Cruise Control Challenge

[cbkid]

I wonder if a cruise control expert could give advise for my 92 automatic. The last time I saw the cruise control work was just before the speedometer quick working - the speedo became erratic, jumping up and down. The cruise would work so long as the speedo work, but when the speedo went to 0 MPH, the cruise control released. Whenever the speedo would reume working, the cruise control would again work. I fixed the speedo problem by replacing the speed sensor. But now the cruise control won't work at all.

In an effort to find the problem, I have proven the following: voltages present at the steering wheel on/off (12 V) and resume/set/accel buttons (less than 12 V. but responding to switch movement), voltages at the cable leading to the servo under the driver's side fender well, significant vacumm to the servo with engine running, proven that the vacumm "dumps" whenever the brake pedal is pressed, inspected the wiring connections inside the servo for broken or corrosion, proven that the vacumm line to the reservoir is not leaking.

I have no errors on the diagnostic code testing scanner.

Can anyone advise what else to try?

[J.D.]

I'm not a cruise control expert since I've never had any cruise control problems, but I'll offer some suggestions all the same.
1) Check the resistance between the resume, set/accel, and coast buttons and ground. Resume button to ground should be around 2.2K ohms, set/accel to ground 680 ohms, and coast to ground 120 ohms. Do you get the same readings when you turn the steering wheel back and forth?
2) Are your brake lights always on (stop lamp switch is always on, causing cruise control to turn off)?

[WRGoudy]

When I got my '91 the cruise control wasn't working until I adjusted the switch operated by the brake pedal (not sure if it was the brakelight switch, and tend to think it was a separate one for the cruise control). Hope this helps.

Bill

[cbkid]

JD: Thanks for your suggestion for something that I haven't tried yet. I checked the voltages and to my surprise my readings are within 10% of yours! And they are stable even with the engine running and the sterring wheel moved. The switch located onthe brake is a brake light activation and as far as I can tell it has nothing to do with the cruise control. The brake disengage for the cruise is the pneumatic "dump" valve that is attached just above the brake light switch. The dump valve and the brake light switch both seem to work fine.

What puzzles me about my cruise control problem is that it worked fine until the speed sensor went out and the speedo quit. Now that I have a new speed sensor, the speedo resumed working but the cruise did not come back.

I am ordering a factory manual on this car with the hope that I can find more information about the cruise control.

Thanks,
Cliff

[cbkid]

Originally Posted by WRGoudy

When I got my '91 the cruise control wasn't working until I adjusted the switch operated by the brake pedal (not sure if it was the brakelight switch, and tend to think it was a separate one for the cruise control). Hope this helps.

Bill

Thanks for the suggestion. I think the pneumatic switch on the brake pedal is fine since it does not leak under pressure (or vacuum) but will release to the atmosphere whenever the brake pedal is pressed. I think this is how the cruise Control shuts down under braking. I don't think the pedal mounted electric brake switch affects the cruise control.

[J.D.]

When the stop lamp switch on the brake pedal is closed, it signals the speed control amplifier to turn off the cruise control. The vacuum dump valve on the brake pedal is a backup in case of a malfunction. So if the brake lights are always on (stop lamp switch is always closed), your cruise control won't work, as Bill experienced first-hand.
Here's something I found - the electrical manual says to check the two coils in the modulating valve (at the servo): 40-75 ohms between the orange/yellow and gray/black wires and 100-150 ohms between the orange/yellow and white/pink wires.

[cbkid]

Originally Posted by J.D.

When the stop lamp switch on the brake pedal is closed, it signals the speed control amplifier to turn off the cruise control. The vacuum dump valve on the brake pedal is a backup in case of a malfunction. So if the brake lights are always on (stop lamp switch is always closed), your cruise control won't work, as Bill experienced first-hand.
Here's something I found - the electrical manual says to check the two coils in the modulating valve (at the servo): 40-75 ohms between the orange/yellow and gray/black wires and 100-150 ohms between the orange/yellow and white/pink wires.

I must follow up and tell you that my only problem was a blown 15 amp fuse that supplies power to the control amplifier (the device mounted next to the brake pedal). I obtained a service manual on CD from ebay at $10. The CD had full step by step troubleshooting for the cruise control and when it directed me to check voltages at the amplifier, I discoverer abscense of 12 VDC on one of the leads, although voltage coming in from the steering wheel switches was present.

I feel a bit foolish having figured out and inspected the entire system before discovering the fuse problem.

I replaced the fuse and plan to cruise to grandma's house for Thanksgiving.

THANKS FOR YOUR HELP.

CB

[J.D.]

I thought of the fuse, but ruled it out because you were getting voltage at the cable leading to the servo. The voltage at the switches were probably from the horn relay. Anyway, I'm glad it's fixed.