# low pressure switch question? chevy tahoe



## Al White

I have a 2000 chevy tahoe, 4.8l v8.  I noticed about two weeks ago that the a/c wasn't cooling like it had been and figured that it was low on freon.  Today it quit cooling all together.  The clutch on the compressor is not 'deploying' when i push the a/c button.  I checked the freon, and the range is normal.  After messing with it for awhile, I found out that there is no fire getting to the low pressure switch.  All of the fuses in the panels appear to be fine.  So... my question is, do low pressure switches go bad often, or is there a better possibility that the problem is in the line going to the switch?  Any suggestions would be appreciated.


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## bull0ne

Yes, they do go bad quite often.

It's a ''snatch off & plug in'' job you can do yourself that costs about 15 bucks.


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## Al White

thanks man, i'll replace it and see what happens


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## Slug-Gunner

*"Low Pressure Switch" Operation*

On most A/C systems the "Low Pressure Switch" functions simply _'as a ground source'_ for the a/c clutch assy. If you have freon pressure in the system (they're usually rated at 15 psi for the 'low limit' - may be higher for R-134) then the low pressure switch will be grounded and no voltage will be present on the wire going to it.... if you read voltage, then the switch is open. The 'low-pressure switch' can also be checked with an ohm meter by unplugging the connector to it and checked for continuity thru the switch to ground (with at least 15 lbs psi in the a/c system).

If the 'low-pressure switch' test GOOD, then your problem is then probably in the wiring going to the a/c compressor. Check for 12 vdc going to the twin-lead connector plug going to the compressor with the a/c turned ON _with the connector unplugged_. If 12 vdc is present, then everything is GOOD in the wiring back to the a/c control switch.... if not, you have a wiring problem between the a/c compressor plug and a/c control panel switch (or fuse panel). Try to get a wiring diagram (found in some Haynes manuals) for your vehicle to see what components are used in your a/c wiring system (be sure you're using the correct diagram for your year model vehicle). 

CAUTION:
On some a/c systems the 'low-pressure switch' is screwed into the compressor body or near the receiver-drier and removing it will release/loose all the freon from the system.      Some late model vehicles use a Schraeder valve to isolate the switch from the freon (will not lose freon).... be sure which one you have BEFORE REMOVING  the 'low-pressure switch' from the compressor/receiver-drier. If you have the first type you will have to use a 'freon recovery system' before removing the switch and then 'purge/vacuum' the system again before recharging it.


Some vehicles may also have an a/c cut-out switch incorporated into the power steering system which cuts off the a/c system momentarily when the power steering is being used (under pressure).... this is mainly used on low powered cars (4 cylinder).... these same vehicles may also have a 'manifold pressure sensor' to cut off the a/c when under 'heavy load' (like when going up a steep grade). A lot of these functions are now controlled by the 'body/climate-control computor' on late model vehicles.

Slug-Gunner


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## bull0ne

Slug-Gunner said:


> On most A/C systems the "Low Pressure Switch" functions simply _'as a ground source'_ for the a/c clutch assy. If you have freon pressure in the system (they're usually rated at 15 psi for the 'low limit' - may be higher for R-134) then the low pressure switch will be grounded and no voltage will be present on the wire going to it.... if you read voltage, then the switch is open. The 'low-pressure switch' can also be checked with an ohm meter by unplugging the connector to it and checked for continuity thru the switch to ground (with at least 15 lbs psi in the a/c system).
> 
> If the 'low-pressure switch' test GOOD, then your problem is then probably in the wiring going to the a/c compressor. Check for 12 vdc going to the twin-lead connector plug going to the compressor with the a/c turned ON _with the connector unplugged_. If 12 vdc is present, then everything is GOOD in the wiring back to the a/c control switch.... if not, you have a wiring problem between the a/c compressor plug and a/c control panel switch (or fuse panel). Try to get a wiring diagram (found in some Haynes manuals) for your vehicle to see what components are used in your a/c wiring system (be sure you're using the correct diagram for your year model vehicle).
> 
> CAUTION:
> On some a/c systems the 'low-pressure switch' is screwed into the compressor body and removing it will release/loose all the freon from the system.      Some late model vehicles use a diaphram (sp?) to isolate the switch from the freon.... be sure which one you have BEFORE REMOVING  the 'low-pressure switch' from the compressor. If you have the first type you will have to use a 'freon recovery system' before removing the switch and then 'purge/vacuum' the system again before recharging it.
> 
> 
> Some vehicles may also have an a/c cut-out switch incorporated into the power steering system which cuts off the a/c system momentarily when the power steering is being used (under pressure).... this is mainly used on low powered cars (4 cylinder).... these same vehicles may also have a 'manifold pressure sensor' to cut off the a/c when under 'heavy load' (like when going up a steep grade). A lot of these functions are now controlled by the 'body/climate-control computor' on late model vehicles.
> 
> Slug-Gunner



Al...........better listen to Slug-Gunner rather than me. He's a mechanic and I'm a  trial & error parts changer.


