A/C intermittent problem

[TechNova]

My 89 has had a 134 conversion about 4 yrs ago, all was working well.
Lately it will occasionally stop blowing cold air. It is not bowing ambient but noticeably warmer air than normal. It happens sitting or driving, it might be OK for two hours one day or act up 3 times in a 15 minute drive the next.
I have checked the diverter doors, connections and ground at compressor and the pressure switch on the drier.
I recently discovered that slipping into neutral and revving the engine a few times cures it. I hear no squeeling and see no evidence of belt slippage.
What would cause this?

 

[PlayingWithTBI]

What would cause this?

I had pretty much the same issue with my 88. It turned out to be low voltage at the compressor clutch, I was getting ~11.7 Volts. After wring in a relay to get full battery voltage (14+ V), it worked a lot better.

I would also check your air gap on the clutch, it s/b ~0.020" - 0.030". I had a new one from NAPA come with ~0.080" and promptly, within a week, smoked it. I took a bad compressor that had a good clutch and swapped it to the smoked one and have been rockin' it for a couple years now.

Here's a thread I started about swapping the clutch but also has where I wired in the relay and the high pressure cutout switch which my truck never had. Go to page 3, post #22 for the schematic, notations in blue are the upgrades I did.

Swapping the Clutch on an R4 A/C Compressor

The other day when I was waiting for curb side delivery at Total Wine my A/C started blowing warmer to the point it was cooler than the vent but not cool enough to keep me from sweating. It got a little cooler when driving home on the freeway then, when I got off at my exit it started blowing...

www.gmt400.com

This may not be your issue but, maybe worth checking into?

 

[TechNova]

I forgot about the clutch air gap, I quit doing AC work when these trucks were new. I have changed clutches before, I think I still have the tools. I did not see any slippage.
I am pretty sure I checked voltage at the clutch last year, I remember also checking the ground since I have seen grounds check fine then get worse with heat.
We just drove over 100 miles and the AC acted up, when I got home I saw the clutch was disengaged, I revved the engine and clutch engaged.
I'll check gap and recheck voltages tomorrow or Sat.

 

[letitsnow]

Do the hvac doors operate by vacuum on those trucks?

 

[TechNova]

12. 9 volts and .045" gap

 

[evilunclegrimace]

Do the hvac doors operate by vacuum on those trucks?

If you are refering to the '88-'98 GMT 400 trucks the answer is no, they have electric actuators.

 

[PlayingWithTBI]

12. 9 volts and .045" gap

IMO 0.045" is a little too big. It may or may not be the root cause of your issue though.