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A lot of the aftermarket companies used York compressors or Sanden. Post pics
My '80 has the same unit. Same compressor, same controls. I bought a '79 GMC new off the lot without factory AC and had the dealer install AC. It was the same as well.
MX1 is TH 350 trans.topp, I worked at a GMC dealer in 84-85', we sold several trucks with dealer installed a/c, controls just like zieg85 has pictured.
Hear are 3 pictures, I like the simplicity enough it may wind up on my 1986 that is under construction... The only thing I didn't like was how the glove box had to re-configured and covered up the options list... What is MX1 transmission RPO code BTW???