Diagnosing AC Trouble

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avslash

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Anyone have any relevant experience diagnosing hvac problems? I'm conversant in auto ac systems, but don't have a lot of hands on experience.

While driving, my truck will normally blow cold air as expected. However, it will intermittently start blowing warm air despite no changes being made to the temperature controls. When it does this, completely turning off the hvac system and then restarting it will usually resolve the issue, but sometimes you might need to cycle it on/off several times for it to resume blowing cold air again. Once it is blowing cold again, it might work for hours, or you might have to do it again after a few minutes.

When it is blowing cold, it blows as cold as it ever has, so there has been no reduction in cooling performance. The system has also been charged by a reputable shop within the last couple months after the shop repaired a leak in the refrigerant lines going to the rear hvac unit, so it is very unlikely to be a refrigerant charge issue.

Because the system can operate normally, I'm wondering if it is an intermittent problem with compressor engagement. I know these trucks don't use a traditional ac clutch, but some type of solenoid system to engage/disengage the compressor.

I do have a IID tool, and no relevant faults are being thrown, so what should I be looking at?
 

Nechaken

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Is there any relation to vehicle speed? I had an issue on my 2012 where sitting in stop/go traffic, it would start blowing warm air, then cool off again when I started driving. Turned out the freon (r22, 134, whatever) was just a tiny bit too low (a few ounces) and sitting/driving 5-10 mph or whatever, the airflow from the fan wasn't sufficient to lower the temp enough to prevent whatever high or low pressure switch from triggering. I know it was just refilled, but either possibly another leak exists, or maybe they got the charge wrong by a bit.

I also want to say I remember the compressor was variable output -- is there a duty cycle / PCM % value to monitor in the GAP app?
 
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f1racer328

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I’ve had the same problem, except it is super rare.

Usually only happens after I’m driving for a very long time (3+ hours) - cycling the AC button and going back into auto fixes it for me too.

Hasn’t been too big of a problem though, but I’d love to fix it if it gets worse.
 

djkaosone

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Mine did this a while back with similar symptoms. I recharged the r134 freon and it was on point with no leaks, but the warm air symptoms were still there. Turns out that my AC compressor was the issue even though you can see/hear the clutch engaging. I ended up replacing it with a reman AC compressor from https://www.rycompressors.com/. I changed out the drier too while I was at it. Fixed my issues.
 

Jimmy Brooks

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funny Enough is I’ve had this issue before too. It used to happen a lot after I replaced my compressor, then a year later it froze up and tore my serpentine belt because of “metal flakes going through the condenser filter” so the condenser was replaced along with the belt and the compressor for free. I rarely have the issue now so I think it is an intermittent glitch. Like everyone else said cycling the AC button normally fixes it.
 

avslash

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Is there any relation to vehicle speed? I had an issue on my 2012 where sitting in stop/go traffic, it would start blowing warm air, then cool off again when I started driving. Turned out the freon (r22, 134, whatever) was just a tiny bit too low (a few ounces) and sitting/driving 5-10 mph or whatever, the airflow from the fan wasn't sufficient to lower the temp enough to prevent whatever high or low pressure switch from triggering. I know it was just refilled, but either possibly another leak exists, or maybe they got the charge wrong by a bit.

I also want to say I remember the compressor was variable output -- is there a duty cycle / PCM % value to monitor in the GAP app?
Haven't seen a duty cycle value in Gap tool, but I could be wrong.

Mine has always blown warmer air when sitting still, but most vehicles I have had exhibit the same behavior.

Thanks.
 

avslash

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Mine did this a while back with similar symptoms. I recharged the r134 freon and it was on point with no leaks, but the warm air symptoms were still there. Turns out that my AC compressor was the issue even though you can see/hear the clutch engaging. I ended up replacing it with a reman AC compressor from https://www.rycompressors.com/. I changed out the drier too while I was at it. Fixed my issues.

Thanks for the link. I'm guessing it is the same for me. Called my Land Rover indie and he felt like it was probably the same thing as well.
 

f1racer328

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Mine has always blown warmer air when sitting still
This is normal. When it's 105 ˚F + outside its noticeable too.

Our fan is a viscous fan and needs a little bit of engine speed to get the air flowing.

It'll bump the idle up in park but can't really do much in drive.
 

RoverTide

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Thanks for the link. I'm guessing it is the same for me. Called my Land Rover indie and he felt like it was probably the same thing as well.
Reviving an old thread... Hope you guys are still around. To the OP, did you ever determine if it was just the compressor? or a combination of things?

I am experiencing the same issue, where the cold air turns cool/warm for no apparent reason. Sometimes I can cycle the ac off/on and it starts blowing cold again, but not always. I have noticed there isn't a direct relationship to how fast I am driving, or sitting still, and the temp that is blowing from the vents... the issue seems random. I put a set of manifold gauges on the system and I have a low side pressure of 60 psi, and high side pressure of 170 psi (engine was idling and outside air temp was 90 degrees). The system was just evac'd, vacuumed, pressure tested (no leaks), and charged with 2lbs of 134a. I am torn between the following being the problem... 1. Expansion valve not functioning correctly, 2. Compressor is weak, and 3. a pressure switch not reading the pressure correctly and sending bad signals to the compressor. I have a GAP tool, and the pressure reads between 120 and 140 when driving. I know the pressure should be much higher, especially when driving, whats strange is, the manifold gauges read higher even when its at idle. Any thoughts or help is much appreciated.
 

f1racer328

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Reviving an old thread... Hope you guys are still around. To the OP, did you ever determine if it was just the compressor? or a combination of things?

I am experiencing the same issue, where the cold air turns cool/warm for no apparent reason. Sometimes I can cycle the ac off/on and it starts blowing cold again, but not always. I have noticed there isn't a direct relationship to how fast I am driving, or sitting still, and the temp that is blowing from the vents... the issue seems random. I put a set of manifold gauges on the system and I have a low side pressure of 60 psi, and high side pressure of 170 psi (engine was idling and outside air temp was 90 degrees). The system was just evac'd, vacuumed, pressure tested (no leaks), and charged with 2lbs of 134a. I am torn between the following being the problem... 1. Expansion valve not functioning correctly, 2. Compressor is weak, and 3. a pressure switch not reading the pressure correctly and sending bad signals to the compressor. I have a GAP tool, and the pressure reads between 120 and 140 when driving. I know the pressure should be much higher, especially when driving, whats strange is, the manifold gauges read higher even when its at idle. Any thoughts or help is much appreciated.
I’m actually experiencing the same exact problems. Have also put a set of manifold gauges on and checked pressure with my IID tool.

IID showing around 140 driving.
Manifold gauges were showing 55/240 at around 100° F.

I haven’t added any refrigerant or done any leak testing.

I also only experience the problem after my car has been running a long time, however, my AC compressor hisses after I shutdown my car now. It hisses for about 30 seconds after shutdown.

I’m pretty sure it’s the compressor but it seems like it’s a royal pain in the ass to replace, and not exactly cheap.

I’m going to see if it’ll go for the rest of summer here and proceed from there. It does seem to be getting worse though.
 

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