Gas Guage

[claude]

1985 K20 was just given to me. Runs great with fairly straight body that desperately needs a paint job.  My issue is the gas gauge. Originally gauge always registered empty on both tanks. Tanks would not switch but a new switch fixed that, but of course both tanks still registered empty  Both wires from gauge to tanks were melted by headers. I fixed that issue and now gauge for both tanks register 1/8 when both tanks are full. Once in a while gauge may go to 1/2 for half an hour then go back to 1/8. With the bed on it is extremely difficult to get to the ground wires for each tank but is probably doable and will work on that next. If that does not work I know I should start testing wires from tank to gauge to see if problem is gauge, wire, or sending units.  How do I do this and can it be done without removing bed or dropping tanks. With time I am a decent mechanic on engine/drivetrain. I am a complete novice on electrical but can usually understand and follow advice/directions. Any help is appreciated.

[bake74]

     BD wrote up a article in the Tech Section on testing GM factory gauges.
     BD also wrote up a article on common causes of bouncy gauges in the tech section.
     Here is a link to Back Country's thread where there is listed a electrical schematic for a dual tank set up, and where he was having problems with his gas gauge.
     Lastly but not least, in the tech section is listed the Service and Unit Repair Manuals for our trucks.
     Look through this material and if you have more questions or need help we can help out.

[rich weyand]

That they both read wrong the same makes me think of a failure in the common part of the wiring.  If they read 1/8 to 1/4 when full, it is probably the resistor on the back of the gauge is broken.  PITA to replace because you have to dig four layers deep into the dash to get to it.

Here is the resistor that worked on my 78.
http://www.classicmuscle.com/p-6237-fuel-gauge-resistor.aspx

[claude]

Thanks rich weyand and bake 74.  As soon as the rain stops I can start working on the pickup in my driveway by doing a process of elimination.