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2002 chevy trailblazer_ls
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Discussion Starter #1
4.2L 2002 Trailblazer 2WD

YouTube video of gauges:
https://youtu.be/Ys7vkRQW7Go

Issue: A/C freezing up after 45 minutes of driving

Things I’ve replaced:
Low pressure switch
Fan Clutch

I rented some gauges and these numbers came up

Ambient Air Temperature: 85
Low pressure: 40
High Pressure: 225

Engine revd to 2000 RPMs
Low: Dives down to 13
High: Remains at 225

My thought is a clogged Orifice tube, but hopefully you all can help me out! Thanks everyone.
 

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2004 chevy trailblazer_lt
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102 Posts
I am by no means an expert in the field of automotive HVAC, but I think your system is low on refrigerant. At 2000 rpm, ambient temp of 85 deg F, "normal" low side pressure should be 45 - 55 psi, and high side pressure should be 225 - 250 psi.

In watching your video, it looks like when the compressor engages, you are getting the pressures you cite at 2000 rpm - 13 on the low side and 225 on the high side. However, when the compressor disengages (what you call the 85 deg ambient air temp, the low side pressure rises and the high side pressure drops slightly, which is normal behavior.

Does this correlate with what you are observing relative to the compressor engaging and disengaging?

Is the cycling of the compressor frequent?

If yes, then you are likely low on refrigerant.

Here is link to a chart for R-134a Temperature vs Pressure and it includes some troubleshooting information:
http://rechargeac.com/how-to/ac-system-pressure-chart

Good luck!
 

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Discussion Starter #3
Thanks for the help! In my video the a/c compressor isn’t disengaged at all. The A/c compressor is on and a/c blowing but then I’m just reving the engine. I don’t think the a/c compressor is switching off at all. It’s staying on all the time. Could that be the issue? Does anyone know when the a/c compressor is suppose to disengage?
 

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2004 chevy trailblazer_lt
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If the compressor is not disengaging at all, then you need to find out why. You'll need to determine if it is electrical or mechanical. I don't know if the varying pressures are a result of the system freezing up.

I had this (system freeze up) happen after I replaced the AC compressor in my 99 Silverado. The compressor was always engaged, even after unplugging the electrical connector to the compressor. Turns out the clutch was pressed on too far (mechanical problem, not electrical) so it was always "on" and would freeze everything up.

Replacement compressor for that one had a vacuum leak and would not hold a vacuum. So returned that compressor to the parts store and they warranted it and fortunately compressor number 3 held vacuum and is still working great today (4 years later). I don't buy no-name cheap parts either. All 3 were from a top name aftermarket HVAC manufacturer so even the big boys have QC problems every now and then.

Good luck!
 

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2002 chevy trailblazer_ls
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Discussion Starter #5
Thank you! When I rev the engine to 1500 RPMs the pressure drops from 45-13 and the clutch begins a 5 second cycle of off and on. To me that’s saying that something is obstructed and it’s pulling a vacuum on the low side. Somethin like an Orifice tube. Would that make sense? Is the low side suppose to go down when rpms go up?
 

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Make sure you have the correct freon charge in the system. The low pressure switch cycles the compressor, turns it on at about 45psi, off around 30.
Also, check the condenser for debris (leaves and such).
I've seen this happen on a properly working system when the outside temperature was high with real high humidity.
What I'm not getting is why the low pressure switch isn't turning the compressor off. That's why I'm suggesting checking for the proper charge.
 

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Discussion Starter #7
Thanks, but again the compressor IS disengaging when the pressure gets below 25. But then kicks back on when it gets above 25 then just starts the process again and drops below 25...
 

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The compressor cycles off because the system cannot maintain the minimum pressure required to keep the low side pressure switch in an electrically closed position and the switch opens up and the clutch disengages.

When the system warms up and the low side pressure increases, the low side pressure switch once again closes and the compressor engages until the low side pressure drops again and the compressor disengages.

A classic sign of low refrigerant charge as NJTB04 mentioned.
 

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2008 chevy trailblazer_lt
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If there were a clog, the high side pressure would skyrocket, especially with any revving of the motor. I'd simply add more refrigerant as a start. If airflow is too low across evaporators, it would be more prone to ice up. So, being sure vents are open and blowing harder would make ice less likely. We used to have a Suburban that needed to be way over charged in order to cool. Not sure what that issue was exactly.

Rob in AZ
 
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