Why does the idle jump to 3000 rpm when ac is turned on?


Asked by Jul 05, 2014 at 10:05 PM about the 2000 Ford Taurus SE

Question type: Maintenance & Repair

I work at an automotive repair shop, we have a 2000 Ford Taurus 3.0L Duratec Vin "S".  Customer complaint was originally misdiagnosed and an IAC valve was replaced.  Customer returned and I found a vacuum leak at a broken PCV valve in the lower intake. We removed the intake and replaced the broken PCV valve.  After reassembling the engine, we test drove vehicle and found no problems and gave it back to the customer.  The customer brought it back complaining of an intermittent high throttle.  After several attempts to recreate the problem with little success, we returned the vehicle to the customer.  The customer returned several days later with the same complaint.  This day was hot and the AC was on. The engine was racing at 3000 plus RPM.  We turned off the ac and everything was normal.  suspecting maybe a defective IAC or even possibly the wrong IAC from a Vin "U" motor, we replaced the IAC again verifying the correct part. With the Scanner we can activate and command the IAC with no abnormal results to 60 percent with a slowly increasing idle. Once we command the IAC to 70 percent, the engine accelerates like a rocket ship to in excess of 3000 RPM.  We have verified part type, quality, operation, coolant levels, checking refrigerant levels now and even attempting to disconnect ac compressor from circuit to see if computer is solely responsible for idle jump.  Any Ideas?

4 Answers


Sure sounds like you still might have a vacuum leak somewhere. Have seen several fords with the hard plastic vacuum lines get cracks in them.

1 out of 1 people think this is helpful.

When the A/C is switched on a request signal is sent to the computer. One of the things that then happens is the computer slightly increases idle speed to compensate for the added load of the A/C compressor. Sounds like the speed up device or it's wiring is shorted. I'd be looking at that circuit. HTH. -Jim

2 out of 2 people think this is helpful.

just to follow up, the answer was a bad alternator. When running without the a/c on, the alternator appeared fine. when you turned on the a/c, the alternator output dropped to 12.5v. After investigation, we found that Ford programmed the ecm to raise the engine rpm to raise alternator output rather than cut off the a/c. Replaced the Alternator and all is well!! Thanks for the help

5 out of 5 people think this is helpful.

You're welcome. Glad to help. Thank you for letting us know. Glad you got it fixed! -Jim

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