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AC compressor won't engage

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Hi folks: I'm the original owner of a '96 T-100 SR5 4WD Extra Cab. About 3 months ago, it started to stall at idle when the AC switch was depressed. The simple solution was to not run the AC. We continued to drive it with no problems (and no AC). I've done other maintenance recently to pass state inspection (brake rotors, control arms, 1 CV axle, power steering pump and lines) and am now focused back on the AC problem. Since it had a slow refrigerant leak over the last few years, I decided to replace all the major components (evaporator, pressure switch, expansion valve, receiver/drier, compressor, and hoses). I left the condenser and the high pressure line running between the compressor and the drier. Install seemed to go smoothly and it holds a vacuum. I was able to charge the system with a single 12oz can of R134 and my static pressure is 60 on both the low and high sides. But, I can't get the system to take any more refrigerant and my compressor doesn't engage. I'm stumped and am seeking advice from the experts here. I'm thinking electrical but need some guidance. I've replaced the AC switch as my resistance readings didn't match the specs in the FSM but the new switch's readings didn't either so I think that was a red herring.

Thanks in advance for any help.
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Discussion Starter · #3 ·
Thanks echo. Sorry, should have mentioned that: yes, I've checked the 10A fuses under the hood and in the inside fuse box. Both look good and measure accurately. But, I haven't checked any relays. I'm not aware of anything else that is not working so I figured relays were good.
 

· just a nobody
Echo
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Check and made sure the relay is getting signal to turn on and the output is getting power to engage the compressor clutch!
Also, there might be more than one pressure switch on some system, so check for that as well!
 

· just a nobody
Echo
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· just a nobody
Echo
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Check for power to the clutch! If that is getting power then you may have a bad clutch or poor ground connection!
If the clutch is not getting power, check the dual pressure switch in the heater box, if that is open, the compressor clutch won't work either!
 

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Discussion Starter · #10 ·
Thanks again echo. I'll check the power tonight. I was also thinking ground--any idea where the ground connection is for the clutch? I replaced the dual pressure switch and will double check the connections there but since this was a problem before I swapped the parts, I'm thinking the new switch is ok.
 

· just a nobody
Echo
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Thanks again echo. I'll check the power tonight. I was also thinking ground--any idea where the ground connection is for the clutch? I replaced the dual pressure switch and will double check the connections there but since this was a problem before I swapped the parts, I'm thinking the new switch is ok.
Can said for sure where the ground is, sometimes they just grounded to the compressor itself. If you have a two wire connector at the compressor, just use a test light to see if you are getting a good ground at the other wire; the black/white is the feed!
 

· Super Moderator
1995 T100 2WD & 1993 MR2
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Put 12v to the compressor and once you add another can of refrigerant the pressure switch will be out of the equation and things should be back to normal. Capacity is 22.92 oz, +/-1.76 oz.
 

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Discussion Starter · #14 ·
Put 12v to the compressor and once you add another can of refrigerant the pressure switch will be out of the equation and things should be back to normal. Capacity is 22.92 oz, +/-1.76 oz.
Thanks sdspeed. I thought of that but was gun-shy about putting in another can--I figured one can was sufficient to kick off the compressor. I'll start with the power check. If there's no power, I'll go that route.
 

· Super Moderator
1995 T100 2WD & 1993 MR2
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With one can your below 50%, I crawled under my T100, shouldn't be too tuff to get power to it, mine is so damn disgusting under there I didn't do much more than look. But when I was in a shop it was SOP if the pressure switch was a pain to get to.
 

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Discussion Starter · #16 ·
Update: no power to the clutch. I put 12V to the compressor, it engaged and I was able to charge the system with the second can. But, still no power to the clutch. Hmm.
 

· just a nobody
Echo
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Check the dual pressure switch, should have power at blue/red wire when the AC is on, same for the red/green wire if the switch contacts are closed.
If those are good, check to see if the signal is getting to the AC amplifier. And if the AC amp is sending power to the clutch via the black/white wire.
 

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Discussion Starter · #19 · (Edited)
it could be there is too much pressure, was it filled with gauges?
Yes, I filled with gauges. If anything, I think I'm probably a little low as the spec calls for 2 cans (22.92 oz, +/-1.76 oz) and I don't think I got a full 2 cans worth in the system. The pressure definitely went up by adding the second can but still no power to the clutch. I'll check the power to the pressure switch, amplifier, and then out based on echo's suggestions...probably a weekend activity. But, I'll report back with my findings.
 
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