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Clutch always engaged even when ACC is off

Serge PETIT on Mon August 09, 2010 11:19 AM User is offlineView users profile

Year: 2000
Make: Chevy
Model: Blazer
Engine Size: 4.3l
Refrigerant Type: R134A
Country of Origin: France

A French guy need your help................
When engine running the compressor clutch stay engaged even if the AC switch is off on the cab ACC module is off the temp of the air is cold as normal with AC engaged..........
Thinking that it's an electrical issue, I have checked :
1) low pressure sensor disconnected ..........Clutch engaged and compressor running
2) high pressure sensor disconnected...........Clutch engaged and compressor running
3)- HVAC relay in the engine compartment fuse case pulled out ........clutch disconnected and compressor off
4) Connector on clutch disconnected..........clutch disconnected
5) Clutch coil disconnected ......................clutch not engaged and compressor off
6) Automatic Climate Control module on the dash:
- AC switch off (no light) clutch engaged and compressor on
- AC switch on (light on) clutch engaged and compressor on
Due to the cold air , I think we may consider that the compressor and clutch are functioning right .

All fuses for A/C are OK .
I have checked the automatic ACC control module connector......nothing special or disconnected

When I start the engine the drier il covered with frost in 1 or 2 minutes ..........?

At the moment it's not very hot in France (around 75 °F) and I have no long travel planed so I have , to protect the compressor disconnected the coil of the clutch and will reconnect if necessary .

Today , With the owner manual in hand I have test all the positions of the automatic ACC control module .
I think if I well remember that normally when button on the front defrost position , the compressor is on and the AC light on AC switch is on to .
Am I right ?
In my case the light on AC switch don't come on when on front defrost position.

When turning the button back on the auto position the switch light come on and all HVAC flaps are moving back to their normal position (same if I place the button on an other position .

So I can say that the HVAC flaps are correctly functioning independently of the electrical management of the compressor I am wondering if my failure is not here in the ACC control module ?
Meaning that the defrost electrical position is , on an "electronic point of view" , stuck which may explain that the clutch is engage continuously ?

Will a TECH2 reader be able to check that point and give a code ?

Thanks in advance for your support because now there is no more importer for the US Chevrolet car (only the Daewo ones) in France ..

Edited: Mon August 09, 2010 at 11:24 AM by Serge PETIT

mk378 on Mon August 09, 2010 11:42 AM User is offline

Most likely the relay is stuck (contacts have welded themselves together). Pull the relay and check for continuity through the contacts. With the relay removed it is of course de-energized, so the contacts should be open. If it is stuck closed, replace it.

Edited: Mon August 09, 2010 at 11:43 AM by mk378

Serge PETIT on Mon August 09, 2010 4:51 PM User is offlineView users profile

I have swapped the relay with the horn relay which is the same and the clutch stayed engaged ...............
I have made a phone call with the guy in charge of maintaining my Blazer and considering all tests that have been made he has suggested that I disconnect the ACC control module and check if the clutch stay engaged .
He said that if clutch is engaged ,it's a short and if clutch disengaged the module is faulty because giving a bad order to it and in such a case I would have to change it .
What is your opinion ?

mk378 on Mon August 09, 2010 6:18 PM User is offline

If the control panel were keeping it on, it would still cycle normally on the pressure switch.

Does the clutch stay pulled in all the time--
even with the key off?
with the key on but engine not started?
or only after starting the engine?

Would have to pull a diagram but I think the PCM controls the compressor relay.

Serge PETIT on Mon August 09, 2010 6:26 PM User is offlineView users profile

The clutch is engaged only when the engine is running .
Concerning the ACCcontrol module , the "bad order may comme from the front defrost position failing because when on that position the AC is running but the light on the AC switch is not lighted ???? Its the same as when the AC switch is off and that the clutch stay engaged .

bohica2xo on Mon August 09, 2010 7:32 PM User is offline

Pull the connector off of the high pressure switch on the rear of the compressor.

If the compressor stays on, the fault is with the PCM, or the wiring at the relay socket.

If the compressor stops, the command is coming from the ATC panel.

