No cruisin' in my '85 Chevy Van

I have owned this 85 Chevy Van all its life and when I take it on trips I like to use the cruise control. The last few years the cruise engages and works except that it seems to hunt or vary the speed up and down a couple of miles an hour. I replaced the servo figuring that it was just old and leaking vacuum. Well the new one behaved exactly the same. So what else is there? The electronics module? OK?!? I can see where you can buy a replacement but my only problem is; where the !#$? is it? I have looked everywhere under the hood and under the dash with no luck. Manuals don’t even include the cruise system. Does anyone have a solution to the hunting issue or at least know the location of the module or any other suggestion?

I tried to link some info, but was unsuccessful, however I did find the schematic for this cruise control system by Googling, and according to the site I found the speed signal for the system apparently originates in the instrument cluster. That may be the next place to look. I would have suspected that it originated directly in the vehicle speed sensor, which is what drives the instrument cluster, but apparently the signals are “daisy chained”.

Google “schematic for 1985 Chevy Van cruise control system”. That’s how I found it.

Thanks so much

Maybe there’s a vacuum leak in any one of a number of places and that is having an affect on the vacuum servo. A leak could be anywhere in the engine compartment or inside the dashboard.

The best way to determine if there is a leak is with a vacuum gauge. If a leak exists according to the gauge then it’s a matter of pinching hoses off one at a time until the right path to the problem is located.

One other possibility did occur to me. If the van has a factory cruise control unit on it and it has the inner fender mounted transducer (2 speedometer cables-one from trans to transducer, other from transducer to speedo head) it’s possible for hunting to be caused by binding cables or an iffy transducer.

Did someone mention the vacuum check valve?