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resident pyromaniac
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Discussion Starter #1
As some of you may know, I sold my 02 rcsb a few months back and replaced it with a 2005 GMC Sierra crew cab. When I first got the truck, the air-conditioning didn't seem as cold as I thought it should. The A/C in my 02 used to freeze me out of the truck on high. For that reason, I took the 05 in to get gassed up. IIRC, it was slightly low, so after vacuuming it out, they filled it back up per GM spec. Since I bought the truck in Feb, I haven't really had many days where I use AC. It's now getting in the 90s in here SC and time to start turning on the AC.

I've noticed in the few months that I've had it, the air blows warm intermittently (with the A/C on). The truck has the digitial dual-climate control. Sometimes, when I turn the AC on, it will blow cold, other times it will blow warm. The kicker is ALL of the vents do it, not just one side like the issue with the blend doors. I can turn the truck off and start it back up, and SOMETIMES this will fix the issue. Other times, it will just keep blowing warm air. I haven't really paid much attention to it until today, but I did notice that when the air was warm, the compressor wasn't on. As soon as it did kick on, the truck started to cool down. I was going to jump the low-pressure switch on it tonight to see if it would work, but of course, I couldn't get it to blow hot air. :guns:

Also, I'm not sure if this is related but I'm starting to think that it may be. It seems like the efans on this truck run more often at idle than the 05 efan setup that I had on my 02 (yes, even using the same tables in EFILive). I'm wondering if one of the pressure switches is bad and is causing the A/C compressor to kick-off (and also activating the 2nd efan via a/c pressure).

Any help is appreciated! I need cold air. :bawl:
 

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well the efans seem to always run on my 06 Sierra with the auto climate control activated. don't notice it as much in the 06 Yukon with auto ac though. I also think my fans on the Sierra have run on high more so after I replaced my water pump (thanks for your help). It may be b/c I have a few air pockets, or it may be all in my head. In your case, I'd check the pressure switch. Also, there is another ambient temp reader, just like the one for your rear view mirror, mounted behind the right head light. I think this works with the auto ac. might be something to check also.

Mr. 960
 

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05 Sierra Crew
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987 Posts
the ambient air temp reader like mentioned above is what went out on my mom's trailblazer. It's about a $40 part if I remember correctly. She lives in HI and her TB would only blow hot air even though you would turn the temp down all the way on the electronic controls...I would def look into this part.
 

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im not sure if silverado have these but i assume they do, a expansion valve, happened to my cousins car due to imbalance of pressure. so when the pressure allowed it the clutch would kick on and super cold air, then it would stop and just blow hot air..

it still does, im think hers needs a new condensers to keep system clean, and a new exap vavle but i dont really feel like doing it

best of luck hope it works out, u have a gauge set to see your pressures ?
 

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00Silverado
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Almost sounds like it low on charge kicking the pump on and off, or the pressure switch is bad. When they charged it did they put dye in the system? If not it will have to be charged with dye to see where its leaking if it is low. The fans should always kick on with the AC
 

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hey i have an issue with my a/c also. but you halfway mentioned it in you post. something about "blend doors" when one side of the vents blow cold and others are warm. can you explain more because ive been stumped about that since i got my 06 cc.
 

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resident pyromaniac
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Discussion Starter #7
Mr. 960 said:
Also, there is another ambient temp reader, just like the one for your rear view mirror, mounted behind the right head light. I think this works with the auto ac. might be something to check also.

Mr. 960
Hrmm, is it just mounted on the core-support/header panel? I need to do some digging and see what I can find. Matt (Denaliano) is going to try and get me the full text copies of some TSBs that I found.



I guess I should've been clearer. Yes, the AC system was recharged and has dye in it. Keep in mind, this has been going on before and after system was charged back to GM spec so I don't believe it is related to the amount of refrigerant in the system.

What I was mentioning about efans, I've not been using AC for the last few months. I realize that with the AC on, the fans will be on once AC pressure triggers it. However, for the last few months, I've just been riding with the windows down without the AC on and I've still noticed that the fans run quite often when I come to an idle. This doesn't make any sense as I'm normally pulling into my driveway and the air from driving 55mph should be sufficient enough to keep the truck cool. I flushed the cooling system after I got the truck and installed a new t-stat, so I know that nothing is wrong with the cooling system. This is why I'm thinking the fans running and my A/C issue are related.

I'm going to run it today and see if it will blow hot air. If so, I'm going to jump the LPS and see if it causes the compressor to kick on.
 

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I had the same problem and jumed my lps and it worked fine just tried to freeze up a lil. replace lps and runs fine
 

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resident pyromaniac
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Discussion Starter #9
Yeah, unfortunately, something that simple wouldn't be the issue I'm having. I attempted to jump the LPS on mine earlier when the compressor was off. No such luck. Like I was thinking before, the efans have been running quite frequently. I got on my old computer earlier to look at EFILive tables. Apparently, AC HIGH pressures activate the efans to power on at idle. So now I'm wondering if my issue is with the high-pressure sensor/connector. I need to get a voltmeter and check it out this weekend.
 

