Please help me figure this one out:

I've got a '91 240 with an AC system that recently stopped working. The first thing that I noticed was that the compressor will click, engage, start spinning and then will disengage itself. This will repeat endlessly but for some reason the compressor will not stay engaged. It will engage for about three seconds, then disengage and continue the cycle.

Any ideas on what I should check? There is one green wire coming from the compressor which looks like it goes to the electronic colling fan on the AC radiator. I can't seem to come up with any ideas on testing... I'm thinking that perhaps the electronic cooling fan is not working and the compressor is turning off because it perhaps determines that the AC system shouldn't be running if the cooling fan is not sending a signal? My 240 manual shows that there is a relay involved with the compressor but I can't seem to identify it. I see this part but this looks more like a V(+) node than anything else.

Please help me with this one...

Now I'm thinking that either this relay or the pressure sensor is malfunctioning. The reason for this is that I believe that I traced the green wire mentioned earlier to a pressure sensor, which would agree with this diagram . Perhaps this pressure sensor is sending the wrong signal to the compressor, resulting in the compressor shutting off. Either that or the relay (which I still haven't located on this car) is malfunctioning (Item K).

Can anyone tell me where this relay is?

sounds like low gas pressure to me-bridge out the switch on the top of the drier on the right hand side of the radiator with some thick wire strap, and see if that stops it cycling. do NOT prolong this test, as a low gas compressor is damagable by continuous running-hence the switch. The aircon relay is in behind the central dash, near the thermostat. expect gas conversion to R134A, regardless. R12 is illegal/difficult to obtain.