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2002 chevy trailblazer_ltz
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Discussion Starter · #1 ·
Hi all,
hoping to get some suggestions on what to check for my A/C system. This is long but this helps me lay out all the details. Really scratching my head here.


Problem: A/C compressor turns on for approx. 15s after engine start, then disengages. While charging system, compressor shuts of entirely before system is fullly charged.


Background: Last week I broke down and bought a vacuum pump and manifold gauge set. I properly recharged the system with 30oz of refrigerant, and the A/C worked great. Then I noticed compressor clutch wasnt always engaging, it would get hung up and needed to be poked to engage.

So I decided to replace the compressor, orifice tube, accumulator, and decided to throw a new condenser in there while I was at it. When I had the lines off I ran some flush solvent through the two main lines that could easily be removed (low pressure line from accumulator to compressor, high pressure line from condenser up to orifice tube. Cleaned out with shop air until oil droplets stopped misting out of lines. Replaced gaskets and buttoned everything back up. I did not add more oil to the compressor. There may have been some oil in the evaporator as I did not flush that out.

Applied fresh vacuum to lines then started to recharge. During recharge, while still well below 30oz refrigerant, the compressor clutch engages and disengages every few seconds. I could hear a click in the low side pressure switch each time the compressor would turn on and then back off. I believe it was click on and off at around 55 psi and then again at 25-30psi. As I neared the full 30 oz of refrigerant, the compressor shuts off entirely.

With a full charge, while jumping the relay to run the compressor, the high side pressure was ~225psi and the low side was ~55psi (~85*, humid). Its almost like I’ve overcharged it, but I’m certain I haven’t. I was also careful to vacuum air out of the lines before I attached the next can of refrigerant, to avoid introducing air into the system. Either way the cooling performance is pretty poor when the compressor is jumped, not at the same cooling level that I had achieved last week.



Couple things come to mind:
1) I might have damaged the high side pressure sensor during cleaning, and it’s reporting a overly high pressure - but why would cooling be subpar with compressor forced to run? So I don't think this is happening.

2) I think the clicking in the low side switch during charging was kind of unusual. If it’s a normally open switch, I would expect it to stay closed but I guess I don’t know the pressure threshold to close the switch. It could be working perfectly fine. I’ll have to go and check continuity across the switch here in a little bit with the low side at 55 psi. I suspect the switch will be closed. Even with the compressor jumped, the low side reading is too high. I think this is an indicator of another problem.

3) Perhaps the pressure the high side sensor is seeing is greater than what’s measured at the high side port, indicating a constriction between compressor and the port (In condenser or line to condenser from compressor). I am going to try and get Torque up and running to read the A/C high side PID. Based on the poor system cooling performance, I think something else is affecting the system and the sensor is not just reading a falsely high value.


Not really sure where to go from here. I don’t see anything yet that justifies swinging more parts, but I’d like to see what others think.
 

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2004 chevy trailblazer_ls
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571 Posts
I had a bad wire on the high pressure switch drive me nuts, intermittent connection and took me a while to find. Watch the scan tool and make sure it's within range.
Check the distance on the compressor clutch, if it's too great it will give you problems as you described.
I also had problems getting the proper amount of charge into the system, wound up jumping the low pressure switch (with the guages hooked up) until it all went in. Found out later there may have been a hose with a 'ice' restriction. Don't know why it happened.
 

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2002 chevy trailblazer_ltz
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Discussion Starter · #3 ·
What data monitor do you use?

I think the compressor clutch is okay. When I jump the relay it engages fine. This was the issue I had on the compressor I just pulled off though.


Barring an issue with the high side pressure sensor, I’m leaning away from an electrical issue. Everything seems to work at a partial charge/lower pressure. I even start to get some cooling effect. It’s when I complete the charge that the system shuts down.

Hopefully my OBD2 reader shows up today, I’ll try to monitor that PID.

I also was thinking a hose might have gotten constricted/damaged, so maybe I’ll try swinging the hoses in the near future.
 

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Premium Member
'05 Chevy TB EXT
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5,156 Posts
You are SERIOUSLY overthinking this situation .... stop thinking Zebras --- OK?

The Low Pressure switch won't let the compressor run at all unless there's sufficient gas in the lines --- it's a pressure thing.

Cycling after charging usually indicates a low charge and needs more gas. Using gauges won't work and if you believe that those Freon cans hold 16 ounces -- you're wrong --- they hold 12 ounces.

