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Help me with this damn AC!

Brian468

Red Skull Member
Joined
May 19, 2020
Member Number
351
Messages
213
Long story short on my 96 Silverado the ac compressor only comes on when I jump the relay, the system was totally jacked when I bought this truck so as of today it has a new control module, new cycle switch, new accumulator, new condenser, new high pressure cutoff switch and new manifold line, the high pressure switch at the manifold is the old one but I jumped it and it still isn`t my issue, system charges up okay and holds pressure but the compressor clutch will only engage when I jump the relay, anybody ever have this issue?

My interweb searchs have mentioned a problem inside the ecm on a few different websites but I have no codes so I can`t rule it out 100%.
 
081FF1CF-25C1-4798-BF2F-97682B7B675C.png
 
Do you have power to the compressor relay?
 
DMG said:
Do you have power to the compressor relay?
Click to expand...
Yeah, if I jump it at the ac relay it works perfect, I jumped all 3 switches (low pressure, high pressure and cycle) individually and simultaneously to see if one was bad and it made no difference, the relay fits a bit loose in the pins so maybe thats my issue but I doubt it, gonna double check for power at the low pressure switch to make sure the new hvac control is working properly but i`m pretty confident that`s not my issue.
 
I would start with the light green wire out of the control module like you mentioned and just see where you lose power. Also, if the relay is loose, a little twist on each prong will ensure connection.
 
Do you have 12V at relay terminal A4?
Do you have continuity relay terminal A4 to VCM C2-9?
Is the VCM receiving the signal from the HVAC control module on C3-25?

If yes to all 3 then you probably have a bad VCM. If no to any of those 3 then that's your problem.
 
[486] said:
hackfuck kyle answer:
both pressure switches in series with compressor coil, to a stupid toggle switch stuck in the dash
no relays no computers no lords no masters
Click to expand...
i had the same thought but was resisting the urge to post it. :laughing:
 
I'd rather fix it right but I'll be air conditioned either way this summer.

Just aggravated at this point because I'm rarely stumped, I'm kinda leaning towards a wire being fucked up somewhere, I have power coming out of the new controller head to the light green wire going to the low pressure switch so I'm ruling out the new head being the issue and I have all the pressure switches jumped so I'm ruling them out as well.
 
does the relay go into a PDC? I don’t remember. If it does, flip it over nad look for corrosion, pushed out wires or broken wires.
 
That sensor has no power going through it, the guy at vato zone says thats the clutch coil cycling switch but I am not positive, both the low pressure switch on the accumulator and the high pressure cutoff switch at the bottom of the compressor have power going in and coming out, pin 85 at the relay that should signal the relay to close and send power to the compressor has no power coming from the ecm with ac switch on.

20210417_140305.jpg
 
Chkn said:
Do you have 12V at relay terminal A4?
Do you have continuity relay terminal A4 to VCM C2-9?
Is the VCM receiving the signal from the HVAC control module on C3-25?

If yes to all 3 then you probably have a bad VCM. If no to any of those 3 then that's your problem.
Click to expand...
I have power at all 3 of those spots with the key on
 
If that’s the case then likely your VCM is bad and isn’t grounding the coil side of the relay.
 
Does anybody know where that switch in the picture gets its power from? is that the cycling switch? I have no power going to it.

I tried swapping pcm`s today and it still didn`t fix the issue, I had one out of a same model year same optioned suburban so I`m still leaning towards fucked up wire somewhere.
 
that's on the high pressure line, so look at your diagram right there and see it gets power from the low pressure switch, follow it one step further and it's getting it from the "a/c control module" whatever the fuck that is
 
[486] said:
that's on the high pressure line, so look at your diagram right there and see it gets power from the low pressure switch, follow it one step further and it's getting it from the "a/c control module" whatever the fuck that is
Click to expand...
I thought the round sensor on the back of the compressor held in with a snap ring was the high pressure switch getting power from the low pressure switch, dammit!
 
I'm not really a GM guy, could that be a thermal switch like the hondas got?

check the low pressure line (the big one) for more switches like the one that's jammed into the HP line. I remember seeing a lot of them on the big can dingus off the evaporator. Wanna call it the reciever, but maybe that's the accumulator? Either way. That big aluminum can thingie up by the firewall.

ETA: oh, and the "cycling switch" is the one on the low pressure line.
they open the clutch before the low side gets to a low enough pressure for the evaporator to frost over
 
High pressure cutout is the switch on the back of the compressor. Low pressure is on the accumulator on the firewall.
 
that big can is the drier/accumulator, it only has one switch the low pressure one that power comes out of the hvac module to first, then apparently goes to the high pressure switch which I thought was the one on the bottom of the compressor snap ringed in place I was checking for power at, especially since the wires are supposedly the right colors coming from the lp switch, think i`m just gonna strip the weatherproofing off the harness tomorrow and see where everything is originating/terminating.
 
That switch I posted isn`t the high pressure switch it`s the one on the back of the compressor like I originally thought, nobody seems to know what the fuck this thing does or is supposed to do, all I know is I don`t have power going to it at all and the other side of it seems like a ground.

23 years of being a mechanic and almost stumped by my own shitbox
 
Brian468 said:
That switch I posted isn`t the high pressure switch it`s the one on the back of the compressor like I originally thought, nobody seems to know what the fuck this thing does or is supposed to do, all I know is I don`t have power going to it at all and the other side of it seems like a ground.

23 years of being a mechanic and almost stumped by my own shitbox
Click to expand...
The switch on the high pressure line? It seems to be the only one with dark green and black wires which matches the high pressure switch.
 
DMG said:
The switch on the high pressure line? It seems to be the only one with dark green and black wires which matches the high pressure switch.
Click to expand...
yes the one on the line, the dark green wire from the lp switch goes to high pressure switch on the back of the compressor then the dark green with white stripe wire coming out of that sensr goes to the vcm, that all checks out, the sensor on the high pressure line with the green and black wire has no power going to it and I don`t know where it`s supposed to be getting power from i don`t see it on that schematic
 
that is called the cycling switch
 
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