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## Al White

Thanks for the info.  I unplugged the plug going into the switch and it did not show anything.  However, i did not use a meter but used a tool (can't think of the name) - it looks like a scredriver with a light bulb and a ground wire - to check the line (plug).  That's where i'm not seeing any fire - but you are saying that this is normal?  I made sure that I had a good ground, but it would never light up.  The switch that I am referring to is not on the compressor itself, but on the condeser/drier? - where the freon lines are.



> On most A/C systems the "Low Pressure Switch" functions simply 'as a ground source' for the a/c clutch assy. If you have freon pressure in the system (they're usually rated at 15 psi for the 'low limit' - may be higher for R-134) then the low pressure switch will be grounded and no voltage will be present on the wire going to it.... if you read voltage, then the switch is open. The 'low-pressure switch' can also be checked with an ohm meter by unplugging the connector to it and checked for continuity thru the switch to ground (with at least 15 lbs psi in the a/c system).


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## Slug-Gunner

*Need Wiring Diagram....*



Al White said:


> Thanks for the info.  I unplugged the plug going into the switch and it did not show anything.  However, i did not use a meter but used a tool (can't think of the name) - it looks like a scredriver with a light bulb and a ground wire - to check the line (plug).  That's where i'm not seeing any fire - but you are saying that this is normal?  I made sure that I had a good ground, but it would never light up.  The switch that I am referring to is not on the compressor itself, but on the condeser/drier? - where the freon lines are.




Al White, 

I'd need to see a wiring diagram for your vehicle to be 100 % sure of how the 'low-pressure switch' is incorporated into the a/c system. It can be located anywhere in the a/c system that is _under freon pressure_. The switch is then CLOSED and will supply a GROUND to the a/c control system. Its purpose is to protect the compressor from being cycled with insufficient freon pressure AND LUBRICANT in the system, which could damage the compressor and contaminate the a/c system with metal particles (major expense to repair a/c system). If your vehicle has a climate control computor system, then your body control computor probably controls all a/c functions (which makes it more complicated to evaluate). 

_IF YOU'RE SURE THE SYSTEM HAS AN ADEQUATE FREON CHARGE_, then you can momentarily bypass the 'low pressure switch' by grounding the connector going to it. If the connector has only ONE (1) wire going to it, GROUND THE CONNECTOR TO A GOOD METAL GROUND (the ground is INTERNAL by the switch itself when under pressure) .... if it has TWO (2) wires on the connector, it is getting its ground somewhere else and you can put a jumper across the connector lead terminals to TEMPORARILY bypass the 'low-pressure switch' to see if the a/c then works. DO NOT OPERATE THE A/C SYSTEM with this switch bypassed except to test the a/c system. If the a/c system works with the 'low-pressure switch' bypassed, then IT IS THE PROBLEM and needs replacement.

NOTE:
I've been retired for over 10 years now and haven't actually worked on many different late model a/c systems. Many things have changed since going to "Body/Climate-Control Computer" systems and late model vehicle systems can be much more difficult and complicated to troubleshoot without the proper diagnostic equipment (ScanTool, etc.), wiring diagrams and troubleshooting charts/tree. Same 'basic' a/c operating principles apply but they make it more difficult for the DIYer to troubleshoot and repair their own vehicles. 

Maybe someone who owns/works on a similar vehicle to yours can provide more accurate input.

Slug-Gunner


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## Al White

Thanks Slug-Gunner, i might just end up taking it somewhere


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## W4DSB

unplug the plug from the low pressure switch and use a short section of wire to jump the pins in the connector. if the compressor starts you have found your problem. The pressure switch on the chevy just opens or shorts those same 2 wires.
also the pressure switch on my 99 silverado can be changed without loosing any freon. there is a schrader valve behind it that seals up when you remove the switch
hope this helps


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## Al White

turned out that the low pressure switch was the culprit   thanks for the replies.


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## yanknga

jump ut pressure switch.


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