To charge an empty system easier - and correctly --- jumper the Low Pressure switch with a bent paper clip to make the compressor run so you can fill the system easier and in seriously shorter time too.

When the charge is correct, the compressor will have an overall temperature of between 140°F to 160°F --- hot enough that you don't want to touch it for too long, but it won't burn the meat off your hand.

If the compressor is cool - (below 140°F) add more gas.​
If the compressor will fry your hand (above 160°F) -- it's overfull.​
The compressor tells all!​

It's really THAT simple.

AND -----> Oil will never "sit in the lines" if the compressor is running and it has a mostly-full charge.
The oil will move, believe me. <more Zebra-thinking>
 

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2002 chevy trailblazer_ltz
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Discussion Starter · #7 ·
Well that’s what I was hoping to hear - overthinking it.

However, I believe the low pressure switch is closed and isn’t the culprit. The system cycles the compressor on at partial fill, which leads me to think the ECM sees that the low side switch is actually closed (rules out wiring/sensor). The cycling makes sense - the system was only partially charged at that time. After completing the charge, the compressor just stays off. So one of the switches is telling the system the pressure is out of range. I also jumped the low switch, which didn’t result in the compressor engaging at full charge - this makes me lean towards something on the high pressure side of the circuit.

I was measuring the weight of the cans during the fill process, and after 30oz of refrigerant had been transferred, considered the system charged. This process resulted in excellent cooling ~2 weeks ago, so I’m pretty sure the right amount of refrigerant was transferred.

Everything worked up until I changed parts, which makes me think it’s related to something I didn’t do right during the compressor swap.
 

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'05 Chevy TB EXT
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5,156 Posts
You also said that you "touched" the clutch and it engaged. Did you check that 12V power was at the connex AT the clutch --- not any of the sensors or switches and at that time?

The clutch air gap may be too large for the electromagnet to draw the clutch friction material tight.... or at all.
 

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'05 Chevy TB EXT
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5,156 Posts
I’m really not following. I think I’m good on checking 12V at the terminal. The clutch engages after engine crank and when I jump the relay.
Do you even remember what you said before?

Then I noticed compressor clutch wasnt always engaging, it would get hung up and needed to be poked to engage.
Poking a clutch sure sounds like you had to whack it to make it work --- right?

I won't tolerate lying.
 

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2002 chevy trailblazer_ltz
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Discussion Starter · #13 ·
The clutch has been replaced and the new one can be made to engage at will. The electromagnet generates sufficient force to draw in the clutch. That is not what is preventing the controller from commanding the compressor to engage.
 

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'05 Chevy TB EXT
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5,156 Posts
If you defy the logic tree, you'll run in circles. Start from where the problem manefests itself and go backwards.

Some questions only require a Yes or No answer.

But you need to answer the questions as they come so people can form a logical deduction to your problem. You are there .... we are not.
 

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'05 Chevy TB EXT
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5,156 Posts
I also had problems getting the proper amount of charge into the system, wound up jumping the low pressure switch (with the guages hooked up) until it all went in. Found out later there may have been a hose with a 'ice' restriction. Don't know why it happened.
If you have "ice" forming inside a hose --- you have serious problems.

Who would offer such a diagnosis?
 

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2004 chevy trailblazer_ls
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If you have "ice" forming inside a hose --- you have serious problems.

Who would offer such a diagnosis?

I was skeptical at first, too, but the person sent me a video explaining the circumstances /conditions that would cause it WHILE CHARGING, and it made sense. Don't remember the details, seem to recall it happened at the orifice tube. This happened about 2-3 years ago.
I did hundreds of A/C repairs while working at dealerships, gas stations, etc., and really don't recall any time I had that much trouble getting the charge in a car.
After getting out of the buisness (mid 90's) and buying this TB, there's a learning curve on some things between the 90's and 2004.
 

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'05 Chevy TB EXT
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5,156 Posts
Well that’s what I was hoping to hear - overthinking it.

However, I believe the low pressure switch is closed and isn’t the culprit.

Everything worked up until I changed parts, which makes me think it’s related to something I didn’t do right during the compressor swap.
How do you know the LPS is actually closed? "Cycling" can indeed be the result of a low charge and then that LPS will drop the daisychain and the compressor goes off-on-off-on, etc.
 
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