mingtech

Normally the idel speed is around 650~750
After I turn on AC, the idel speed will down to 300, and the car will shake.
Anyone has such experience? I am a totally newbie
thanks for your help!

Roddimus Prime

whenever you engage the ac compressor clutch it's going to put a strain/drag on the engine. This is common. What is uncommon is for it to drop that much. I also think your curb idle may be too low.

Your car is a 93 model? If you haven't done so I would suggest going in and doing a complete "tune up" on your car to make sure everything else is running like it should. Change your plugs, check your wires to make sure they aren't corroded, change all of the fluids, oil, coolant, transmission (never hurts!), clean your MAF or VAM, check all vaccuum hoses for cracks, brittleness or leaks.....replace as needed, check your hoses, check your distributor cap for signs of heat damage or burning.

After this is up to speed you should notice the car running better. The idle should be smoother and more adjusted. Then try switching on the AC and see what it drops to. If the engine is running stronger it may not cause as much of a drag on it as it does right now.

macdaddyslomo

does it bog and then the idle comes back up?? If so this is pretty normal..If it isn't then Matty is right you have a problem....

raldo_gtm

First thing I would check is your TPS. Get a multimeter and you need to check it while it's still on the Throttle Body. Sometimes the TPS would start to develop a little bit of play while over the years, it gets to a point where the car still idles, but when the AC is one, it idles really rough.

Roddimus Prime

another good piece of advice.

mingtech

yes, after turn the AC off, the spped back to normal, thanks for you guys!!

Roddimus Prime

macdaddy meant "when you turn the ac on, does the idle drop down and then come back up??" We know the idle will go back up when you turn it OFF....will it go back up on it's own while the AC is running?

mingtech

no, the idle speed will keep around 300 if ac is on, and back to normal if ac is off,
thank you again! :-)

macdaddyslomo

"danger will robinson, danger".....something is definitely wrong...You definitely should not idle around 300...check all vacuum lines and the TPS...

exexx

Two of the inputs to the ECU are the a/c on status from the a/c switch and the refrigerant pressure switch. The ECU uses them to compensate the idle rpm when the a/c is putting a load on the engine. You will need a wiring diagram to check them out.