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MB190D 12-05-2005 07:36 PM

Testing pressure-switches on AC?
I'm slowly gathering all the remaining parts needed for my AC-project (boy did that take a whole lot longer than expected!), but I'm wondering whether there is a way to test the pressure-switches on the dryer before I put everything together?

Matt L 12-06-2005 01:52 PM

You don't say whether you intend to run CFC12 or HFC134a in your system. It matters.

CFC12 systems only need one switch, to prevent the compressor from starting if you are low on refridgerant. Some systems use a switch on the low-pressure line which also cycles the compressor, but I don't think yours is one.

HFC134a systems are required to have a high-pressure cutout switch, which will not be a stock item for a system originally speced with CFC12. But don't worry, if you buy a replacement low-pressure switch for your dryer (which is on the high-pressure line), it will also incorporate a high-presure cutout switch.

If you intend to run CFC12 and merely want to test the low-pressure cutout, just check its conductivity at ambient pressure (should be infinite resistance), and slowly increase the pressure until it conducts. Should be about 30 psig or so.

Testing whether the switch will hold pressure while in use, or whether the HFC134a high-pressure cutout works is much harder for those of us who lack 300+ psi air compressors.

Note that there's only one pressure switch on the dryer. The other device is a temperature sensor, to turn on the auxiliary fan.

MB190D 12-06-2005 04:07 PM

Hi Matt, thanks a lot for your reply! Hm, at least I got something else to think about - the system is from a '91, but I believe that was the year Mercedes changed from CFC12 to HFC134a? I really have no idea as to what was in it originally, but as far as I know I cannot get CFC12 around here anymore, so I guess I will be running HFC134a.

On the dryer that came with the system, there are two sensors, both of which I was under the impression were pressure switches due to the German wiring diagrams (Both are referred to as 'Druckshalter'). One is set to control the fan and the other one to engage/disengage the compressor like you say. Is there any way to find out whether the pressure switch already present is a low-pressure only, or high/low-pressure switch?

Hm, I might be able to plug one side of the (old and useless) dryer and connect our compressor to the other side - if I'm lucky it should create enough pressure to find out whether it works or not :)

Matt L 12-06-2005 05:16 PM

I don't recall the part number being printed on the dead high-side pressure switch on the '83 around here, and the replacement for CFC12 or HFC134a looked identical. If it were me, and I were unsure or couldn't test the switch, I'd just get a new one. They're not terribly expensive. You may not need the high-pressure cutout in your jurisdiction, but it's a good idea as it can prevent loss of refridgerant.

The "Druck" in the name would refer to pressure, but there is no need for two pressure switches in the same line. If one of them does run the fan, it's the temperature switch.

Your low-pressure cut-out does not cycle the compressor as in some designs. It merely prevents the compressor from starting if there is insufficient refridgerant and thus possibly insufficient lubrication for the compressor. Once the compressor starts, the switch will stay closed. A cycling switch would need to be on the low-pressure line to do anything.

If you use your car's compressor to test the switches, be sure to let it have some lubrication.

MB190D 12-06-2005 07:09 PM

Thanks again! The sensors I was referring to on the dryer, are on both the line in and out, but I presume one of them "measures" the temperature by pressure then?

I think I will follow your advice and just replace it in any case - it will surely be cheaper than to find out that it is defect AFTER having the system recharged :)

Thanks a lot!

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