I have a 300SDL and I think I have a bad relay , but not sure. Any easy way to test myself or is it a lost cause? Car sympton: when car is idling I experience an intermittent RPM spike then drop-off ,then car smooths out and runs fine. I have noticed the problem both at idle when engine is cold , and upon re-start with warm engine. I have gone thru archives but did not find any trouble shooting suggestions.

Johnny Justice
1986 300SDL 171K
1996 Ford Explorer 62K

Testing is possible but as in your case with a intermittent problem it can be difficult and time consuming. If you are going to test you will need a ETM and a good multi-meter with a recording function at min. A recording occiliscope would be better.
Also it would be much easier if you had the breakout boxes that allow testing of circuits without backprobing the connectors.
Good Luck

Thanks for the input. I think I will have the relay tested at a local shop. My electrical expertise is not up to the level I feel comfortable to properly troubleshoot. I am not real sure the shop will either, since the problem is intermittent and of short duration[1-2 seconds].