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Old 08-26-2012, 01:40 PM   #1
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AC compressor always on

99 TJ 4.0

I recently got everything back and running after a front impact accident. Many threads about other issues but what would cause my compressor to constantly run? Prior to the accident and after i got the Jeep running i could hear it cycle. It doesn't cycle any longer. Any thoughts?
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Old 08-26-2012, 01:58 PM   #2
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Sounds like the miswired the compressor so it is no longer powered by the HVAC circuit wiring.

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Old 08-26-2012, 02:23 PM   #3
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Thanks for the reply... I did the work personally so i never had the wiring disconnected. Should i check electrical plugs at the compressor itself? Also, my knee went into the climate control unit pretty good in the accident, could that be a factor? AC blows cold... Almost too cold.

Just a side note - the fan selector switch only has 1 speed now ( resistor was replaced a few months prior to the accident).
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Old 08-26-2012, 02:38 PM   #4
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The HVAC system has a thermostatic switch that controls the compressor, perhaps the accident smashed it and it no longer works.
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Old 08-26-2012, 03:28 PM   #5
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When i turn the selector knob to non-ac the compressor stops. Is that indicative of a bad hvac switch?
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Old 01-10-2016, 12:31 PM   #6
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I realize this is an old post, but it's still returned near the top in web searches and I didn't find others that gave a real answer. So, here's what I can add. I have a 2005 Wrangler TJ 4.0 and the AC ran at all vent settings and, I believe, also when the vent was off (the AC compressor on mine is buried deep and I don't have a clear view of the compressor clutch - just a side-view of the belt).

Anyway - Car ACs have a compressor which has a turn-wheel that is one of the wheels the serpentine belt goes around. When the AC is off, the wheel spins freely. The compressor has a magnetic clutch. When you turn on the AC a signal is sent to a relay located in the main fuse box in the engine compartment (which Jeep calls the Power Control Module or PCM). The relay tells the magnetic clutch on the AC compressor to engage or disengage.

So, with this knowledge the problem could be wiring, a bad relay, or a bad AC compressor clutch (one that is stuck in the engaged position). Easiest way to troubleshoot is to identify and remove the relay (it's labeled). With it removed, the AC should not work. If you still have AC, it's a problem with the AC magnetic clutch. Otherwise, you've narrowed it down to the relay or wiring with the relay being the probable cause.

In my case, just unplugging and jostling the relay and plugging it in again seemed to fix my problem. If it reoccurs, I'll buy a new relay. OEM ones cost ~$15 (<$20), so I wouldn't mess with cheap aftermarket ones.

Hope this helps someone.
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Old 01-10-2016, 12:35 PM   #7
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Meant to included above: There is also some confusion on which vent settings are supposed to trigger the AC and the Owner's Manual doesn't help. As far as I can tell the AC is on with any of the settings that have the snowflake (logical) + the last two settings that have the defog / defrost vent (to help defog). It is not on when the main dash vent or floor vent settings (no snowflake) are selected.

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