Chasing down broken cruise control

UMR_Engnr

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Troubleshooting my cruise control and had a few questions.

I don't think this has ever worked since I had my truck. Today, I went out, got up to 40mph, pushed "On", and then "Set". Nothing happened. I didn't get any sort of light when I pushed on.

When you press "On" are you supposed to get a CRUISE light?
Is there any sort of self-test on the 1989 trucks?

My horn was broken, but I replaced it with a Stebel Nautilus run off a relay that is triggered from the original horn line, and it works fine, so I am assuming the relay and fuse are fine.
 

79jasper

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No light, no self test.
I would start by checking the contact ring behind the wheel.
There's also a cruise control module behind the dash.

First, check to make sure the cable is hooked up with the throttle cable on the pump.

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laserjock

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On an 89 the cruise is tied to the horn relay circuit. The stock horn relay is behind the dash clear against the pass side door post. No horn may be related to no cruise.
 

chris142

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My cable cam off at the injection pump. A hose clamp fixed that
 

Macrobb

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Also, if your BRAKE light is on(e-brake or loss-of-vacuum sensor) the cruise won't work.
 

UMR_Engnr

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Well that was a fun foray into the cruise control system.

I started by tearing down the steering wheel and taking apart the switches. They looked good, and the resistances checked out on the multimeter. I checked slip rings by disconnecting the connector by the fuse box, and checking the resistance. It was very close to zero.

I took the servo motor out and checked the throttle cable. I took the cover off the servo and applied 12V to the solenoids to check them. They actuated fine. I used a hand vacuum pump and checked to make sure the diaphragm moved properly. It did. One of the vacuum lines that connects to the servo goes into the cab. I was curious what this was for so I tracked it down. Turns out that it is connected to a mechanical dump valve right over the brake pedal. The valve has a plunger that seals the the hose when the brake is up, and lets air in when the brake is pressed. Mine was poorly adjusted and the plunger wasn't even touching the brake at all. I hooked up the vacuum pump to the end of the hose in the engine bay, and sure enough it wouldn't hold vacuum. I pushed the valve down so that it contacted the brake pedal, and rechecked. When the brake is up it held vacuum. When the brake is pressed the vacuum dumps.

Took it for a test drive and success! Cruise control is operational.
 
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