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Air Conditioning Diagnostics. No power at compressor

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Air Conditioning Diagnostics. No power at compressor

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Old 04-15-2018, 08:24 PM
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Air Conditioning Diagnostics. No power at compressor

Hi all,
Working on my new '85 EB. This is the first time I've ever worked on a vehicle that has air conditioning, so the systems are new to me. I've noticed that there's no cool air when the A/C is turned on.

I pulled the belt off of the compressor and then started the engine and turned on the A/C, and quickly noticed that the clutch did not engage.
Pulled the plug off of the compressor and checked the voltage. Whether the A/C was on or of, it was getting 0.12 volts. So, no voltage at the compressor.
Traced the wires back to the cylinder over by the firewall on the passenger side (the drier/accumulator?) and checked the voltage at the plug there. It was getting 12v when the A/C was on, and 0v when it was off.

I don't know enough about A/C to know what to do next, or how the system works. But, what would keep the compressor from not getting voltage?

Thanks!
 
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Old 04-16-2018, 06:59 PM
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Pressure switch, to keep the clutch from engaging if refrigerant leaks out?
 
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Old 04-16-2018, 08:41 PM
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Very well could be, I saw this mentioned a few times when doing searches on the net. However, I also read that sometimes it's just pressure switch failure and a new pressure switch fixed it.
I have no idea though since, as I said, I know nothing about A/C systems.
I also am not finding any "pressure switch" parts online for this system, or how to find/locate it.
 
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Old 04-16-2018, 10:32 PM
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That is the clutch cycling/low pressure switch. It's the device that turns the compressor on and off when A/C or Defrost is selected. During normal operation the clutch cycling switch is open below ~26 PSI. When the pressure rises to ~45PSI the switch closes to apply power to the compressor. It stays closed until the the suction pressure drops below 26 PSI again.

If the refrigerant is low the compressor may be energized by this switch momentarily when first selecting A/C or Defrost but will trip off very quickly. This is caused by the high and low side pressures equalizing when A/C is not selected. Once you turn on the A/C the compressor energizes and the low side drops quickly below 26 PSI which shuts off the compressor. If there is enough refrigerant still in the system the high and low side will equalize again given enough time. Too low or no refrigerant will result in that switch always being open. I would not waste any money on a new switch. Verify the high and low side pressures with a manifold gauge set.
 
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Old 04-17-2018, 12:32 PM
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Thanks for the info rla2005, that's the kind of stuff I am looking for. It's all new to me, so I definitely want to know how it works and what makes it tick (especially when it's not working).
I see a few manifold gauge sets on Amazon for about $30. Might be worth grabbing a set.
 
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Old 04-17-2018, 03:51 PM
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I grabbed a cheapo manifold set from Harbor Freight years ago. It has paid for itself many times over. The low side pressure I stated are for a factory R12 system. If someone "downgraded" to R134a they could be a little different. I quoted those from a 1992 Ford service manual.
 
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Old 04-17-2018, 07:04 PM
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Good call on HB. We have one close by.

This is an '85, so I don't know how much its changed throughout the years, or what would be different about it from newer systems. I imagine if I need straight up numbers, my Chiltons will have it.
 
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Old 04-18-2018, 10:01 AM
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The AC system did not change much over the years. Pretty simple setup.
 
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