Cruise control doesn’t work

[87GrandMarq]

Is there any way to diagnose this system? All fuses are good and it doesn’t throw a vss code so my guess would be either the servo or transducer but how could I test either one?


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[gadget73]

on what? the 87? VSS doesn't actually connect to the ECM in 87 so you won't get a code for that. If you have a tripminder that works, that proves the VSS works.

does the horn work? Cruise buttons are powered through the horn relay so if thats dead the CC will be too
do you have LED tail lights? Gets dumb if you do
beyond that check the vacuum dump valve above the brake pedal. Easy way is to unplug it at the cruise servo to see if it holds vacuum. If you look on the servo you'll see a short hose that goes from one side of the can to the other with a Y connector going off someplace else. Thats the vac dump line. If that does not hold vacuum the CC will not function.
also make sure you actually have vacuum supply to the servo, and that the cable from the throttle body to the servo is not broken internally.
also make sure the tail lights work as expected. If the brake light switch failed and the brake lights are stuck on, the CC will not work.

There are more detailed diag processes in the manuals but those are the easy and likely things for it to be. Personally I've had more issues with vacuum dump valves than anything else. usually just out of position after working on something under the dashboard.

[bgreywolf]

I second the vacuum dump valve. Many of the cruise/speed control modules have labels for the wiring connections so you can figure out what goes where pretty easily.
Also make sure the connectors are tight on the controller itself; they aren't the stoutest connectors out there.

Gadget, I think the VSS connection on EEC-IV is used to prove the car isn't coasting (coasting is a different condition from sitting still in neutral or drive) so the computer knows it can pull fuel but not go to full idle mode. I'll have to look that up, though, now that I'm thinking about it.

[87GrandMarq]

I’m sorry yes its for my 87. Its a gs so no tripminder too. No leds, brake light switch and horn work. I’ll check the dump valve when I can, thanks. I bet thats the issue as it seems everything else is fine but with the car being 33 years old I’m sure the diaphram in the servo is leaking.


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[gadget73]

haven't had that, but unless the diaphragm is completely ripped it would still work even with a leak. Might surge tho. Leaky vac dump valve does the same thing. If its not holding at all though it flat out won't work.


as for the vss, I know 86 has no vss connection to the ECM, pretty sure 87 didn't either. I had to add those pins on my 86 when I converted it to mass air in order for the EGR to behave itself.

[87GrandMarq]

The dump valve is where exactly?


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[Arquemann]

Im subbing this thread, my cc worked a couple times and stopped.
The shaft broke later though, so the fix won't be as simple.

Is the cfi cruise setup similar to the efi one?

[87gtVIC]

The dump valve is where exactly?


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They are talking about the dump valve directly above the brake pedal...above steering shaft..shitty area to access.

It has a large vacuum line going to it.

If you follow the brake pedal pad upward its brake pedal lever you will see a white (should be white) piece of plastic next to the brake light switch. that white piece of plastic makes contact with the dump valve in question. The dump valve threads into a hanger. It has a plunger that releases vacuum from the servo mounted under the hood on the drivers side inner fender once the brake pedal is applied. Brake pedal not applied the plunger is closed and vacuum is stored in the servo...I Believe that is the gist of it.

[87GrandMarq]

Oh ok I’ll take a look. There is something leaking under the dash but I thought it was the parking brake release, perhaps its not.


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[gadget73]

the dump valve only has vacuum on it when you engage the cruise control.

[87GrandMarq]

So if I were to engage it and not hear a hiss its probably electrical because its not getting vacuum?


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[sly]

No. There should be no hiss from the dump valve at all until you press the brake with cruise engaged and then only long enough to break the vacuum in the cruise solenoid. If you hear a hiss already near the floor, it will probably be the half moon shaped valve at the bottom of the steering shaft that actuates the parking brake release. The leak could also be coming from the actuator itself. Pull the black line off the valve on the steering shaft and plug/cap it and the leak should stop.

[gadget73]

just unhook it under the hood and pull vacuum on it. If it holds, that isn't your problem.

[jaywish]

I can say that most times the cruise failed on me it was a vacuum leak.

[87GrandMarq]

Hmm ok I’ll do that tomorrow thanks for the tips.


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