Ok here's the deal. I have my idle set low and my cold idle air valve plugged so the only bypass around the throttle blade is determined by the idle air screw. For some reason whenever I have the a\c on and the compressor kicks in, if I push the brakes the idle fluctuates up and down continuously at the same interval. It's NOT vacuum related because I can cap the master cylinder vacuum line at the booster AND the intake and still replicate the problem. Also, for some reason it won't do it if I have the transfer case shifted into 4wd. This leads me to believe it is somehow electrically related. I have read the the wire from the brake light switch runs to the ecu and THEN to the brake lights. This is supposed to be so it can cut fuel during braking for fuel economy reasons or something. It did not do it before so SOMETHING had to change. Does anyone have any idea what could be causing this problem. The idle fluctuation is similar to the one experienced when the idle is set too high during closed loop operation (engine warm) and the ecu cuts fuel back to lower the idle, then restores it, and so on.. WTF would the brake have to do with this though? Hopefully someone can tell me, I don't really want to have to bypass the ecu and wire the brake light switch directly to the taillights (pain in the arse), but I will if I have to.

 

I believe the a/c idle up is adjustable also, meaning your idle is increasing too much when the a/c is on. Many things on your truck will increase idle, ac, p/s,
4wd. There IS a wire to cut to bypass that bullsht but youll have to search, unless someone chimes in with a link, HINT< HINT.

 

I had this problem when the plug to the cold start sensor came loose. It's the green plug in the red box in this pic:



I couldn't figure out why the brake had anything to do with it, but once I made sure that connection was sound, the problem went away.

Rob

 

what about that little plastic check valve between the master cylinder and the motor?

 

Thanks man.. I'll check that as soon as I get home from work.

 

If you're talking about one on the vacuum line, I've already capped off the master cylinder and plugged the vacuum line so the engine (vacuum wise) shouldn't know that the brake is pressed. It still does it when I push the brake..