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FarmerFixEmUp

7220 Magnum Low Voltage to AC Compressor.

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Have a customer's 7220 in here that he's complaining the AC cuts out intermittently for no apparent reason. The pressures look good, it cools good. No yellow light coming on in cab. Never did get it to cut out, but when the compressor's unplugged I have 13.7 volts at the pigtail and when I plug it in it's 12.0. Checked voltage at both sides of the thermostat switch and it 13.7. If I run a jumper wire from the battery to the compressor it's like .2 volts difference off 13.7. Draw on the compressor clutch is 5 amps, similar to a 66 series I have here with a York compressor. I unplugged the engine harness under the cab and checked the compressor feed wire with an ohmmeter and it's good.  The system voltage seems low to me, 13.7 volts and 13.2 with AC on and all the lights. Going to get the alternator checked out, should have 14.2 or better I think. 

Anything else I should look for? I talked to a local AC shop and they have installed Bosch relays on some of these and use the existing compressor wire as an activating wire and get power directly from the battery. I've hooked up extra lights on 66 series tractors with those and it extends the lift of the light switches. Also see mid 90's GM pickups running that to the AC compressors. 

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The relay would be the quick fix, I would back probe the wire for compressor clutch at the cab fire wall connector and keep moving forward from there (everything connected) to find your voltage drop. Would need to review a schematic, but does the wire to CC go thru a pressure switch? Could be a voltage drop at switch.

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I don't have a schematic either. I called AP Air and he said that voltage drop is normal. I should check another Magnum to see. He said take the front dust shield off the compressor and check the air gap. Should be .005 to .030. Also junk likes to build up in there according to them. 

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That's probably the biggest problem I ever have with a Sanden compressor clutch....and even then its rare. Had more than one that will work good up until an 85-90 degree day and then all of a sudden the AC cooling quits. Then maybe later on in the day when it cools off a little bit the AC goes back to working again. You test the voltage at the clutch when it quits engaging and its a nominal 12 volts. Either take some shims out, replace the clutch assy., or replace the compressor because its cheaper to buy a whole compressor with the clutch on it than it is to just buy the clutch itself anymore.

Also seen situations like this where the AC clutch doesn't engage at night once all the lights are turned on on the machine. The voltage drop due to the lights on is just enough to cause the clutch to not engage.

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I had the alternator checked out and it was only putting out 70 amps (110 amp unit.) and 13.7 volts max. He took the regulator out and the rotor was grooved badly where the brushes ride. He had a new Leece Neville 150 amp alternator on hand.  Put it on tonight and at idle with all the lights on and AC at max it shows 14.4 volts. 

I'm going to take the clutch apart tomorrow and check it out  and if needed replace the clutch or as advised the compressor with clutch.

Thanks for the replies! 

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The clutch doesn't look so good. May need to replace the clutch or compressor assembly. 

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