Aux Fans

[DAVID LEE]

This is my first inquiry on the site. I have been reading about aux fans for several days, and using the knowledge gleaned to troubleshoot by A/C problem. I think I'm close to knowing what the problem is, but I would like just a little more info before proceeding with some possible fixes.

I have a 92 300E. My aux fan would not come on when the compressor was running. After reading the archives for several hours, and seeing bubbles in my sight glass, I decided that the first thing I needed to do was to get my A/C charged. That was done today. There are still a few bubbles in the sight glass, but not nearly as many as before, and it seems to cool OK, but it was only in the 80's today.

The aux fan still does not come on at low speed. I jumpered the switch on the redeiver/dryer and the fans came on. I took out the resistor and checked it for continuity, and it checks OK. The fans come on at high speed when temp reaches 100 degress. Is it possible my high side pressure sensor is bad? If so, can the switch be replaced without losing the r-12? My sensor is not on top of the dryer, but comes out the side of the dryer very near the top.

Without pressure gauges is it possible to test the sensor? I have read in one thread about someone wiring a relay so that the fans come on at low speed when the compressor is running. Where would I get such a relay?

Thanks in advance for any help. This site is a gold mine of info.