Wierd problem, I'm not sure how the two functions are related! When the cruise control is set it shuts down randomly within 1-20 seconds if the air conditioning is on, almost like the brakes are being applied. I think it shuts off as soon as the compressor starts cycling. It works perfectly fine if the air conditioning is off. I swapped over to the electric cooling fan which automatically turns on when the A/C is turned on. I'm not sure if this could be a part of the problem.

 

Check for vacuum leaks. Both systems have things that rely on vacuum, so if there was a leak somewhere, that could explain your problem.

 

I don't see any vacuum lines, I think my cruise is electrically controlled... The black box has an electrical plug with a bunch of wires and a cable running to the throttle body.

 

OK, a little update... If I unplug the A/C connection on my efan controller the cruise control works perfectly. I called up Flexalite and they recommended switching the ignition source wire from the coil to the yellow wire on the radio. I did that and the problem is still there.

 

does the cruise kick out if your fans come on? - might be hard to tell since they likely don't usually come on while driving at cruise speeds. either figure out something electrical to force them to come on or maybe block the rad with something and deliberately make the engine breifly! slightly! (over)heat. I'd go the electrical route myself, but I'm sure what to suggest for forcing them to come on - it'll need to be something at the sensor input - leave whatever module controls the fans controlling the fans.

My thought is this: if your efan controller is 'fancy/digital' then whatever frequencies its using/putting out into the fans is messing with the computer in the cruise - similar to a cellphone making your speakers buzz - if this is the case you'll have to play around with grounding/moving components around/RF shielding

EDIT: Just found wiring instructions for flexalite fans - there is option for a manual on switch, so if you jumper from your ignition feed (so fans are running as long as ignition is on) then if thats the trouble your cruise should refuse to engage

 

I'm thinking it actually might be the brake light switch. Maybe the draw from the fans is overloading the ignition circuit and causing a drop in voltage at the brake light switch and killing the cruise control. I'm not sure how to test my theory without throwing money at parts though.