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probably not your issue, but the ambient sensor is behind the parking/day time running light housing. You may also have to remove the grill shell. I can tell you that my old truck that did not have auto ac, did not have the second sensor, so it has something to do with auto ac...... Good luck! keep us posted


Hrmm, is it just mounted on the core-support/header panel? I need to do some digging and see what I can find. Matt (Denaliano) is going to try and get me the full text copies of some TSBs that I found.



I guess I should've been clearer. Yes, the AC system was recharged and has dye in it. Keep in mind, this has been going on before and after system was charged back to GM spec so I don't believe it is related to the amount of refrigerant in the system.

What I was mentioning about efans, I've not been using AC for the last few months. I realize that with the AC on, the fans will be on once AC pressure triggers it. However, for the last few months, I've just been riding with the windows down without the AC on and I've still noticed that the fans run quite often when I come to an idle. This doesn't make any sense as I'm normally pulling into my driveway and the air from driving 55mph should be sufficient enough to keep the truck cool. I flushed the cooling system after I got the truck and installed a new t-stat, so I know that nothing is wrong with the cooling system. This is why I'm thinking the fans running and my A/C issue are related.

I'm going to run it today and see if it will blow hot air. If so, I'm going to jump the LPS and see if it causes the compressor to kick on.
 

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resident pyromaniac
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Discussion Starter #11
probably not your issue, but the ambient sensor is behind the parking/day time running light housing. You may also have to remove the grill shell. I can tell you that my old truck that did not have auto ac, did not have the second sensor, so it has something to do with auto ac...... Good luck! keep us posted
Thanks. I bought an AllDataDIY account yesterday to get some of the wiring schematics and I saw the sensor that you were talking about. The thing is, my A/C does this regardless if it's on "auto" or if i manually control the temps with the HVAC head.
 

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Matt.... gotten down to the bottom of this? Just curious what, if anything, has turned up? On my 06 Sierra, since i changed my water pump, every so often my fans go to warp factor 6, as in super high speed. sounds like a jet airplane. Truck is not hot and ambient temps ar normal. Untill now, i've never seen my fans run at such high speeds.

Mr. 960
 

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resident pyromaniac
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Discussion Starter #13
Nope, I haven't messed with it much recently. i tried jumping the low pressure switch but it didn't seem to engage the compressor. i did notice that the compressor is off when it's blowing hot like this though, so i know that its something that's causing the compressor to kick off. once i get some extra cash, i'm going to see if the dealer can diagnose it with the techII. i've noticed though that if i start the truck with the a/c on hi @ 60* with it turned on vent, it will blow cold non-stop UNLESS i change the settings. if i change them, it will start to blow hot and won't stop until i restart the truck. sometimes restarting doesn't seem to fix it. it makes me wonder if its an issue with the HVAC control module itself.
 

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GMFS Senior Member
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Damn Matt, still doing this? :think: So I'm guessing the thermostats weren't bad when they were changed?

Wasn't it doing the same thing that night we damn near froze in it at the beach? Seems like when we'd try to turn the defrost on the compressor would kick on, then kick off intermittently if I remember right. Also it'd blow heat on startup but switch back to ambient air once the setting was changed. Gotta be that HVAC module or something electronic I would think.
 

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resident pyromaniac
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Discussion Starter #15
Yep, it's doing this. I'm guessing the t-stat wasn't messed up either but its replaced now anyways. I'm tired of throwing parts at it. I have a full wiring diagram from alldatadiy but so much shit runs through the BCM that it's not that easy to chase. I, too, think that its something electronic b/c when the compressor does engage, it air begins to cool. I just hate to spend the cash on a new HVAC controller and that not be my problem. Analog>Digital
 

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"God Damn Buick Regal"
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Yea, makes sense. Any idea how much a diagnostic trip to the dealer will run?
I took mine to the dealer and just for a diagnostic it was 85
 

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mine does the same thing. like exactly the same. sometimes the compressor won't kick on and it blows hot. i jumped the low pressure switch and it kick on right away. i also bought a cheapo harbor freight gauge to check it and it was just at the top of the "safe" range. (according to the gauge).

i can't figure it out. haven't tried replacing the low pressure switch yet. i have found that with the windows down or cracked and the air on high it kicks on 95% of the time. learned that trick from my handy dandy haynes manual.
 

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Sweet Tea Pants
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Mine is doing almost the exact same thing, except I can usually adjust my temp a little without it defaulting to hot. I was going to run it by the dealer yesterday but ended up having to work.
 
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