re: 92 400E. I'm getting the "Electromagnetic A/C compressor clutch blocked" on the base module/pin 8. AC used to come on for a few seconds, then cut out, now it won't come on at all. Freon level is OK. I thought I might try a new hi/low pressure switch largely because it's cheap.

But my question is this - is it possible to replace this switch without evacuating the system? It looks pretty easy to get it out and throw a new one in right away, but I have very little knowledge of AC systems. Is all hell going to break loose when I take the old one out, or some other environmental disaster? If the system has to be evacuated it probably makes sense to switch to a diagnostic approach to the problem.

I don't believe that you can remove that switch without evacuating the system, although it is possible on some cars.

However, is your car among those with a compressor speed sensor? The diagnostic listed seems to point that way.

That code may be due to a missing/noisy speed signal from the compressor.
Examine the electrical connector on the compressor closely. We have seen many broken wires there.

It does have the compressor speed sensor which I will inspect. I was looking at the pressure switch only because it's about $12 whereas the speed sensor seems to be about $175. Hopefully it's just bad wiring. Thanks for the help.

This is just from threads that I've read here, so do your own search, but it seems that the sensor can also accumulate dirt which interferes with the signal, which is another free fix.

I'm glad that my car lacks that particular sensor, although it has its own trouble spots.

If your pressure sensor really is a switch, and I don't know that it is, you can check it with an ohm meter. Pressure switches should be closed (conducting) with the system off, assuming that the system has sufficient refriderant.

When my car did that, it turned out that the compressor was seized! It would start for few seconds, then finally nothing.

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'94 S500 Cpe

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Take a closer look at "dpkreuze" post!!! The 3 wires that plug into the compressor WILL break as the engine mounts sink the wires will be TOO short.

OF course a loose belt will cause those codes as well.

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Just checked it and sure enough I've got a broken wire, the red and blue one, which seems to be the speed sensor. The other two wires don't look any too great either. But re-attaching doesn't look so simple - can't figure out how they go on or how the black junction box comes apart. It looks like a 3 prong plug but I couldn't get it to pull out.

Friend,
The black pin holder/connector unclips at the sides I think. Mark down the wire positions and pull out all the pins. Make yourself an extension harness and solder the connections. Those silver colored wire ends are reusable if you're good with a soldering gun and not afraid of a few 3rd degree burns.
Good luck and happy DIY'in.

I'll give it a try. I finally got the connector apart. Thanks all for the help.