Radiator fan running on high speed when in motion

ARFFAZ · December 30, 2016

My 07 1500 Classic 4.8 radiator cooling fans have been constantly running on high speed. When I start my truck and put the truck in drive the radiator cooling fans will kick into high speed after about 30 secs or so and will run at high speed until I turn the truck off or let the truck idle for a minute or so.

I have replaced the coolant sensor - no luck

Pulled the PCM and checked the pin - clean

Quick tested the fan relays (no meter available) - Relays click over

Besides testing the relays for continuity I am at a loss. Does anyone have an idea of what the problem might be?

Thank you in advance!

rjgti · December 31, 2016

is ac on?

ARFFAZ · January 3, 2017

AC in the off position. I even pulled the AC fuse and Relay and the fan still ran.

This morning when I left for work (40 Deg outside) the fan turned on and the truck has been sitting for 2 days.

ARFFAZ · January 3, 2017

Also my truck has NOT displayed a check engine light yet.

Coby7 · January 3, 2017

If the ECM detects a low or excessive pressure in the A/C irrelevant of the fuse and relay being pulled it will turn fan at maximum. You may have a bad a/c pressure sensor or you are actually low on refrigerant.

ARFFAZ · January 3, 2017

What would be the easy way to check for the ac pressure sensor failing?

I have a cheap A/C pressure gauge, don't know if i would trust it to tell me the system pressure.

Edited January 3, 2017 by ARFFAZ

Coby7 · January 4, 2017

Whenever it comes to A/C I never trust anybody but the experts, often in trying to save money by doing it myself I ended paying more down the road. My A/C guy charges me $20 to put his machine on and test. Then the fix if needed is extra, the last time I took in my red Cobalt it cost me $75 total and almost had to wear gloves in the summer.

ARFFAZ · July 13, 2017

http://www.gm-trucks.com/forums/topic/202155-help-solve-this-ac-mystery/page-2?do=findComment&comment=2010360

Problem solved... see post above.

ARFFAZ · July 13, 2017

If the ECM detects a low or excessive pressure in the A/C irrelevant of the fuse and relay being pulled it will turn fan at maximum. You may have a bad a/c pressure sensor or you are actually low on refrigerant.

You were on the right track. My system was full but I ended vacuuming down the system and filling it back up to spec and still had a problem. The AC pressure switch/AC pressure transducer was being grounded out through the rack and pinion.