1987 Ford F250 6.9 diesel, 4x4, 4 speed, 249K.
Duel fuel tanks. Both tanks read full when the tanks are full. As the fuel is used the guage gradually goes down. The gauge reads about 1/8 used when on empty. If the truck is level and just started, 20 % of the time the gauge shows the correct fuel level. As soon as the truck moves the guages returns to full. Both tanks work, and I get the same behaviour on both tanks. When the truck is turned off the guage goes back to empty. No leaks. Seams like it should be someing not related to the tanks. ( Both have the same behavior ) Whats in between? Any connections / grounds I can check?
Additionally, the tach works 98 % of the time. The remainder it is registering 0 to the correct rpm. Here is the catch. Turn the lights on, and it works 80% of the time. 20 % of the time its registering 0 to the correct rpm. Turning the lightsback off does not correct the issue. The only way to " reset" this back to the 98% working, is to turn the truck off and restart it.
All other electrical equipment works as designed 100 % of the time.
I think its a short in the dash for the tach, not related to the fuel tank issue.