I noticed that when I turn my A/C on and get to a stop light my idle will pulse while the compressor is running. Then I began to notice that it pulses only when the A/C is on and the brake pedal is pressed. As soon as I let go of the brake pedal, the pulsing stops. Now I have noticed that it will do it when the headlights are on as well. This only occurs when the compressor is running. Its almost as if the addition of the compressor (when the compressor kicks on) is too much for the charging system. I thought it may be the wrong size alternator but I haven't gotten that far yet. I haven't ever replaced the alternator, but that isn't to say the previous owner did and just cheaped out with the lowest cost they could get.

Anyone else ever run into this?

 

Does the idle increase WHEN the compressor is running, and then drop back down when the compressor stops? (Even with the A/C switch in the on position, the compressor switches on and off) It's supposed to do that. If the idle DROPS when the compressor comes on, that's just because your A/C idle-up is not working, and the compressor is loading the engine. (which will slow the alternator, and possibly dim the lights)

 

To answer the first question, the idle does increase when the compressor is running and then drops back down when the compressor stops, just as it should. To lend more detail,when I am sitting at a stop light and the truck is idling and the compressor is running, it idles UP to accommodate the extra load. During this time, I put my foot on the brake pedal and the pulsing starts, and if I take my foot off of the brake pedal, the pulsing stops and goes back to high idle until the compressor turns off. The pulsing does not occur when the compressor is off, only when it is on.

This scenario also occurs when the headlights are applied. I think the high idle is working fine, but the extra load of either the brake lights or the headlights creates the pulse.

 

leaky brake booster, maybe. check for vacuum leaks along that line

 

+1 on vacuum leak.