Ac Duct Temp Up And Down

peirek · June 29, 2011

2010 GMC Denali,

AC went out. Took to dealer for warranty repair. Tech found two leaks. One was at the high side fitting weld the other was in the evaporator core as they detected dye coming out the drain. 4 days in the shop. geez!

Get it back and the radio is not working..... forgot to plug in the antenna to the radio head.

Bigger problem - the compressor is cycling every 30 seconds.

Take it back and they discovered a leak - pinched o-ring. Plugged the antenna in. Had two guys, Service Lead and Service Manager, drive it and check it out before saying it was OK.

On the way home, after 2-3 minutes I knew it was not right. The air coming from the ducts is getting ice cold and then warming up in 5-10 second cycles. I thought "Christ the compressor is cycling again?". But when I get home the compressor is NOT cycling on/off. It is staying on but the air is going real cold to not so cold to real cold. Not a drastic difference (maybe 10 degrees) but enough to notice when it is 98 outside and the sun is beating down.

BTW if I put it on 60 (overrides the auto) the temp stays ice cold............. tells me the computer is getting bad data.

So what sensor did they muck up or connector did they miss when they tore apart the dash and replaced the evaporator core? Any thoughts. I would love to go sit with the tech and monitor the temp sensors (all) and the "air blend" door position. It is almost like they swapped a couple of sensors so the computer is totally screwed up. Any thoughts would be appreciated. Other than find another dealer. I am one day away from that now.

peirek · June 29, 2011

Dropped the truck off at the dealer this morning. Asked them to scan the HVAC computer to monitor the temp sensors and the actuator door position to determine what was oscillating. I explained that it was cycling on a 15 second interval. They call back and say works just like a 2011 Sierra. I asked him if he scanned the HVAC computer and his response "I think so..... but it is behaving just like the 2011 Sierra side by side". I asked him if someone drove it down the highway to feel the temp changes. No...............

Had my wife go pick it up and take it to another dealer (I could not get away from the office). I asked her if I was loosing my mind and she said it definitely was acting strange and the air temp from the ducts was getting real cold and then getting warm. Why can't the dealer do the same. Thankfully we live in a big city with more than one GMC dealer. More follow up tomorrow I hope.

I sure hate to think I will need to buy the scanner software and debug this myself. Of course after I fix it I would go back to the original dealer and demand payment for the software.

BTW I am a Hardware Engineer and understand closed loop systems. They are not suppose to oscillate - signifies a bad input or control.

Still waiting for a GM Tech to respond. (Hope they are not from Dallas and from the dealer I just left............)

2010 GMC Denali,

AC went out. Took to dealer for warranty repair. Tech found two leaks. One was at the high side fitting weld the other was in the evaporator core as they detected dye coming out the drain. 4 days in the shop. geez!

Get it back and the radio is not working..... forgot to plug in the antenna to the radio head.

Bigger problem - the compressor is cycling every 30 seconds.

Take it back and they discovered a leak - pinched o-ring. Plugged the antenna in. Had two guys, Service Lead and Service Manager, drive it and check it out before saying it was OK.

On the way home, after 2-3 minutes I knew it was not right. The air coming from the ducts is getting ice cold and then warming up in 5-10 second cycles. I thought "Christ the compressor is cycling again?". But when I get home the compressor is NOT cycling on/off. It is staying on but the air is going real cold to not so cold to real cold. Not a drastic difference (maybe 10 degrees) but enough to notice when it is 98 outside and the sun is beating down.

BTW if I put it on 60 (overrides the auto) the temp stays ice cold............. tells me the computer is getting bad data.

So what sensor did they muck up or connector did they miss when they tore apart the dash and replaced the evaporator core? Any thoughts. I would love to go sit with the tech and monitor the temp sensors (all) and the "air blend" door position. It is almost like they swapped a couple of sensors so the computer is totally screwed up. Any thoughts would be appreciated. Other than find another dealer. I am one day away from that now.