Defroster and A/C Fuse - Anybody tried this?

'90 XJ Limited 4.0

Well I noticed yesterday that my ammeter drops drastically when I switch over to defroster.... I could hear the a/c kick on, and I understand why it does, but the voltage drop seems drastic to me because I have a low idle problem....and that may be my whole problem is the low idle (possibly in need of a TPS adjustment).....Ammeter at idle is usually around 14V - when I switch to defroster the compressor kicks on and voltage drops to roughly 11v - almost in the red..... Idle is around 300 (low huh?) but jumps up to around 750 when the compressor cycles on.....and my lights dim as in the post below....

Would bumping up the throttle stop adjustment screw help any?

Anyway - I did a search and found this thread and since there was no follow-up, I was wondering if anyone had done this - and if it caused any problems??

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Crown 01-13-2005 10:14

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Defroster / A/C question

Hi all,

Evidently, when the defroster is on, it also cycles the a/c on & off to help keep the windows from fogging up.

Every time the a/c cycles on, the volts momentarily drop from 14.5v to 12v, and the headlights momentarily dim/flicker, and the idle drops.

Is there a way to disable the a/c cycling on / off? I thought about pulling the a/c fuse for winter, but wasn’t sure if it would mess anything up in doing so.

BTW, it is a 99 Sport 5-speed.

Thanks!

jldiaz 01-13-2005 10:49

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Re: Defroster / A/C question

The compressor is controlled by the PCM, and that can't be user-modified. It signals the Idle Adjusting Motor at the same time so that RPMs are sustained. Is your IAC Motor allright? How long is "Momentarily"?

If you pull fuses or otherwise disconnect the compressor, you may get a MIL (Check engine light) that should go off after a few driving cycles. Otherwise, do you have a way to turn it off afterwards (scanner)?

Appears you have more than one problem here but for the time being on the 90, I would just unplug the wire to the compressor clutch (wire coming out of the A/C compressor should have a connector). The idle presumably will still kick up when you switch on the defroster but you say your idle is too low anyway.

The voltage drop (that's a voltmeter BTW) may indicate a wiring or alternator problem (5-90 can help you with improved cables if you need them). Or it could just be the alternator delivering inadequate current at low rpm. A stock alt is typically not going to offer a lot of amps at 300 rpm. Does the voltage drop when you switch the A/C on at driving rpm's or only at idle?

And of course you need to correct the idle speed, comments about which I will leave to some one who knows what they are talking about.

You need to start with the base throttle body/tps settings. You can't just go cranking up the idle screw, it will mess up TPS settings and everything else. The CORRECT procedure is like 5 or 6 steps in the proper order, with the proper equipment.

John

The voltage drops when I kick on the a/c as well...and yes, the voltage goes up during regular driving....

I have the procedure to adjust the TPS - just wasn't 100% sure if the problems were related.

Hunting season starts here in Bama Saturday, and since the Jeep is now my daily driver, it makes it hard to tear it down overnight - cause Murphy's Law is always in effect when I pop the hood....lol...

So, I'll take you guys advice and work out the idle problem first, then see what kinda voltage the alternator is putting out....which brings up another question.... Is there a stronger alternator available for these XJ's? I know I can get up into the mean green brand of alternators, but the $$ goes up too... Just wondering if there was an Autozone/NAPA direct fit replacement that put out more juice over stock?.....

Thanks for the help guys....