Last night the speedometer started acting up, would fluctuate when the truck went over bumps and completely cut out on moderate right turns. Looked underneath as well as one can in the dark and couldn't find anything loose.

This morning I searched on here and the VSS came up numerous times as a common failure. $15 and about a half hour later I had it installed, and while I was under there I took a better look at the other connectors to double check harness connections for tightness and cleanliness. It all looked good, so I went ignition on... and the light came back. Tried clearing it by keeping the diag connector unplugged while switching ignition off, but no go. Took it for a quick spin around the parking lot, and the speedo was completely dead.

Hit up my factory ETM, found all the relevant connectors and set up a wiggle test of the VSS circuit with the DVOM on audible continuity check. Found one of the wires had chafed against the body right by the under-cab harness connector and worn almost completely through. Cut out the bad bit, soldered and heat-shrunk a patch in, and took it for a spin. Voila! No more ABS light and a working speedometer.

Also fixed the fuel gauge sending unit wire which was in the same connector grouping and chafed (but not yet broken) in a similar manner.

Ah, the joys of older vehicles.

 

Does this look & sound familiar??? https://www.ford-trucks.com/forums/3...und-today.html

 

Yes! That's exactly it. I found a TSB about water intrusion into the connector but never saw the one about wire chafing.

 

OK, good trouble shooting, repair, feedback & to hear you have a good grip on that common super cab wiring problem.