No A/C

gmtofd

Original poster
Member
Dec 30, 2013
68
Working on a '08 SAAB 9.7, unable to get the a/c compressor to work. First thing I tried was to check all fuses which were all ok. Then attempted to jump the low side pressure switch to get the compressor to work, but with the HVAC control set for A/C there is no voltage at the connection to the switch.

The question is can someone post a HVAC wiring schematic so that it may be determined if there is anything upstream from the low pressure switch that is preventing voltage at the low pressure switch connection? Or is it a common problem that the digital HVAC go bad as my son is suggesting?

Thanks for any help.
 

freddyboy61

Member
Dec 4, 2011
276
Did you check fuses #22 & 30 in the underhood fuse box? Relay #44 is also a suspect. Attached is schematic for 2006 Trailblazer (don't have 2008), should be the same as 2008.
 

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TJBaker57

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Aug 16, 2015
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but with the HVAC control set for A/C there is no voltage at the connection to the switch.

That is because it is not a power circuit like they were in the days of old. Like most everything these days that switch is only a signal input to the HVAC module.
 
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TJBaker57

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Aug 16, 2015
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If you've looked over the schematics you might have noticed that everything is computerized now. The HVAC module asks the PCM to run the compressor. The PCM will only run the compressor if a handful of conditions are OK. Wide open throttle = no compressor, unstable idle = no compressor, an overpressure at the high side pressure sensor = no compressor, and so on.

So what can you test? Check every related fuse and relay. Check refrigerant pressure, verify the A/C clutch works by jumping power between terminals #87 and #30 at the A/C relay (never leave it jumped for long, you will blow off the overpressure device on the high side).

Did it just stop working recently or has it not worked since you got it?
 
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mrrsm

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You've probably already done this ...but for anyone else following the issue... Harbor Freight sells a reasonably priced complete Auto A/C Manifold Gauge Set. Even with the system off...the Gauges would indicate whether or not there is enough R-134A inside the system:


Also... The Low Pressure Switch (Port shown in the attached generic image on the BACK Side of the AC Accumulator) is a Known Failure Point on the GMT-360s and is quite easy to replace as it works by depressing an Internal Schrader Valve. after it screws in and seals up... so performing the R&R does NOT require a Vacuum Pull Down and-or a Re-Fill of the Freon and will not lose much during any Part Swap. It is often the last place that gets this attention and will offer quite a 'cool relief' if this thing turns out to be the Broken Part. Apply just put a little PAG-46 Oil or Blue NYLOG around the Green "O" Ring B4 installing this Thing :>)

 

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6716

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Jul 24, 2012
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To tldr MRRSM's comment: the compressor won't turn on if there's not enough refrigerant.
 

gmtofd

Original poster
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Dec 30, 2013
68
Thanks ever so much for the great responses.

When I'm back with the 9-7X, I'll go thru the fuses again, the relay that which was mentioned. As for checking out the charge, don't know how that can be done if the compressor is not engaged?

It was my wife's ride for the pass couple of years, and when I sold it to my friend, the a/c was working. The wife complained that the a/c was not cool enough, and I thought that my son check the charge but he says he does not remember ever doing that?
 

mrrsm

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You seem unsure about these issues and you're dubious of your Son's abilities ... Well that is Okay... as long as you accurately portray whether or not you feel you have the Skill Set necessary to perform the repairs that may be required as per whatever follows. There are quite a few GMTN Members with an Expert Level of HVAC Skill ready and willing to help and advise you along the way:

In a properly sealed A/C System, there is always System Pressure inside that gradually balances out between the High and Low sides as the refrigerant re-distributes itself. The Manifold Gauges will shows these two coming into rough balance AFTER the Engine has been turned off for a while.

Of course this dynamic changes whenever the Engine Starts and the Magnetic Clutch engages and the Compressor begins to work. The Manifold Gauges will show that the Pressure on the (Blue) Low Side Vapor Return Line side Decreases while the (Red) High Side Liquid Line side Pressure Increases as the Compressor keeps on running. This is Normal A/C functionality on a working system.

However, with the Engine turned OFF, if the entire A/C system is either empty of Refrigerant or fairly close to it, then the Manifold Gauges will STILL show the actual system condition as having Very Low Pressures on BOTH sides. Variable Internal Line Pressures and Temperatures (inside and out) are always intimately involved in all uses of all types of HVAC Refrigerants, but the Compressor in your Saab will NEVER engage if the R-134A has been reduced below 40 PSI or has become completely lost from within the closed A/C system.

