I have the same symptoms and did the same testing as I did on this thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=59170&highlight=compressor+relay

Background:
About a week ago, it was raining really hard, I drove over some water on the road. The A/C quit working. I re-started the car and the A/C worked againg.

A couple of days ago, I started the car in the morning after a heavy rain overnight. The A/C has not been working eversince.

I have two theories:
1. Water is coming in behind the battery, where the relays are, and shorting the relay. It is raining a lot in Florida.
2. The belt tensioner is weak and the speed sensor is killing the relay somehow. Is that possible?

One thing I did not notice before, or can not remember, is when I jumped the relay with 5 and 7, the compressor did engaged but, the jumping wire was real hot.
BTW, I do have the bottom engine shield.

I do not want to buy a realy every six months. Any ideas?

__________________
J.H.
'86 300E