Defroster..

WylieD · Posted: Sat Apr 02, 2005 4:42 pm Post subject: Defroster..

I've learned that when I turn my defroster on, my engine idles higher?

I really don't think thats normal, but was just wondering if anyone knew why, or if anyone elses did this.

btw its an 81 Elc

spoonplugger · Posted: Sat Apr 02, 2005 11:28 pm Post subject:

Yes, it is normal. When the defroster is running, so is the AC compressor. The carburetor is equipped with a fast idle solenoid that is energized anytime the AC compressor is on. The solenoid prevents the carb linkage from traveling all the way to the low idle position. Of course, the engine idles faster with the AC system running also.

You can test this for yourself. Turn on the defroster. Come to a complete stop. Listen carefully to the engine RPM's. Turn off the defroster. Observe the drop in idle speed. By the way, it's a good thing. Be grateful it works correctly. One less thing to fix!!!
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86 caballero, 350 cid, TH350C

WylieD · Posted: Tue Apr 05, 2005 3:26 pm Post subject:

Ahhhh...I see now thanks, that makes sense.
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vrooom3440 · Posted: Tue Apr 05, 2005 4:45 pm Post subject:

You need some idle step up to compensate for the extra load on the engine with the A/C compressor on.

Actually in a perfect world your idle would not change, but in an analog/mechanical world you only get so close. With more recent cars with computerized EFI you get no idle speed change, just throttle change.

Still depending on how far it moves, I would look into verifying that the A/C system is really working. Your idle would go too high if there is no load from the A/C compressor for example. If you get cold air on A/C then you are probably fine here. Next after that I would check if the idle stepup mechanism has any adjustability built in. You may be able to reduce the amount of throttle movement at the stepup position.
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Project: '68 El Camino SS 396 clone w/4spd
Daily driver: 2001 Mustang Bullitt