78 Chevy K10 PU- AC question

[DSS]

My78 Chevy K10 has stock AC which typically works fine- Last year around this time it abruptly quit blowing cold air one day- -I drove it  again a few days later and it had cured itself and worked fine. A few days ago it did the same thing and abruptly  quit blowing cold air, but now it hasn't fixed itself. I am by no means an AC expert- Anyone out there have  suggestions on some things I should check/troubleshoot before I take it to a shop?

Thanks!

[K20Detroit]

Is the clutch still kicking on, just not cold?
Is it not kicking on at all?

In either case, check the pressure in the system. If it's converted to 134, a cheap refill kit has the proper connection to check it.
If it's R12, you'd need an A/C gauge set - Harbor Freight sells them - that come with R12 and 134 gauges.
I'm not 100% familiar with these truck's A/C, however; on newer cars, under a certain pressure, the A/C will not kick on. Since temperature affects the pressure, it is possible for conditions to be just right for the pressure to go just below the threshold for a bit.

The line pressure and clutch engagement are the two main things to check.

[DSS]

Thanks for the reply- I did check and the clutch is not moving when AC selected- I don't have any AC gauges so I'll look into that next

Thanks again

[hatzie]

Before you hook up gauges you can do a quick test.
With the key on engine off, AC selected on the mode lever, and Cold on the temp lever. Bypass the Low Pressure Cutoff switch on the accumulator with a piece of wire and 2 male disconnects.  The LPCO has 2 green wires. 
If the electromagnet on the compressor engages you'll hear it.

If the magnet engages proceed with checking your charge level.
If the magnet doesn't engage proceed with checking the wiring and switches.

My money is on electrical.

[LTZ C20]

A note to DSS, Hatzie is a professional. Your in good hands. It does sound like an electrical intermittent malfunction to me also.

[DSS]

Ok I did the test as you suggested- no go on the clutch engaging. So I buttoned the low pressure switch back up , made sure any other visible connections were tight and,  for grins tried AC again with engine off- the clutch engaged- so I started up truck and switched AC on again and it works fine- blows cold air and seems to cycle on/off normally.....??

[hatzie]

Ok I did the test as you suggested- no go on the clutch engaging. So I buttoned the low pressure switch back up , made sure any other visible connections were tight and,  for grins tried AC again with engine off- the clutch engaged- so I started up truck and switched AC on again and it works fine- blows cold air and seems to cycle on/off normally.....??

The switches in the mode/temp control head are probably flaky from plain old dirt.
You can probably clean up the contacts with Caig Deoxit D5 & Deoxit Gold using Q-Tips & mascara brush.  D5 is cleaner Gold is a lubricant & protectant.

The other place to clean up is the LPCO plug & switch contacts.  It's probably in the control panel.

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[DSS]

Thanks again for the info! I have the LPCO plug location figured out and will clean those contacts- but where/what is the mode/temp control head?

[hatzie]

Thanks again for the info! I have the LPCO plug location figured out and will clean those contacts- but where/what is the mode/temp control head?

Mode is the AC Vent Heat Def lever.
Temp is obvious.
They are in the control head in the dash,

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[DSS]

OK, Got it-Thanks again!