca/dankai/Cruise%20Control%209C.pdf
http://members.shaw.ca/dankai/Cruise%20Control.pdf

So...by "module" do you mean the cruise control servo?

If the vacuum input from the vacuum tank (figure 8a-34-1) won't hold a vacuum when cruise is off then wouldn't you be having other vacuum related problems?

If the vacuum valve in the servo is functioning correctly then a leak via the release port or the vent valve is possible. A leak to the vent valve could be an electrical problem in the instrument cluster or fault in the servo. A leak to the release port could be the brake switch or vacuum line. A crude way to test the release port would be to cap it & see if the cruise control functions---this will disable the break release function, though, so be careful. It sounds like you have a way to generate & test vacuum so can likely safely test the line directly.

Re-reading your post: by "dump hose" do you mean the vacuum release port? If so, the schematic shows that this is normally vented through the vent valve in the servo so should bleed down. If your brake switch is working correctly test the vacuum line from the servo to the switch to be sure that's ok. If all the vacuum lines are ok then it's likely an electrical or servo problem & the FSM diagnostics table can help determine which.

Hope this helps.

Disclaimer: I'm just a noob with a FSM using this as a learning opportunity so weigh my inputs accordingly.

 

Thanks for both replies. I'm in a little deep here so I'm taking my time. I did learn to spell vacuum though, so I'm happy about that.

Yes, I'm working from a different set of instructions but the module is the Servo.

In the key on engine off (KO/EO)I have 8 volts at the servo with the cruise control switch on and 0 volts with the switch off.

At the vacuum in port on the servo I have 17 inches of vacuum at idle.

The next step in my trouble shooting instructions was to put 17 inches of vacuum into the dump hose (vacuum release port) and then push the brake to see if the release valve did in fact release. In doing that, I could not get the vacuum release hose to hold the 17 inches of vacuum or any vacuum for that matter. My trouble shooter did not say what to do or suspect in that situation.

So, I'm trying to figure what the problem is. Common sense tells me that the release valve is staying open. Question is: is it being kept open because there is a malfunction in the brake switch or because there is an internal fault in the servo? Goldcylon seems very confident it's in the module/servo. I don't doubt someone who is that certain, but since I have an abundance of free time I think before I replace the servo i'll check the internals and see if I can verify it's failure.

 

Are you putting the vacuum into the hose or the port?

• If into the port then it will vent to the vent valve & shouldn't hold a vacuum
  
• If into the hose then an inability to hold vacuum indicates a problem with the hose or the switch.

Are you checking the vacuum before and after pressing the brake?

Before pressing the brake the vaccum should hold, if not see above. After/while pressing the brake the vacuum should be vented.

To tell whether the problem is the hose or the switch then you need to test each seperately.

Apply vaccum to the switch only---does it hold?

If yes, then the problem is likely a leak in the hose.

P.S. I mistyped vacuum about every 2 to 3 times typed---kept hitting the f instead of v, d'oh!

 

checking into the hose and it will not hold. I like the check the switch idea. This car has been garaged and seldom driven. The hoses look and feel fine. I'll try the switch first.

 

OK, progress made. Problem checking vacuum was identified as operator (me) error. I went to the effort to remove the brake switch so I could check everything better. I could not get the vacuum to hold. As I was driving back from buying some small hose clamps it dawned on me that I had not tried holding the handle in on my little vacuum gun. So, I hooked up the switch and tried setting the vacuum by pumping up the pressure and then holding the handle in. VIOLA. Switch and hose checked out fine. I then tested the servo as per the service manual. The servo did not pass. I will be replacing the servo and will report back if that does it.

 

Well...Goldcylon was right, eh? Experience often trumps theory. Glad you're making progress.

 

This was a tough lesson, I'm soon 69 years old and working under that steering colume was not a pleasant experience. Putting the brake switch back in once I had figured out that the whole removal was wasted effort was especially humbling. Now all I have to do is find a replacement. I read somewhere that GM wants $485 for one. I don't see that happening, at least until I've exhausted all other options.

 

Goldcylon was in fact correct. Found a used servo on ebay for about 10% of what GM dealer wanted. Installed and working great. Thanks Goldcylon.