ac pressure suddenly shoots up to 400 psi

[1998_K1500_Sub]

I still believe the compressor is fine but will look into adding that switch for insurance.

I don't suspect the compressor. You looked and found no debris.

"For insurance" you might consider replacing the condenser, particularly if you reside in one of the warmer states. The modern, parallel-flow condensers were discussed in a recent thread; I'll point you to it if you're interested.

The same thread also discussed cooling fans (9 blade, 11 blade, etc.) and fan clutches as means of improving performance / lowering high-side pressures.

The R4 compressors seem to leak all-too-often, and minimizing the high-side pressures is going to help mitigate that concern.

 

[Supercharged111]

This truck was "converted" to r134 when this compressor was put on with it's stock condensor. And as said, it has been working flawlessly and plenty cold for 8 to 10 years now. The truck was not used much so only a few thousand miles were put on it but since I've had it I have driven it several days a week in the last couple of years. It just now crossed 88,000 actual miles. I still believe the compressor is fine but will look into adding that switch for insurance.

Turn the compressor by hand with a line disconnected. It will be obvious if something is amiss.

 

[Jimmy D]

Maybe releasing R12 to the atmosphere was a contributing factor in outlawing it.

I'll bet that's exactly why they changed it. Good info here. I've been doing all things automotive including ac work since late 1960s and never had one act up like this one. Dad and I had a shop doing collision and mechanical work for a long time. As a matter of fact the reason I was under the hood of this truck was I was hooking up a new manifold gauge set and one of the quick couplers wasn't working when all of a sudden it bled off pressure from the back of the compressor. It happened once more then the compressor locked up. Glad I haven't yet bought a new compressor. Haven't been in much of a hurry as it's HOT out and I have other trucks to drive.

 

[1998_K1500_Sub]

My 1995 S10s (I once owned three, all 4.3L equipped w/ R4 compressor and R134a) had a OE delay circuit for the AC clutch circuit.

Immediately after engine start-up, the delay would wait ~15s before engaging the AC clutch. That's the only time it came into play.

Evidently this feature was added to mitigate at least the underhood heat soak concern mentioned earlier by @PlayingWithTBI

My father had a 1995 K1500 5.7L w/ R4 compressor and R134a and it did NOT have the delay feature.

I wonder where else GM deployed this "feature".

 

[L31MaxExpress]

My 1995 S10s (I once owned three, all 4.3L equipped w/ R4 compressor and R134a) had a OE delay circuit for the AC clutch circuit.

Immediately after engine start-up, the delay would wait ~15s before engaging the AC clutch. That's the only time it came into play.

Evidently this feature was added to mitigate at least the underhood heat soak concern mentioned earlier by @PlayingWithTBI

My father had a 1995 K1500 5.7L and it didn't have the delay feature.

I wonder where else GM deployed this "feature".

The P01s and P59s have a slight delay of around 5 seconds but it is electronic in the PCM. Not sure if it is to allow a head start for cooling airflow or to help prevent the engine from stalling until the engine idle speed controls take over. Potentially could allow for both.

 

[L31MaxExpress]

An electric pusher fan or two will help dramatically as well. The 1987 G20 has one and the 1997 Express has two. My 99 Tahoe had one from GM.

 

[PlayingWithTBI]

An electric pusher fan or two will help dramatically as well.

I noticed with my new pusher fan, I don't get that super high pressure anymore either.

 

[Jimmy D]

An update to this deal. I wired in a high pressure switch, changed the orifice tube, vacuumed the system down, added some oil and charged that sucker up. It ran just right in the shop. I feel good that the compressor is fine and that the reason for the pressure discharges were because of excessive high side pressure at the time.
Thank y'all for your help.