Easiest way to trouble-shoot a gas gauge?

[Bill Crowell]

This isn't about an A-body; it's about my '78 Dodge D300 dually tow vehicle. It has dual gas tanks with the changeover switch on the dashboard. A couple of years ago the gas gauges quit working, so I went all through the wiring and replaced it and replaced one of the tank senders, and then the gauge worked fine on both tanks. Now it has stopped working completely again on both tanks, so whatever is wrong must be in the wiring that is common to both tanks; i.e., between the switch and the instrument panel. Would anybody have some tips for an easy way to troubleshoot it? Sure would appreciate it.

 

[cudajim]

I would start with a good schematic. I am guessing however it's either a relay problem or a SPDT switch which has a common connection to both tanks.

 

[Bill Crowell]

OK, Jim, so I finally broke down and consulted the schematic in the service manual. The gas gauge gets its voltage supply from the voltage limiter mounted to the instrument printed circuit board. The instruments run on 5 volts because 12 volts would draw too much current and would burn out their meter coils. From the +5 volt output of the voltage limiter, the circuit runs through the meter coil and thence via the chassis wiring harness and firewall electrical connector to the sending unit on the tank, the other side of the sending unit being grounded. I am therefore betting that it must be either a bad voltage limiter or a bad connection at the firewall connector. Since the gauge quit working on both tanks simultaneously, I doubt that it would be bad sending units. I guess I'm going to have to pull the instrument panel. While I have the instrument panel out, I can also check the wiring and voltages at the tank selector switch.

 

[cudajim]

Bill,

It very well may be the voltage limiter, they fail all the time and it would certainly be common to both tanks. There are numerous blogs about replacing them with newer solid state units or building your own using a 7805 regulator. You might be able to pull the instrument cluster out and check the regulated voltage if you can get at it. Since I don't have a schematic I can't help much more. Let me know what you find.