So first, you will also need to determine if you have any Refrigerant Leaks that have lead up to this problem, remembering your Wife's earlier advisement of having "Low Cooling" as the most significant indicator. The Best Tool for this part of the investigation is to get an inexpensive Refrigerant Leak Detector available on Amazon:


Assuming that this problem turns out to have a simple in solution (...like a Leaking Seal around a Bad A/C Low Pressure Switch for example) and you manage to isolate and repair any such Leaks that you find... AND the Compressor Engages, then the most direct way to determine what the actual R-134A Charge left inside the system is to literally: Recover the remaining R-134A Charge and then Weigh-In a Brand New Charge of R-134A (in Pounds) according to the amount specified in the Saab Service Manual.

Barring that approach, you would require some level of experience and sophistication with using and interpreting the Auto A/C Manifold Gauges and Tools along with knowing what the Ambient Temperature is once the system is up and running to then figure out if the A/C Charge inside is either Too Low, Correct or even Over-Charged. If you have this Skill Set, you can download and use this Pressure vs. Temperature Chart to help you:
 

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gmtofd

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Dec 30, 2013
68
Great, thanks for the insight, I'll hook up the manifold, and see where it is at with the charge. Pressing in the Schrader valve at the low side, I know that the system is not flat, and will see if its above 40 psi and go from there. Do have access to a leak detector as well as nitrogen.

Thanks again, Tony
 
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mrrsm

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Roger That...

And for anyone else following along.. Check out Rick's Link for this subject matter in all its glorious details and helpful hints on "How to Re-Charge Your Auto A/C System" via the use of the A/C Manifold Gauge Set:

 

gmtofd

Original poster
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Dec 30, 2013
68
Thanks for the info about reading the manifold. Post again next week when I'm again working on the Saab.
 
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gmtofd

Original poster
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Dec 30, 2013
68
Was able to troubleshoot some more on the Saab today and thanks to the suggestion of checking the relay that is in the under the hood fusebox, was able to get the compressor to now engage, its cycling. Hooking up the gauges found that the system 10 lbs on both sides of system while the compress is engaged. Even though the compressor is now cycling, there's only 10# on both sides and air coming from the the system is blowing warm air.
 

TJBaker57

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Aug 16, 2015
2,897
Colorado
Was able to troubleshoot some more on the Saab today and thanks to the suggestion of checking the relay that is in the under the hood fusebox, was able to get the compressor to now engage, its cycling. Hooking up the gauges found that the system 10 lbs on both sides of system while the compress is engaged. Even though the compressor is now cycling, there's only 10# on both sides and air coming from the the system is blowing warm air.
Something significantly wrong there!! At 10 psi the compressor should not be coming on at all!! That's not enough refrigerant. The low pressure switch on the accumulator is NOT jumped out, is it?
 

gmtofd

Original poster
Member
Dec 30, 2013
68
Sorry, forgot to mention that the static reading, before getting able to cycle the compressor, was at 90#s and with the compressor engaged it drops to as mentioned 10#s. Having a problem with the manifold as the low side valve is not working meaning it does not shutting off the r134a that was being used to purge the manifold lines before hook-up to the Saab's fittings.

Once the manifold problem is resolved, will do a retest with the gauges. Suppose it is possible that the static reading of 90#s was actually the reading of the 30# r134a tank due to the above mentioned faulty low side manifold valve.
 

Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
Make sure both valves on the gauge are closed and only open the low side valve if filling the system.
 

c good

Member
Dec 8, 2011
526
It might be time to just do a complete evac, leak test, and recharge. You can take it somewhere to recycle. Then go get a loaner vacuum pump and a set of gauges from the local Autozone or Oreillys. Pull a vacuum on it. Let it sit for 60 minutes or more. If it holds vacuum, you're good to go. Pick up the correct amount of new r134, hook up to the low pressure side and fill away.
 

mrrsm

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The only remaining "Elephant in The Room" is having a Failing Compressor. Once your A/C Manifold Gauge Tool issues are solved... If no Leaks are detected and those running A/C pressures remain well below nominal and prove out that the Compressor is the Bad Guy... Check out THIS Thread next:

 

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