Ac Continously Runs...

webejpn · July 14, 2010

1999 Tahoe LS with 5.7

Just bought this truck and it's my first non-Jeep vehicle so I am a little lost on the mechanics of this truck.

I noticed with the AC controls switched to off and after driving short or long distance, i would get a puddle of water under front passanger tire and passanger rear corner near exhaust outlet.

I then noticed cold air blowing lightly from the front vents by the windshield with the controls still off. If i adjusted the warm/cold switch, the temp would change accordingly. With the system off, if i try changing the vent selection, nothing happens. So the temp control works all the time, the vent selector switch only works when the system is on and the off/low/med/etc switch does adjust the fan speed when its on, but appears it doesn't turn off the ac.

With the system off and truck running, the AC is engaged. I disconnected the plug on top of the ac pulley and it did in fact turn off the ac/clutch. I checked the 10amp fuse and that's good.

So is there a relay or something in the system, or how do i check if it's the controller head??

Thanks in advance guys!!

gmtech4 · July 15, 2010

your big middle paragraph is as clear as mud Matt,

the control head is a module and may have code in it ,get it scanned/diag'd properly

looking at the data in the scanner is very helpfull to diag the motors that control the doors in the heater box

luck

webejpn · July 15, 2010

Ok, let me try an rephrase it a bit more clearly.

With the truck running and the entire temp control unit switched to off, the AC clutch does not disengage and continues to blow cold air out of the defrost vents.

If I leave the fan speed knob on off (system shouldn't be on at all), but I adjust the temp knob in the middle, it will adjust the temperature.

If I leave the fan speed knob on off, but I adjust the vent selector knob on the right, it will not blow out any other vent but the defroster vents up by the windshield.

When I turn the fan speed knob on anything but off, the systems work correctly although the AC clutch from my knowledge shouldn't be engaging unless I select the AC button between the temp control and vent selector knobs.

Is that better?

bossier62guy · July 15, 2010

Ok, let me try an rephrase it a bit more clearly.

With the truck running and the entire temp control unit switched to off, the AC clutch does not disengage and continues to blow cold air out of the defrost vents.

If I leave the fan speed knob on off (system shouldn't be on at all), but I adjust the temp knob in the middle, it will adjust the temperature.

If I leave the fan speed knob on off, but I adjust the vent selector knob on the right, it will not blow out any other vent but the defroster vents up by the windshield.

When I turn the fan speed knob on anything but off, the systems work correctly although the AC clutch from my knowledge shouldn't be engaging unless I select the AC button between the temp control and vent selector knobs.

Is that better?

Do you have it in the recirculate mode? If so the A/C is going to continue to run regardless if you have it selected or not. The airflow never stops completely even w/ the switch turned off....it's normal. Your other problem sounds like it could be caused by a bad actuator.

gmtech4 · July 15, 2010

it is better ,get it scanned

webejpn · September 3, 2010

nothing on scanner, zipppo.

here is a recap...

This is our tow vehicle that i bought used about 2 months ago. The AC worked fine when i bought it. About a month ago i was taking it to work in the early AM i noticed when the sun got just right, the vents had ice cold air coming out....kicker is the controls were all set to off.

So i looked under the hood, sure enough, the AC pump clutch was engaged with all the controls set to off. So i just unpluged the little wiring harness at the pump and the clutch disengaged.

So i ran a scanner, no errors/codes.

About 3 weeks ago we went to use it for camping and i plugged it back in so we would have ac for the ride. Noticed it wasn't blowing very cold at all, so i just left it run and rolled the windows down. About 30mins later a horrendous squealing was coming from the hood...thought i blew the damn header off As soon as i disconnected the ac pump it went away.

txab · September 3, 2010

Makes me wonder if part of the problem is the low pressure switch is bad or has been bypassed

KMGZ400 · September 3, 2010

both of my current silverados blow air out of the vents even when its off but its not AC cool air. my old 99' GMT400 did it too. it might be normal.. are you sure its AC cold air blowing out?

cwendel4 · September 4, 2010

had a similar issue with the recic door. Seemed to clear up when I had the rear done and the battery was disconnected. Try the reset steps for the HVAC system on this site, it may help. Also, make sure to remove the fuses as well when disconnecting the battery as this seems to help.