Fuel Gauge Repair

From the beginning the fuel gauge showed wrong. A full tank is slightly over half full and as soon as the tank is half empty it shows empty. Not ideal as the gas mileage in offroad use is more related to hours then miles and I always have an uneasy feeling running the truck with the needle on empty.

The Pinz has a VDO sender with 10 Ohms full and 110 Ohms empty resistance. As the gauge shows movement and a test with a 100 Ohm and 10 Ohm resistor showed the desired reading there are two possible reasons why it’s wrong. The sender can be damaged or the connecting wires are corroded and have higher resistance. Again, as the gauge shows movement the most likely cause are some rotten cables/connections (ground or sender-wire). This has to be found out.

Fortunately the Pinz has a plate above the sender which allows (lousy) access to the sender. The plate is located just in front of the rear seats on the elevated floor.

The first surprise was that there is no ground connection wire from the tank to the gauge! This is original as the schematic revealed, something which should never be done as the chassis ground connection will deteriorate over time (and it did!). The resistance between tank and chassis was measured to be ~70 Ohms, no wonder it never showed full.

The solution is simple, a piece of wire with ring lugs connected from one of the bolts which hold the sender in the tank to a more reliable ground point (i.e. alternator ground or even somewhere in the dashboard) and the gauge reads as it should...

While looking at the sender an unused pin was detected. As the sender is a generic part it may be an output for an “idiot light”... Will have to test that after I ran down the tank (which usually doesn’t happen) as it could be connected to an additional indicator on the dashboard......

© 2002-2005, K. Juergen Schoepf