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My '78's A/C worked when I got the car, but looked a little cobbled and I'm trying to get it back to factory looking. Most 78-87 cars appear to use a pressure cycling switch located on the side of the A/C accumulator. My '78 came with a crude looking aftermarket Cutler-Hammer temperature switch connected into the electrical circuit at the same point. It had a capillary tube clamped to the A/C refrigerant line. I was going to replace the non factory temperature switch with a spare factory pressure switch I had, assuming that that was the factory setup. However, the fan relay was mounted higher on the '78 than other years, blocking the installation of a pressure switch. The higher loaction appears to be a factory error-proofing method so you can't install the wrong switch.
Did '78 systems operate on temperature cycling switches rather than pressure switches? If so, I imagine then there is a factory temperature switch I should be looking for.
 

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The first generation of CCOT systems used thermostatic cycling switches. The later systems used pressure cycling switches. The only difference is one senses evaporator outlet temperature, and the other senses outlet pressure...which, equates to the same thing.
Your 78 used a Delco 15-5181 thermostatic switch with a Delco 15-1255 accumulator. The switch is still available at your local delco distributor. You could also change it to a newer system by using a Delco 15-1589 accumulator and a Delco 15-2151 pressure switch. There is really no advantage of one over the other.
 
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