Jump to content

ac compressor relay control question

hgarraway

By hgarraway
in Ask A GM Technician

Recommended Posts

hgarraway New Member

hgarraway

Posted
Posted

2000 2500 5.7, compressor relay stops activating when ac has run for a while. I have tested the relay by swapping, it's good. The pin that supplies 12v to the relay coil (#86 on the relay) always has 12v when ignition is on, the 12v to the contact on relay always has 12v (pin 87). The circuit to compressor clutch is good, It always engages if I jumper pin 87 to it (pin 30). The problem is in the circuit back to the switch from the relay coil (pin 85). This is the "ground" path for the coil. When the truck has been off for a while and the ac is turned on (with relay out) the resistance through pin/socket 85 back to ground is beteen 9 and 25 ohms. If I plug in the relay at this time the comp clutch engages as it should. After it runs a while it will disengage and when I remove it and check the resistance it is at infinity as it would be if switch were turned off or the thermostat sensed freeze and opened. It usually stays in this state until I turn off truck and let it rest. If I had a schematic of this circuit I could probably find it.

 

Have you seen this before or could you make an educated guess as to whether it could be a faulty switch or faulty thermstat? Also, do you know if this system is under computer control or just a good ole switch and mechanical thermostat? Any help would be greatly appreciated. (In an earlier post, to which you responded, I thought the problem was a bad connection in the relay socket but symptoms have gotten more consistent.)

 

Thanks in advance.

 

Hugh

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted

The PCM (engine control computer) controls the A/C compressor clutch relay through pin 85 on the relay. There can be any number of reasons why the PCM would shut off the compressor, but the most common problem we see is a bad low pressure cycling switch. There is no thermostat for the a/c, it determines if the evaporator is potentially too cold by pressure via the low pressure switch.

 

This is the switch that screws into the accumulator. It's a simple set of on/offcontacts that open below a certain pressure (25 psi or so) or close above that pressure. Closed the compressor is on, open it's off... assuming all other parameters the PCM wants to see to allow compressor operation are okay.

hgarraway New Member

hgarraway

Posted
  • Author
Posted

Thanks GM-Tech. That seems to have been the problem. I replaced the low pressure cutoff switch (after testing old pressure switch upon cutoff) and all is cool now.

 

You saved me a lot of work and money as I had incorrectly thought that the pressure switch close to the compressor was the cutoff and had incorrectly ruled out the switch as a possible problem.

 

I really appreciate your assistance.

 

Hugh

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.

  • Forum Statistics

    250.5k
    Total Topics
    2.7m
    Total Posts

  • Member Statistics

    342,972
    Total Members
    8,960
    Most Online
    Whit.the.COB
    Newest Member
    Whit.the.COB
    Joined

  • Who's Online   0 Members, 0 Anonymous, 1,206 Guests (See full list)

    • There are no registered users currently online

  • Latest Articles

  • Posts

    • MikeBMW
      Installing tow mirrors

      By MikeBMW · Posted

      Yep! This should be a pin somewhere as @pgamboa really dives into all of this. Thanks for reposting the link!
    • Grumpy Bear
      TCM Tuning 6L80E

      By Grumpy Bear · Posted

      I did the new thermostat after doing a 'flip' for a year. Runs as cool one way as the other in the summer but in the winter there is a bit of a floor. Yellow with the new 70 C thermostat. My data over the life of the unit to date. Still need more cooler. Anyway that floor helped both winter fuel economy and shift quality in the winter. I found it a good balance.       
    • Grumpy Bear
      Food & Drink

      By Grumpy Bear · Posted

      Evan Williams Bottled in Bond   So pouring the last of a bottle of Evan Williams Black label last night I set off the the state store for a refill only to be denied the by an empty shelf. Rats.    What the did have was a bottle of Evan Williams Bottled in Bond. 100 proof. Hum, a few bucks more but a known distiller. Lets have a swing at it. ( I have before )    Cracked it open and pour a serving over ice. Hum.... more heat, yes. More alcohol naturally. But the barreling not the proof brings the taste and this one came up short.    Oh is was familiar but the heat ran the barrel over like a Peterbilt runs over a squirrel. Could use a heavier toasting of the oak me thinks or perhaps a longer barreling. Who knows. But what I can say is ice or a water back isn't the answer. What kills one kills them both. Sad. It is a pleasant drink....if you have little to compare it to.  
    • Grumpy Bear
      Bridgestone vs Goodyear tires?

      By Grumpy Bear · Posted

      I've had exactly one set of Bridgestone. Lasted five year and 125,000 miles and had I known better 15oK would have been a cake walk. I would have bought a like set had they not tripled in price since the first set. I removed them at said mileage with 5/32" remaining in dead even wear all four. They were fuel efficient, quite, comfortable and predictable.    Early on I got a very good alignment checked at least once a year, sometimes twice; kept the tires properly inflated, balanced and rotated. And I've found that many a tire issue can be avoided with great maintenance practices. Lot's of tire dealers will do rotation and balance (simple) for free for as long as you own the tire they sold you so no excuse not to.    Bead prep, alloy integrity and stem health have a huge impact on inflation stability. There are more mountings done poorly that perfectly. Finding a good tire guy is gold and as important as finding a tire you like.    That 3X price increase now two sets of "other tires' later is starting look like a bargain.      Bad maintenance kills more tires than bad tires. 
    • Grumpy Bear
      Wow I'm in a pickle. (Broke down truck) and a story.

      By Grumpy Bear · Posted

      Atlas, you kill me As aggravating as a project like this can be you have been unable to hide your fascination with learning and using that angst against this beast like a Ninjitsu Master. It amuses you, admit it now.      BTW, love your writing style. 😉       

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...