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A/C light comes on for unknown reason


Bushwack

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Let me give an approximate timeline regarding the a/c light coming on.

Day 1: Replace compressor and evaporator and convert to R134A

Day 2-5: Routine driving of the car with the a/c used in 10-20 minute increments. All systems are normal.

Day 6-16: Car not used.

Day 17: After a 2 minute idle warm up, the a/c light comes on a few yards after putting the tranny into drive (a/c not turned on). Drive for 30 minutes w/ the light on. Return an hour later and the light stays off all the way home.

Day 18: Duplicate of day 17 but took the car straight to the mechanic once the light came on. Mechanic looked the car over and found nothing.

Day 19. After a 2 minute idle to warm up, a/c light does NOT come on. Ran 2-3 errands that day with the car either warm or hot and the light never came on (didn't use the a/c).

Day 20. After a 2 minute idle warm up, a/c light does NOT go on. I use the a/c for about 15 minutes w/o any problems (light stays off).

Days 20-25: Car not used.

Yesterday: After a 2 minute warm up idle, a/c light comes on after driving a few yards. After a five minute drive, I turn off the car. 10 minutes later car is turned back on and the a/c light stays off for 15 minutes of driving.

Today: After a 2 minute warm up idle, a/c light does not come on. I've started/stopped the car four times today running errands (not using the a/c) and the a/c light has stayed off.

Kind of stumped. It seems the light, when it comes on, does so after a couple minutes of a warm up and then placed into gear. For the most part, light stays off when the engine is warm (w/ one exception). What should I look for to solve this problem? On a side note, unless I'm hearing things, I hear a slight hum that seems to be coming from the a/c vents (w/ the a/c turned off). I don't hear this hum on my other cars. Not sure if its a/c related or its just paranoia-related from that darn a/c light.

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Have you checked for codes (h-history) that might give a clue to what's going on?

Haven't checked any codes yet. Although that may be a start, I don't have much faith that the codes given are accurate.

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Sounds like it's on the threashold of full charge.

Per the mechanic, its fully charged. Besides, I understand that the a/c light (if there was a problem w/ the compressor) should only go on if the a/c was turned on. I have the light going on w/o using the a/c.

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Guest Squire Tom

mine is switched to r134a . if the air temp is below 60 the service ac light comes on. when it warms up some the light will extinguish and the air works fine @ 48F.

for awhile in cooler weather the light stayed on and no ac. went in and cleared codes ac works fine . got to be a little less than full charge to do this.

clear all codes and see what happens.

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...the a/c light comes on a few yards after putting the tranny into drive (a/c not turned on)...

... On a side note, unless I'm hearing things, I hear a slight hum that seems to be coming from the a/c vents (w/ the a/c turned off)....

... Not sure if its a/c related or its just paranoia-related from that darn a/c light.

...Although that may be a start, I don't have much faith that the codes given are accurate.
Let's see what we know so far.

  1. AC light coming on when AC is not running.
  2. Hearing things.
  3. Don't trust computer codes.

Sure sounds like paranoia to me. :D

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"After a 2 minute idle warm up, the a/c light comes on a few yards after putting the tranny into drive (a/c not turned on)."

That part is bothering me also. Is the climate control OFF or in ECON ? Anything else will engage the compressor.

Reason is that I have never seen the a/c light come on with the a/c off. If it is then I suspect something is shorting.

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Guest Mc_Reatta

Light on is commanded on by the BCM as a response to some stimuli from one or more of the sensors / switches.

It will set a trouble code also as result. Check for codes.

If there is no AC related code new or in history, then you have a problem with the BCM or IPC as the command is given over the data bus.

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