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post #1 of 7 (permalink) Old 04-22-2011, 11:46 PM Thread Starter
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ac high pressure cutout switch question

I've searched the forums and can't find the answer I'm looking for.
I'm putting in an ac system from a 95 3.8 to my 93 3.8; compressor, hoses, and receiver/dryer. Do I need to hook up the high pressure cutout switch, or can I leave that unplugged? Otherwise I will need to gather some more parts.

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post #2 of 7 (permalink) Old 04-23-2011, 07:27 AM
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The 93 used R-12 whereas the 95 used R-134a. The pressures and cutoff levels are different, the seals and o-rings are different, the oil is different and the fittings are different. You'll need to flush the evaporator and condenser of all the oil before you reassemble everything. IIRC the high pressure switch is the one that senses when the pressure is high enough to warrant turning on the radiator fan and when to stop the compressor if the pressure is too high, so you'll need to hook everything up.

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post #3 of 7 (permalink) Old 04-23-2011, 09:31 AM
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Pre-'94 didn't use a high pressure switch, as you know, it relied on the low pressure switch to cause compressor cycling and then there was a high-pressure blow-off on the back of the compressors. I don't know if the '94-'97 have a blow-off valve (I assume they do) but if they do, you need to at least see what the max psi of that valve is.

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post #4 of 7 (permalink) Old 04-23-2011, 10:04 AM
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The blow-off valve is also the high side charging valve. IIRC the pressure is somewhere in the neighborhood of 450 PSI.

-Brandon
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post #5 of 7 (permalink) Old 04-23-2011, 02:25 PM Thread Starter
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that is the whole point of this conversion, to go from r12 to 134a. so i will need an electric fan and the CCRM?

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post #6 of 7 (permalink) Old 04-23-2011, 07:47 PM
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I would stick with the hardware you have, flush out all of the old r-12 oil and refill (6-8 oz) of Pag oil. If the older models dont incorporate a high pressure cutoff switch, just leave it unhooked (i assume you are swapping the newer model lines onto the older car). Also I will have to check but I nearly know the blow off valve is on the back of the manifold that bolts to the compressor. Practically every newer model A/C system I have been in has some type of blow off valve somewhere in the high side (usually in the compressor or manifold).

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post #7 of 7 (permalink) Old 04-26-2011, 02:49 PM
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The high-pressure cutout switch serves 2 purposes.
1) It kicks on the high speed fan when the pressure gets sufficiently high.
2) It cuts off the compressor clutch just before the refrigerant is about to discharge from the relief valve.

You don't have to use this switch, but you will want an electric fan for sure. You will also want to swap out your condenser because the newer systems use many more fins to dissipate heat as R-134a operates at higher pressures and temperatures.

Also, the relief valve is on the manifold assembly very near to the compressor itself. It does not discharge out of the high pressure charging port, not under normal circumstances anyway.

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