HT4100 Cruise Control

[Mike Shawgo]

Hi everyone,

Took a long Memorial Day drive in my 1985 Eldo.  I'm having a little problem where occasionally the cruise control seems to get out of synch, like it can't settle on a steady speed--it keeps pumping the accelerator.  You can actually see the accelerator moving up and down.  It maintains a fairly steady speed, but it accelerates, then lets off, then accelerates again, like just about every second, and it will continue doing this until you disengage it.  This is intermittent.  When it does it, if I disengage the cruise, then re-engage it, it is usually OK, but sometimes it will be OK for quite a while--20 minutes or so, then all of a sudden it will start jerking, or bouncing.  I recently had the Idle Speed Control replaced, it wouldn't have anything to do with that would it?  I was having an occasional very high idle, and replacing the ISC solved that problem.

--Mike Shawgo

[Dave Shepherd]

Surging could be caused by excess slack in the cable connection to the throttle.

[TJ Hopland]

I think starting around 1980 the cruise control and climate control vacuum source was a belt driven vacuum pump.   From memory I think its on the left side of the engine just above the power steering pump.   It may have its own belt, I think on the 368's it was a short belt driven off the power steering.   I don't remember if they stayed with that on the 4100 or if the belt went to its own groove on the water pump. 

Perhaps you have a leak in that part of the vacuum system?  Or that vacuum pump is on its way out.   Those are commonly used on diesels and that was how mine failed this winter, things were intermittent.   I had checked it when I first started having issues and it was working fine.   Went through a bunch of other testing and parts swapping over the next week or so and finally concluded that it had to be a vacuum problem and that time when I checked it was totally dead.    Once clue may have been that the old one seemed to take about 30 seconds to a minute to start building vacuum.   I would just see the needle on my test gauge bounce during that initial period.   With the replacement pump (a used one from my stash) it pretty much immediately started building.   Perhaps that was the sign of a valve problem.     

[joecaristo]

Check your electronic controller. My 77 Eldo did the same thing and it was the controller.