Cruise Control 79 model

The three wires are one power wire to the switch (which comes through that switch on the brake pedal) and two from the switch to the two terminals on the controller. There's a terminal for "hold" and another for "engage". Power is red and turns to brown at the steering column. "Engage" is white and connects to black on the steering column. "Hold" is a resistance wire (may be coiled up under the hood) and connects to dark blue on the column. GM didn't publish wiring diagrams for the 79, and you're supposed to use 77, so don't believe these wire colors too strongly. But anyway, start with power. You should have power to one terminal any time the ignition is on. Get to that point first. The wiring is extremely short. It's an add-on that plugs into the front of the fuse box. From there, it goes directly to the switch on your brake pedal. From there, it goes to the steering column button. It does not mix with the main wiring harness.

When the switch is out, only the "hold" terminal should be powered, so you should have continuity from brown to dark blue. This will allow the cruise to be "on". When you push the button all the way down, power is cut off and that kills the setpoint. The way the terminals were located in the switch, this state may connect "hold" to "engage" but keep in mind that doesn't do anything. When you release the button, you must pass a point halfway up (or whatever) where the cruise is able to go from its dead state to its set state. At that point, it has to energize "engage" and "hold" at the same time. So halfway up you must have continuity on all 3. Here are some items that are internal to the cruise controller:

1. The low speed switch opens below 35 mph and prevents it from engaging
2. Mechanical governor just like a steam engine
3. Tiny orifice with a valve that can adjust to vary the vacuum going to move your throttle
4. Solenoid coil sets the speed by connecting (2) to (3), closes contacts to latch in "hold" power, and also closes a vacuum dump valve. It's a very busy solenoid.

Leave the controller until last. If you can get power to "hold" and "engage" when you want it then you'll have it narrowed down to the controller, and you find another one and swap. From the wiring diagrams, it appears the controller body needs to be grounded to work.

Thanks Joe for the response. That is just the info i was wanting. Very helpful in giving me the right start. I have power at the hold. The brake switch isnt an electrical one, it is a vacum hose to a valve that when the brake is depressed it opens and leaks all the vacum out as far as i can tell. Thanks again.

I'm glad to help. In correcting me, you are mistaken. When you step on the brakes, the power to the cruise "hold" will be interrupted, and it will not latch back in. If you've ever driven a car and used cruise control you already know this. The vacuum dump valve is there to close the throttle quickly. It seals back up when you stop braking, but the cruise doesn't come back on and hammer the gas. That's because power has been interruped when you step on the brake. There are two sets of contacts there. One for your brake lights and one for cruise. They're separate. circuits on a 79.