You can use a voltmeter connected to the Light Blue wire in the connector to determine when the panel is requesting compressor operation. There will be +volts on the LtBlu wire when the system is running the compressor.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Serge PETIT on Tue August 10, 2010 2:08 PM User is offlineView users profile

Here are the last bad news !
The clutch stay engaged with the dash ACC control module disconnected .................

I Have checked with the voltmeter on ohm position between the 86 socket position of the AC relay under the hood and the - at battery and the needle of the ohmmeter moved over limit ..................

Is that checking meaning that it's coming from what is called on my diagram the "AC computer relay controller (PCM) ?
In such a case , if I find a TECH2 for GM cars (very difficult in France ) , I thing that it will give a code confirming that point ?
I have checked between 86 and 30 pins on the relay and no deviation on ohmmeter so the relay is definitely not stuck;I
havecheckeddatttheeclutchhcoill connector andwhenn ignitioniss onengineenottrunningg and ACswitchh off I have 12vattthee connector .
I havedisconnecteddthee connector onrearrofftheecompressorr and the clutch stay engaged
Any opinion ?

bohica2xo on Tue August 10, 2010 2:30 PM User is offline

If the compressor kept running with the high pressure switch connector off, the fault is either the PCM, or a wiring fault between the PCM & the compressor clutch relay.

The ATC signal for A/C request runs through the HPCO, to the PCM. If the clutch stayed engaged, the wiring between the ATC & the PCM is not at fault, and the ATC is not causing this problem.

At this point you need either a code scanner, or a subscription to Mitchell for the factory wiring diagram. You will need to work backwards from the relay to see if the signal is actually coming from the PCM, or from a short someplace.

Does the system cycle normally on the low pressure switch when you are actually running the A/C?


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Tue August 10, 2010 3:01 PM User is offline

More than likely the hub is not engaged but crept closed due to what I consider the infamous interference fit. Need a special puller/pusher tool to reset the gap. Well, just the puller part if careful to open up the gap to about 0.5 mm. This can be verified with the ignition off and trying to hand turn the hub and also checking the gap with a feeler gauge.

If you suspect electrical problems, just pull the plug off the compressor coil. With the hub, can either creep in or out, haven't figured that out yet, but normally when you set it again, it stays that way due to the rust on the shaft. This time, rust is good.

mk378 on Tue August 10, 2010 3:18 PM User is offline

It only pulls in after starting the engine-- that means the fault is in the input circuit to the PCM. The PCM thinks that A/C has been commanded on all the time. Once the engine comes up to speed, it pulls the clutch in. Either the wiring between the pressure switch and PCM is shorted, or the PCM itself is defective. Shorted wiring is rare, but it's quite possible someone didn't fix the real problem, they hot-wired it to make the A/C work long enough to get the truck sold.

If there were a blown output driver or short in the wiring after the PCM, the relay and clutch would pull in immediately upon turning the key on.

Serge PETIT on Fri August 13, 2010 5:22 PM User is offlineView users profile

Thanks to all for these strong support but it is out of my competencies to check all that so I have taken the décision to let the car in hands of a specialist in AC with good reputation .
Concerning what NickD explain , when running the engine with unplugged clutch connector The front disc of the clutch don't turn and engaged as soon I plug it again ; It's more like looking as a constant 12 volts applied to the clutch .
The gap is of about 0.5 mm . With engine off , I have tried to hand turn the front plate of the clutch but after 1/8 of a turn I have had the feeling to be stopped by something like a tooth
pour mk378 , there has never been any repair on the AC so no possible hot wiring to have been installed .
2 questions :
1°) what happen if the clutch diode fails ?
2°) If the PCM give a constant order of functioning to the compressor , is it a question of faulty component or of bug in the program ?

Edited: Fri August 13, 2010 at 5:46 PM by Serge PETIT

Tennessee_Timmy on Fri August 13, 2010 9:14 PM User is offlineView users profile

Originally posted by: Serge PETIT

... 1°) what happen if the clutch diode fails ?

Depends. If it fails shorted, a fuse is going to blow somewhere. If it fails open, the voltage spike that occurs each time the clutch disengages may be messing other stuff up.

Good Luck 2 ya,

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