Automotive Air Conditioning Information Forum (Archives)

backfocus on Tue August 10, 2010 10:14 PM

Year: 2004
Make: Chevy
Model: Silverado 1500
Engine Size: 4.8L
Refrigerant Type: 134a
Ambient Temp: 96
Pressure Low: 49
Pressure High: 260
Country of Origin: United States

So I have been having this weird problem going on with the AC in my truck. Going down the road it will suddenly go from nice cold air to feeling like the heater is on. This does not seem to happen at any certain time or because of any certain thing. I can not get it to reproduce the problem sitting still. When it does happen, I did find out that if I come to a stop turn off the engine, then turn it back on the compressor comes back on. I am taking it that the compressor goes off, when this happens, I can hear it click back in when I restart the engine. Whether or not the heat is really coming on or not I am not sure, but the air is really hot, even when this happened back in the spring when the outside temps where cooler.

I am not sure where to even begin looking since I can't get it to do it at home and the Chevy dealership was puzzled as well. It was only doing it on a rare occasion, say once a month since April but it has done it now 3 times in the last 2 days.

Any ideas.

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Michael McCall

Edited: Wed August 11, 2010 at 8:32 PM by backfocus

77shovelhead on Wed August 11, 2010 12:24 AM

I am no expert and I'm new to AC repair, so I will only guess as to the problem.

My guess is that the hot/cold switch is losing vacuum. Possible vacuum leak under the hood or dash.

If I'm not mistaken, the hot/cold switch moves the door between the heater core and evaporator.

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Quality is never an accident it is always the result of intelligent effort.

GM Tech on Wed August 11, 2010 10:24 AM

It is not controlled by vacuum! It is all electrical actuators....If it were mine,I'd rule out freeze-up and wide air gap---try to understand if compressor is on or off when this happens,,,,next time it happens- pull over do not turn off a/c or engine--pop hood--- is compressor running? are a/c lines frosty white? Is the air heated air or just outside ambient air? These will all be big hints.....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

backfocus on Wed August 11, 2010 9:05 PM

Well it cutout again going home today and I was able to pull into a parking lot and check things out a little while it was not working. The system shows no signs of freeze up. The compressor clutch is engaged and turning. I can turn off the AC while it is not working and the clutch stops, as normal, turn it back on clutch kicks back in, but it is still blowing hot air. I am more convinced that it is just ambient air and not the heater as it felt just a little hotter than outside, but I know early this spring it blew much hotter air than outside.

I brought it home and checked pressures and here is what I found.

While not blowing cold air:

Low side 60
High Side 305
Coil Temp 100
Outside Temp 96

I shut the system off for 20 minutes or so and checked static pressures

Both sides are at 105


Turn truck on and AC working

Low Side 49
High Side 260
Interior Vent Temp 53
Coil Temp 97
Outside Temp 95

I am not super experience with Auto Ac system and most of it is with older R12 systems, but the Low side seem high to me, what do you guys think, and what could cause that.

I have also read on some bulletins that I could be the control unit???

Hopefully someone has more ideas how to help me now, I am still not sure where to start.



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Michael McCall

padgett on Wed August 11, 2010 9:14 PM

Pressures look similar to when the electric fan (radiator) in my TranSport quit.

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Many Carpets

Edited: Wed August 11, 2010 at 9:16 PM by padgett

GM Tech on Wed August 11, 2010 9:38 PM

Did it cut out upon an acceleration -- like from a stop light? If so, you probably are experiencing high pressure cut-outs due to sluggish clutch fan--- when you stopped and looked- compressor was running- it may have just come back on after the HPCO event. Did it start cooling again while you saw it running?

Head presasure runs high at idle and while topped in traffic-- then upon acceleration- it goes even higher until some good air flow comes across condenser-- HPCO is around 425psi and return on at about 250. seen it often upon accells...

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

backfocus on Wed August 11, 2010 10:02 PM

It did do it when accelerating today, but it usually occurs while driving normal speeds on the highway. When I stopped and looked today and compressor was working, but no cold air. I can turn the AC switch to off and the clutch disengages, turn it on and in engages, but still no cold air. The only way it will start blowing cold air again is to turn off the engine then start it back up.

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Michael McCall

Mudshack on Thu August 12, 2010 9:38 AM

I am very interested in this thread. My truck is doing a very similar thing. What could be the coorlation with turning the truck off and turning it back on? That to me sounds like some sort of computer issue or vacuum issue. But again, if I knew, I wouldn't have my problem.

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1999 Jeep XJ Sport

backfocus on Thu August 12, 2010 2:00 PM

I am thinking that if I could scan for DTCs I would find the answer. After doing some in-depth reading on alldata.com, a scan tool can run the system through the paces, and show codes if there is something wrong. Does anyone know if a regular code reader will do this, or would it need to be a GM tool? I seems to me more computer related, since all I have to do is turn off the engine and restart it, possibly resetting something computer related.

I have manual temp controls if this matters for the codes.

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Michael McCall