I have a 94 Disco I, 3.9L v8, 174,000 miles.

I have been working on the cruise control system. I printed out the Cruise Control diagnosis pages from the Rave CD, and have been using their testing procedures as a guideline. First, I made sure my vacuum lines were intact, and that the lines were securely attached at all points, including the switches on both clutch and brake pedals. I removed and tested each of these switches for electrical function as well as their vacuum holding function and made sure upon reinstallation that they were adjusted to work. I removed the electric vacuum pump and tested it and both the motor and the solenoid (in the pump) work fine when connected directly to a 12v source. With the pump running and the solenoid closed it pulls vacuum. I can suck on the vacuum line normally attached to the vacuum pump and the throttle servo will draw in, so it is not leaking. Using my multi-tester, I have confirmed the wire from the speedometer (speed sensor) to the cruise control ECU is fine, and of course the Speedometer works fine, so Vehicle Speed Sensor is working. By measuring voltages at the Cruise Control ECU plug on the ECU according to the various test procedures I can confirm that the clutch and brake switches close sending voltage to the ECU, the main power switch on the dash works, and the steering wheel set and decelerate switches work just fine. I tested the cruise control diode, and it is fine. I still can't get the cruise control to work. So far, I am diagnosing that I have a bad Cruise Control ECU, and I have ordered a salvage one from Roverland Parts. The only things that I am not sure about are the "Set" relay, and the "Cut out" relay. Any advice at this point would be appreciated.

 

I have a 94 Disco and have been having issues with the cruise control also. Is your's just flat out not working, or what is it doing? With mine, I checked and replaced all of the vacume hoses, but that did not fix it. For a while it would work every now and then. And when it did work it was for just a few miles then would quit. I researched this site for along time reference the cruise control and finally found a guy who had the same problems you and I have. He replaced his cruise control ECU and said that fixed it. I have one being shipped to me soon so we'll see. I bet that when you replace your ECU it will work fine.

 

Sometimes its the brake switch on the brake pedal that the vacuum line is on,,,it will leak or it will get pushed back when its pushed back its as if the brake pedal is pushed down.

 

how do you keep the switch from getting pushed back?

 

I have made sure my cruise control switches on my brake and clutch pedals (I have the 5-speed manual transmission) are seated properly. They work as specified. My cruise was working a teeny bit, after I corrected all the connection problems and now it won't at all, so I suspect it is the ECU.

 

The switch "snaps" into a hole in a metal bracket which positions it such that when the brake pedal is at rest, the switch contacts are closed and the vacuum is sealed. If it won't stay snapped into the bracket, then the plastic "tabs" on the switch may be worn or broken, requiring a replacement switch, which is only $12.95 from Atlantic British (as an example).

 

Thanks. My cruise control worked the whole time on my way from work today. What the crap, I'm guessing it's the cruise control ecu.

 

I have a 96 Disco with a similar problem. Did the ECU replacement work?

Ed

 

Hey edh, I replaced the ECU and the cruise works great now. Fairly cheap fix. The ECU cost me 60 bucks from roverland parts.