I got a lot of answers from this forum....so, I would like to share something....
I bought a 1990 300 SE two month ago. The car is great, however, two items the previous owner could not resolve:
1) Check engine light - code indicates O2 sensor. He changed it and could never clear the check engine light. Of course, in California, this will not pass the smog.
2) A/C compressor does not engage even when the refrigerant is full.
I took the car to a mechanic, and he was able to jump the A/C compressor. He told me the compressor is good. There is something that is controlling the compressor and it's not working properly.
After research and looking through the repair manual, I found out that the Electronic Control Unit (ECU) (also called MAS Control Unit) controls the A/C compressor and the O2 sensor. I ordered one from Auto Parts at AutohausAZ - OEM Auto Parts - Discount Replacement Parts, Resources and Car Care Tips
( $ 260.00 ). I replaced it, unplug the battery for an hour or so, and guess what? It's all good.
AC works, Check engine light is out. I have been driving it for two day now and it's all good.