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1991 S, A/c hasn't worked in about 5 years, just now trying to get back online. I evacuated system which was low anyhow. Now the compressor will not engage. I have no power at the grn/black wire connected to red wire on compressor. I have jumped the grn/black wire at the trinary switch and compressor engages. I jumped the white connector at trinary switch and the cooling fan comes on. Thinking that it needs some charge to come on I tried charging with the red connector jumped and compressor engaged but the system will not take any charge. I also applied 12 volts directly to compressor and it engages. Any ideas where to go from here?. Thanks
 

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Check the fuse? Had the fuse blow a couple of times in my 91S a/c circuit.
 

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Discussion Starter #3
Thanks Del, the ten amp fuse in box is good, 40 amp at fire wall is good, I haven't tackled the relay behind instrument panel yet. Am I right thinking that the relay is working if I have 12 volts at the trinary switch? Also the clutch engages when I grounded the wire at the frost switch.
 

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Ok. In my case, I remember now that the a/c clutch at the pump shorted out and that blew the fuse. Was a few years ago.
 

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1991 S, A/c hasn't worked in about 5 years, just now trying to get back online. I evacuated system which was low anyhow. Now the compressor will not engage. I have no power at the grn/black wire connected to red wire on compressor. I have jumped the grn/black wire at the trinary switch and compressor engages. I jumped the white connector at trinary switch and the cooling fan comes on. Thinking that it needs some charge to come on I tried charging with the red connector jumped and compressor engaged but the system will not take any charge. I also applied 12 volts directly to compressor and it engages. Any ideas where to go from here?. Thanks
First off was system converted correctly to R134a 5 years ago?

So do you have a proper a/c servicing gauge set? I would think if you had a vacuum pump hooked up to low side and pulled down system to hold -30 on low side gauge and system held you should have keep vacuum trapped in blue side and then hooked can of R134a to yellow hose that was hooked to vacuum pump. If you start with vacuum trapped on low side and use use hot water bucket to help system take in the freon as soon as low side pressure builds up to over 25psi trinary
switch closes and compressor engages with engine running at idle and pulls in freon.

 

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Expansion valve often seizes which can prevent the compressor from engaging. Anti frost switch and the tri switch also suspects.

The AC pressure detection system is the usual problem. Too much or too little refrigerant, no expansion into the evaporator (that is, no exit of decompressed gas from the expansion valve which is what refrigerates the air ) or the antifrost valve failing to operate.
 

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Discussion Starter #7
Thanks, the system was converted by previous owner. Needed charge when I purchased car, did so and it worked good. It eventually lost charge and I haven't had time to fix until now. I do have a set of gauges and pulled vacuum on system. I will give it another try tomorrow. I am not familiar with the hot water bucket, could you explain so I can try that as well. Thanks again
 

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Thanks, the system was converted by previous owner. Needed charge when I purchased car, did so and it worked good. It eventually lost charge and I haven't had time to fix until now. I do have a set of gauges and pulled vacuum on system. I will give it another try tomorrow. I am not familiar with the hot water bucket, could you explain so I can try that as well. Thanks again
Once you blue hose valve at low pressure service fitting screwed in to open valve core and have vacuum pulled down, turn off blue gauge valve at manifold to trap vacuum in system. Now remove yellow hose from vac pump, hook up can of R134a to yellow hose. Open valve on can and loosen yellow hose at gauge manifold slightly and bleed air out of hose so only R134a now at manifold. Open blue gauge valve and see that blue gauge pressure rises up to maybe 80 psi.
Now, start engine and press on a/c panel on and auto buttons. When R134a enters low pressure service fitting and pressure builds up to about 25psi compressor clutch should engage and start to turn compressor and pull in the R134a in a gaseous state. I just use the hot water to help warm the cold can of R134a and raise the pressure of the gas to help the compressor suck in the gas. When one 12 oz can empty shut off blue valve, install new can, bled air yellow hose and open blue valve again. System should take in three 12oz cans. See my pictures in above post.
 

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Remember that you need R134a, not freon.

Also, it is critical that you ensure your existing system is not leaking. If the AC failed originally because it leaked refrigerant then recharging it without locating and fixing the leak isn't going to work.

If the conversion was done incorrectly (old R12 compatible oil must be completely removed) then that could have caused the system failure.
 

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Let's hope he used Ester oil for original conversion. If system has original 1991 hoses for the old R12 Freon yes some leakage is going to occur but not a game stopper at this point.
 

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I did find that using a couple of cans with a/c stop leak in the working fluid of the converted 91S did take care of minor seepage without causing problems. The 94LS has never been fiddled with, works fine since we bought the car. The Milano? Unconverted and empty. Just a little used local driver, so we've not bothered to change it over from original.
 

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Discussion Starter #13
I appreciate all the help. I made another attempt at charging this morning with no luck. I had a new expansion valve and receiver I was going to use on my other car but went ahead and changed them out on the S. Vacuumed the system, made sure I had no leaks and it took a charge and is working fine now. Again thanks for the help, it sure makes maintaining these cars a lot easier.
 

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Seized expansion valve is a common cause of AC failure to operate. Symptoms when the AC is charged are frequent compressor cycling with no or very poor cooling effect. The compressor overpressures the system repeatedly due to the stuck valve. If the system is discharged a stuck expansion valve prevents recharging.
 
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