Washed my 1991 Dodge Cummins engine at the car wash yesterday
and now the "water in Fuel light" is on. Had a full tank of fuel and
had the alternator and relays covered up during the pressure washing.
I left the hood up in the sun for a while and the light went out and stayed out for the rest of the day yesterday. Sounds like a wet electrical connection
but where do I look ? Fuel filters are new.

 

Had the same problem. Look at the bottom of your filter bowl where the water sensor plug goes. Unplug it, 10 to 1 chance thats your problem, blow it out with an air compressor or dry it the way you prefer. I took and put grease around the seal when I put it back in, have not had the problem since. Hope that helps.

 

Use dielectric grease. Comes in a small spray can. Made to keep electrical connections clean and dry.