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2002 chevy trailblazer_ls
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31 Posts
Discussion Starter · #1 ·
Sorry if this is a repost but I have done some searching and can’t find any info on exactly what I am looking for. During my searches I found this and this thread that talk about issues related to the HVAC system and the transfer case vacuum switch. Apparently Consumer Reports found it to be an issue and a couple of people here have posted about it, so maybe my search function skills just suck.

Anyway, I have checked the charge on the A/C system and it is fine (I work in the HVAC industry, am EPA certified and know how to check a refrigerant charge). The first symptom I noticed was my A/C is “fading” in and out. Sometimes nice and cold and sometimes not so much. This is while driving at highway speeds, so from what I can tell, the Sticky about poor A/C at an idle doesn’t apply. One of the things I found when checking things out is the compressor is running regardless of the position of the push button switch but it will turn off when I shut the fan completely off. I checked the compressor rely as neelskit recommended and even after it tested Ok I replaced it anyway (it was a cheap attempt and weird things can happened when relys weld themselves together), no luck. I do know the compressor shouldn't run all the time.

Another thing that is leading me to think the transfer case switch could be the culprit is; a few weeks ago (prior to having and A/C issues when it wasn’t as hot out) I noticed when I first started my truck the 2wd/4wd switch would blink for a few seconds. It has since stopped but it is connected to the transfer case (I would assume), so who knows. So, does anyone have any experience with the transfer case switch failing and symptoms leading up to failure? Could this be my issue? Is there a way to test it? I don’t want to change out any more parts without some type of reasoning or testing.

BTW – The air blowing out the vents seems to be good and I don’t hear any “clicking” so I am assuming the actuators that have been notoriously lame are not a problem. I could be wrong on that too though.

And oh yeah, this is an 02' LS.

Thank in advance for any input or help.
 

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2009 chevy
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6,431 Posts
Ok, before we start, you have 2 separate issues. The HVAC and the 4x4 transfer case have nothing in common.

Yep, I know in one of your selected threads, I posted something, that I found using a google search; but I also posted that it did not make any sense to me.

It's hot, so lets get the ac working first.

A couple of questions:

You said that regardless of the position of the ac switch, the compressor is running. How do you know that?

Do you have the recirculation button operated?

Or the switch in the defrost position?

(both of these positions will engage the ac clutch)

Just one hint before I drop off, you posted a long paragraph about your vehicle and at the end (as an after thought), you said "And oh yeah, this is an 02' LS."

This saved you from me skipping over the thread.

Please post the vehicle information under User CP, so that it shows in the upper right hand corner of your posts. I, and a number of others, are tired of guessing about vehicle information.
 

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2004 gmc
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26,181 Posts
I'm on a trip and don't have time anyway to repost the theory of operation of the 4WD system anyway, but suffice it to say there are no "vacuum actuators", they're both electric, and I've posted how to diagnose them by listening to the noises they make. If you haven't been exercising the switch, you need to run it from end to end a few dozen times and see if it gets better.
 

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2002 chevy trailblazer_ls
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31 Posts
Discussion Starter · #4 ·
Ok, before we start, you have 2 separate issues. The HVAC and the 4x4 transfer case have nothing in common.

Yep, I know in one of your selected threads, I posted something, that I found using a google search; but I also posted that it did not make any sense to me.
Good to hear, I was starting to think I was going nuts.


A couple of questions:

You said that regardless of the position of the ac switch, the compressor is running. How do you know that?
When I hook up the gauges to check the charge I can see that it is running.

Do you have the recirculation button operated?

Or the switch in the defrost position?

(both of these positions will engage the ac clutch)
Jackpot, that is why the A/C is running. I had it on recirc. due to the inside of the truck being hotter than the outside air temp and I was trying to put largest load on it when checking the charge. :thumbsup:

Just one hint before I drop off, you posted a long paragraph about your vehicle and at the end (as an after thought), you said "And oh yeah, this is an 02' LS."
Will be done shortly.


So, any idea why it is "fading" in and out after running for long periods of time at highway speed? I obviously have no way to check the charge in this condition so my only guess is; over time it could be cycling off the low pressure switch? I guess given my access to a recovery machine a vacuum pump I’m going to try and recharge it by weight. I’ll post the results of the recharge. Thanks again for eliminating some confusion.

and I've posted how to diagnose them by listening to the noises they make. If you haven't been exercising the switch, you need to run it from end to end a few dozen times and see if it gets better.
Thanks, I’ll search for your post. The switch was blinking a couple of days after the use of the 4wd, but again, it has stopped now. Fortunately they do get exercised regularly.
 

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2002 chevy trailblazer_lt_ext
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365 Posts
...I found this and this thread that talk about issues related to the HVAC system and the transfer case vacuum switch...
This was true on the older "S-10 Blazers". The front differential was locked by a vacuum actuator that sat under the battery. The HVAC mode doors on these older trucks were also vacuum operated. Like Roadie and RayVoy said, the front differential is locked electronically by a solenoid and the HVAC mode doors are operated by servo motors on Trailvoys- no relation between the two systems.

As far as your A/C fading in and out- have you checked the low side cycling switch? It is located on the accumulator and it can be changed without discharging the system. (There is a schrader valve behind the switch.) It is an inexpensive part and is common to failure on GM vehicles.

As far as your transfer case switch issue, it could be the switch, the TCM, (Transfer Case Control Module) or the Transfer Case Encoder Motor. Most likely it is the switch- the TCM or Encoder Motor would have most likley lit up the "Service 4X4" Light on the dash.
 
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