Cruise Control - 1992 240 Wagon

Locate the vacuum pump for the cruise control. Make three jumper wires that will connect to the pump. There is one power and two grounds. One gound controls the pump, the other controls a check valve for the pump. when the pump is energized and the check valve energized the vacuum will pull on the diaphragm at the throttle spool and open the throttle when the key is on engine off. By removing the pump ground the system will hold vacuum. If it doesn't pinch off the hoses until a leak is found. Usually the brake pedal vacuum safety switch will be the culprit. Otherwise additional electrical checks will need to be done to isolate the brake pedal safety switch electrical portion, the turn signal switch or the control unit.

A failing with the speedometer pick up at the differential is that the machine that molded the waterproof wire end connectors applied too much clamping pressure on the wire insulation and the conductors were damaged. Over time corrosion entered the insulation and made the copper brittle. Fatigue and vibration breaks individual strands of the conductor until very few are left. Once this occurs ther is not enough signal strength for the speedometer to convert the analog signal to a digital source for the cruise control unit. Repair pigtails are available and may be spliced into the harness. Maintain the polarity and twist in the cable to reduce EMI, since the signal is also for the ABS.

I'd look first at the wires at the diff, since that is where the last work was done.

Bob

Start with the easiest test - do your brake lights work? The cruise computer is grounded through the brake light bulbs, if they all don't work you have no cruise.

Thanks! I've just gone out to check and the brake lights did NOT work! I replaced the brake light/ABS fuse, which was not blown, with another new one and they now work. Unfortunately, that hasn't fixed the cruise control. But working brake lights are probably worth having, anyway.

Next up is vaccum leaks, I expect.

Thanks again. Patrick KEenan

Thanks! I will check through these. It's actually been six years since any of this stuff worked, and I'm glad that it's only the cruise that isn't working now. It did all work up until the wrong-year replacement axle went in, so it won't surprise me if there is corrosion or leakage.

thanks again. Patrick Keenan

If it's been that long I would guess that the vacuum portion of the brake safety switch is not holding vacuum. That's the most likely cause of early failure.

Bob