GON Magazine | GON Marketplace

Go Back   Georgia Outdoor News Forum > Woody's Campfire Talk > Everything Motorized - Boats - Trucks - ATV's - etc


Reply
 
Thread Tools Display Modes
  #1  
Old 07-10-2008, 11:44 PM
Al White's Avatar
Al White Al White is offline
 
Join Date: Sep 2004
Location: Bullard, GA
iTrader: (0) Check/Add Feedback
Default low pressure switch question? chevy tahoe

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.
__________________
Proud member of the Southeastern Primitive Skills Association
Reply With Quote
  #2  
Old 07-11-2008, 12:27 AM
bull0ne's Avatar
bull0ne bull0ne is offline
 
Join Date: Feb 2005
Location: Dodge county
iTrader: (1) Check/Add Feedback
Default

Yes, they do go bad quite often.

It's a ''snatch off & plug in'' job you can do yourself that costs about 15 bucks.
__________________
Any fool can criticize, condemn and complain, and most fools do
~ Benjamin Franklin ~
Reply With Quote
  #3  
Old 07-11-2008, 10:39 AM
Al White's Avatar
Al White Al White is offline
 
Join Date: Sep 2004
Location: Bullard, GA
iTrader: (0) Check/Add Feedback
Default

thanks man, i'll replace it and see what happens
__________________
Proud member of the Southeastern Primitive Skills Association
Reply With Quote
  #4  
Old 07-11-2008, 10:42 AM
Slug-Gunner's Avatar
Slug-Gunner Slug-Gunner is offline
 
Join Date: Sep 2004
Location: Augusta, GA
iTrader: (0) Check/Add Feedback
Exclamation "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
__________________
Slug-Gunner - - Keep an "Open Mind" - You'll NEVER STOP LEARNING!

Last edited by Slug-Gunner; 08-22-2008 at 12:11 PM.
Reply With Quote
  #5  
Old 07-11-2008, 11:12 AM
bull0ne's Avatar
bull0ne bull0ne is offline
 
Join Date: Feb 2005
Location: Dodge county
iTrader: (1) Check/Add Feedback
Default

Quote:
Originally Posted by Slug-Gunner View Post
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.
__________________
Any fool can criticize, condemn and complain, and most fools do
~ Benjamin Franklin ~
Reply With Quote
  #6  
Old 07-11-2008, 11:41 AM
Al White's Avatar
Al White Al White is offline
 
Join Date: Sep 2004
Location: Bullard, GA
iTrader: (0) Check/Add Feedback
Default

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.

Quote:
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).
__________________
Proud member of the Southeastern Primitive Skills Association
Reply With Quote
  #7  
Old 07-11-2008, 01:13 PM
Slug-Gunner's Avatar
Slug-Gunner Slug-Gunner is offline
 
Join Date: Sep 2004
Location: Augusta, GA
iTrader: (0) Check/Add Feedback
Arrow Need Wiring Diagram....

Quote:
Originally Posted by Al White View Post
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
__________________
Slug-Gunner - - Keep an "Open Mind" - You'll NEVER STOP LEARNING!

Last edited by Slug-Gunner; 07-12-2008 at 01:22 PM.
Reply With Quote
  #8  
Old 07-11-2008, 02:16 PM
Al White's Avatar
Al White Al White is offline
 
Join Date: Sep 2004
Location: Bullard, GA
iTrader: (0) Check/Add Feedback
Default

Thanks Slug-Gunner, i might just end up taking it somewhere
__________________
Proud member of the Southeastern Primitive Skills Association
Reply With Quote
  #9  
Old 07-11-2008, 11:51 PM
W4DSB's Avatar
W4DSB W4DSB is offline
 
Join Date: Sep 2004
Location: Where ever I am
iTrader: (3) Check/Add Feedback
Default

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
__________________
"After a shooting spree, they always want to take the guns away from the people who didn't do it."
--William S. Burroughs
Reply With Quote
  #10  
Old 07-16-2008, 03:28 PM
Al White's Avatar
Al White Al White is offline
 
Join Date: Sep 2004
Location: Bullard, GA
iTrader: (0) Check/Add Feedback
Default

turned out that the low pressure switch was the culprit thanks for the replies.
__________________
Proud member of the Southeastern Primitive Skills Association
Reply With Quote
  #11  
Old 07-22-2008, 12:09 AM
yanknga yanknga is offline
 
Join Date: Oct 2007
Location: hampton, ga
iTrader: (1) Check/Add Feedback
Default

jump ut pressure switch.
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Forum Jump


All times are GMT -4. The time now is 09:58 AM.


Powered by vBulletin® Version 3.7.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2004 Georgia Outdoor News, Inc.Ad Management by RedTyger