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.
1986 300SDL 171K
1996 Ford Explorer 62K