I noticed this morning on the way to work that the Cruise Control doesn't work when the AC is on. It slowly decelerates. The Cruise works really good when the AC is off, I tried it and it would maintain speed. That seems like an odd coincidence. Could it be the Cruise vacuum line or something electrical? I'll check it out further tonight when I get home from work.

 

I'm thinking it is a vacuum issue. When the a/c is switched on in most vehicles there is an idle up device which is vacuum controlled. Not sure about the Rover though as I am new to them. My money is on a vac leak though.

 

I just got done visually inspecting all of the vacuum lines. They look good, no cracks. I have some spare line laying around, I'll probably replace the line just to be sure. I'll let you know my results.

 

Its a simple problem, the POS V8 doesnt have enough ***** to maintain speed with the AC on. LOL

 

Haha, good point.

 

Not funny or true.
If you knew anything about a Disco you would know that the cruise control makes it's on vacuum.


As for his problem, neither system is related or tied together electrically, I will be interested to see what he finds.

 

I'll probably replace the vacuum line this weekend and I'll let you know the results. Last night i inspected the line, and it looks good. I trimmed a little around both ends hoping to get a bit tighter fit, but no change. I replicated the cruise not working with the AC and working without again this morning to be sure.

It's definitely an odd coincidence and I am probably missing a crack or something on the line, so replacement is probably the best bet.

 

Does the cruise maintain a constant speed with the a/c off? If so, it won't be the cruise controls vacuum system causing this problem.
Look and see if the a/c shares a common fuse with the cruise.

 

Yes, it does maintain a reasonably constant speed (within 2 mph depending on hills). with the AC off I tested it both at 55 and at 75 this morning.

With the AC on, it slowly decelerates (at a rate of about 10mph per minute). The engine RPMs stay up (also slowly spin down), so it's not an immediate loss of speed/rpm at cruise activation/set.

I went through the wiring diagrams a bit yesterday and didn't see anything common in the electrical systems. That's part of what led me to decide to replace the vacuum line regardless of me not seeing anything wrong with it.

 

Most vehicles have an IAC idle air control to bump RPM when the A/ is switched on. This uses vacuum. The combination of the cruise and A/C puling o the vacuum system together might be your problem.