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Intermittent AC availability 1991

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Very recently our 1991 NSX AC has been not coming on about half the time I use it. I can start the car and the AC doesn't come on, then shut if off and restart it and most of the time it will come on. It seems random, sometimes it will start multiple times in a row, sometime not.
  • The CCU has been rebuilt
  • Both the CCU and climate control self tests seem fine.
  • There are no oddities in the climate control display
  • The system was just recharged professionally
  • When it runs correctly, cold air comes out all the vents
  • The temperature of the cold air doesn't seem as cold as it should be - about 45F with 75F ambient but it definitely cools the car when it does come on.
  • All the fuses are fine
  • A mechanic checked that when it doesn't come on, the compressor will start by jumping the solenoid, but not by itself. When it works, at idle, I can hear the motor slow down when the compressor kicks in.
  • It doesn't seem to make any difference if the car is moving or parked, behaves the same, sometimes it starts, sometimes not.
  • I've not gone more than half an hour with it running, but it didn't stop on that trip.
  • The system was converted a long time ago to 134A
  • When I switch from AC to heat I can hear something moving. Vent door?
I can't find the sight glass. I wonder if that was removed when the conversion was done. There's a black cylinder at the front of the radiator with a solid hex connector and two lines coming out of it, one going to the refill port, but nothing that looks like a sight glass.

Anyone have any thoughts?
 
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Saskatchewan, Canada
If it works correctly (blows cold) 'when' it works, it is unlikely that you have a refrigerant problem. However, you seem to be 'iffy' about the performance of the AC. To confirm that the AC is up to snuff (when it is running) you need to measure the ambient air temperature entering the condensers and the air temperature exiting the dash vents after it has been running for 10 minutes and you correct for the ambient relative humidity. There are performance nomograms in the service manual along with the complete details of the procedure. Without knowing all the details of your test, it looks like your temperature measurements are within reason - when the AC is running.

As to why the AC is running intermittently. Since you have had the CCU rebuilt and the mechanic confirmed that the clutch engages, my initial gut reaction is that you have an intermittently failing AC clutch relay or a bad electrical connection between the clutch and relay. Check for obvious signs of a bad connection at the AC clutch connector. If there is no obvious damage then remove the clutch relay from its socket and check for signs of damage in the socket itself. Sometimes as the sockets age the electrical connection becomes flakey leading to heating on the pins and damage to the socket / relay. With the relay out you can carry out a test of the relay as described in the service manual; however, if its operating intermittently its hit or miss as to whether the test will confirm a problem. The AC clutch relay is a SPST relay and is likely a Denso. Have a look around in the relay boxes and you may be able to find another identical relay - go by the Denso part number on the cover. I am going to bet its 39794-SE0-003. If you can find another identical relay in a non critical function, swap it in and see if the problem goes away. If the problem goes away, you know you have an intermittent relay failure. If the problem remains, then life is more complicated.
 
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