Hi, I have a 1995 Disco I with a cruise control problem. It works intermittently.

Sometimes it will initially not work after starting the car, then decide it wants to start working again next time I try it during the same trip.

Occasionally, I will be cruising along fine, and then it decides to disengage. Has exhibited this last behavior after hitting a bump in the road, but a bump isn't always required.

Sometimes it will be working fine, I come to a stop, and then it will stop working again.


What I have done so far:
1. Checked the dash on/off switch (works fine)
2. Checked vacuum tubing for leaks. None detected, but changed out anyway (its cheap and easy and gives me a sense of accomplishing something useful.)
3. Replaced plunger on brake pad. Strictly precautionary and gives same sense of accomplishment.
4. Examined vacuum pump. No apparent damage noted.

Any ideas what to try next? My gut, and past experience with the LORD OF DARKNESS (Lucas), tells me it is electrical in nature. However, all suggestions will be gladly accepted.

 

As far as I know a bump and just touching the brake pedal will make the computer stop the cruise control. I might missunderstood your text tought.

 

Pablo,

Thanks, but the cruise should and does disengage when you press the brake. Unfortunately, I often cannot get it to reingauge afterwards. The other symptoms are as mentioned above. Basically, it is some erratic behaviour.

 

Good post with lots of details. Hmm... You have replaced all the vacuum lines. You say you have replaced the plunger. Is this the stop light switch or the the cruise vacuum dump valve ? It has A double back up system. The other possibility is the ECU. Rare, but not unheard of.

 

Where is the vacuum dump valve and the stop light switch? I have the Workshop manual, and don't see a reference to that in the section on the cruise control system.

I was leaning towards the ECU, but I can hear a relay behind the passenger dash click when in park with the motor idling when I depress the on/off button. Although this could be the neutral lock-out relay.

BTW forgot to mention that I also checked the actuator, and it appears fine. I also checked to see that the plug to the vehicle speed sensor on the transfer box was properly attached. It seemed O.K.

 

Vac dump valve and brake light switch are located under the dash on the brake pedal mechanism. So what plunger did you replace ?

The clicking sound helps A lot. Do you here A clicking noise from the same area while driving above 30 MPH and pressing the set button ?

The VSS is prone to early failure. Have you noticed the speedometer jumping around at times ?

 

Thanks for the thoughts.

I double checked my workshop manual, and I think that the vacuum dump switch you are referring to is the plunger mechanism I replaced (only cost about $15, so easy decision to try it). It has a vacuum line on it and a set of electrical leads on an electrical switch. Essentially, disengages the cruise when the pedal is depressed.

I haven't heard the clicking noise while driving above 30, but there is so much road noise it is hard to tell if it is working or not.

The speedometer has not been jumping around at all, and seems to be working fine. Am I to assume that if this is the case, then it is unlikely to be the VSS?

That seems to indicate the Cruise Control unit (AMR1173?), the neutral switch, or the vacuum pump as the problem. Only other possibility I can think of is a bad/loose wire.

Are there any functional tests I can do on the Cruise Control unit, the neutral switch or the vacuum pump to deterimine which it is? The Workshop manual is pretty vague on the electricals and testing.

 

Yes, that is the vac dump switch. You can not assume the VSS is good based on the speedo. One last basic check. Depress the throttle actuator and crimp off the vac hose to the pump. It should hold vac. If so, you may have A bad cruise ECU. Although it is very unlikely. There is more diagnostics but it won't help you if it is an intermittent problem.

 

Thanks for the help. I crimped the hose and it holds vacuum.

I am leaning toward the ECU or VSS. I listened again for clicking behind the dash when engaging the cruise while driving, but couldn't determine anything different when it was working, and when it wasn't. No noise noted at all.

One last thought/comment: I notice the cruise never works after about 30 or 40 miles, which indicates a possible temperature related issue. I tried an experiment with the heating controls by turning the passenger side to cold to see if perhaps the ECU was overheating, but had inconsistent results.

If any of this gives you any further ideas, please let me know.

 

Did you replace the vac line from the pump itself to the Tee ? The pump is hard to get to. The cruise has worked since you replaced the hoses, correct ? Also you are aware the cruise will not engage under 35 mph ?

How mechanically inclined are you ? Do you have access to A volt/ohm meter ? I can go thru the pin outs with you. Just hate to see you buy An $$ ECU and it not rectify the problem. There are so many things it could be. Sounds like you can consistently duplicate the problem. This is A good thing. Then you can check things.