95 Freightlinet FLD120 Speedometer not working

For the speedo, if it works sometimes, then there is a loose connection somewhere. Make sure the VSS plug is clean and plugged in tight. Does this have and adjustable (threaded in) sensor? If so, is it adjusted correctly?

The tach should run off a flywheel sensor, similar to you vehicle speed sensor. If this is a cummins with a diagnostic switch, make sure that switch is not on, sometimes the tach doesn't work right in diagnostic mode.

Did both the speedo and tach stop working at the same time? If so, they're probably related, like a common power or ground in the dash.

 

follow the speed wires along the side of the tranns where they go from tranny to engine the wires rub there alot and can get some corrosion in them u can cut and but in heat shrink but connectors or you can run new wires from the ecm to the vss sencor if u get new pins from frightliner if you run new wires u have to twist them and put them in a loom its easy to do if u put the wires in a drill to spin but get about 3 times the linth of wire u think u need should be pretty close start at ecm and cut the rest at the tranny leave it a little long so u can tie it up around everything so it dont rub again

 

Curious how do you adjust the adjustable speed sensors?
What is the proper procedure for setting the depth?

 

. Loosen the lock nut
Screw n the sensor till it bottoms out. Then back if off a bit. 1/2-turn or so. Then tighten the lock nut. If it’s backed off too much, it’ll act erratically,