why does my engine surge when my a/c or defrost sw is set to a/c or defrost. on 2000 Dodge Ram 2500

when sw is off everything is normal.switch the selector to a/c or to defrost and the engine will seem to back off and then surge about 40 to 50 rpm. thought it was my transmittion slipping at first but only does this when the sw. is turned to a/c or defrost. it does this every few seconds not contineously. just intermettently. thanks, hope i gave enough info.

1 answer 4 comments
ac compressor puts a load on the motor.

i realize the compressor loads the engine but i can't figure out why in the defrost position it affects the engine. also why it continues to affects it intermitenly as long as the a/c or defrost sw is on pos. it feels like the trans is slipping when it comes on. not sure what or where to check. thanks for your reply.
Defrost automaticlly turns A/C on in most newer vehicles (past 1990 qualifies as newer to me).40-50 RPMs is normal change. If the compressor is cycling more frequently then it used to then you might consider having the system checked out for leaks or electrical problems.
Thanks for your input. I now realize that the defrost sw can actuate the a/c compressor. after checking more closely and trouble shooting I have found out that the compressor clutch is causing my problems.I think I will change out the compressor and clutch because, if the clutch is bad the compressor will soon follow.Thanks again to each one that has replied.::::::::::::::: Tom from Oregon
The clutch is somply on-off. The compressor Would be what causes the drag. If you haven't replaced the clutch already, I would suggest getting it to a shop and have guages/machine hooked up to observe whats happening when compressor engages. several things besides a faulty compressor can cause excessive load on the system.