Fuel Guage broken ?

[darmentle]

So i have been reading threw the forums to see if my issue is a bad sending unit in the tank . But from what i read no one is having there gauge needle go blow empty. So i am trying to figure out if its a bad ground or if the gauge is broken. The needle never moves.

[DnStClr]

Do you have one tank or two? If two, is the gauge inop with both tanks?
Very often, an inop gauge is caused by the loss of elect. signal at the sender, which is located in the tank. It can also be caused by a stuck sending unit arm.
Better troubleshooting has been described by some of the more knowledgeable guys on the forum- here's a good link to help you-
http://forum.73-87chevytrucks.com/smforum/index.php?topic=24127.msg201708;topicseen#msg201708

[rich weyand]

Reading below empty cannot be the sender.  One of the supply leads to the gauge is not connected.  I think no connect on the +12 will do that.

[darmentle]

dual tanks the passenger side tank i don't use its messed up . I only use the driver side. I am trying to get around having to pull the tank because my truck is on a 6"lift with 37 tires i don't really have the tools to support the tank when i am unbolting it.

[DnStClr]

Just measure to the ground and make a plywood box for it to sit on..

[darmentle]

cool if i need to pull the tank i will built a little wood table thanks for the idea. Rich i will hit the connection that the fuel gauge plugs into with a test lite  and see if i got power there.

[rich weyand]

If you have the panel open and the gauge out, measure across terminals B and C in Photo C on this page.  Note that B will be on your LEFT when looking forward into the instrument basket.  Measure from there to the center (lower) terminal.  You should have 12 volts with the ignition on.  The break can also be that the gauge posts are not making good contact in the spring clips due to oxide, but that's less likely.
http://www.chevyclassicsclub.com/the-gm-fuel-gauge-mystery-1965-newer/

[darmentle]

ok i will give it a try

[darmentle]

Ok so i am get 12Volts there. I physically moved the the gauge needle to full and then turn the key and it moved back down to where it was at and stopped.

[rich weyand]

And terminal B was +12 with respect to C, right?  Not backwards?

Does your gauge have the resistor on the back?  See photo B in the link above.  The resistor is the little ceramic doodad between terminals A and B.

With the meter on "ohms", measure between terminals A and C in photo C in the link above.  This is the terminal on the right when looking forward, and the terminal in the middle (lower).

Note the troubleshooting chart says the cause of needle below "Empty" is a break in the ground between the gauge and the chassis, so it's time to talk about grounds.

Do you have a ground wire in each of the following locations:
- between the battery and the alternator bracket?
- between the battery and the radiator support upper crossmember?
- between the passenger side rear head of the engine and the firewall?
- between the driver side front head of the engine and the right-hand-side frame rail at the steering box?

[bd]

I studied a circuit diagram of the fuel gauge internal wiring for a moment and it seems that for the gauge to pin below Empty would require an open or very high resistance ground path, or a broken wire filament inside the gauge.  That notion seems to be in agreement with the link that Rich posted. 

Use your test light connected to B+ to probe the "ground clip" of the gauge.  Any ground issue, under the circumstances you're experiencing, is probably restricted to the instrument cluster or the cluster ground connection to the dash sheetmetal.  Bear in mind, there are multiple junctions in the gauge ground path within the dash: the gauge pins-to-clips, clips-to-copper foil, copper foil-to-harness connector ground wire, harness ground wire-to-cab sheetmetal.  Unwanted resistance in the gauge ground path can be cumulative among all of those connections.  Focus on the connections between the gauge pins, the silver clips, and the copper foil of the PC board (PCB).

Dust and tarnish accumulation in the PCB connections is so commonplace that gently burnishing the copper foil and 'gauge clips,' and perhaps retensioning the clips, may resolve your gauge issue.  Read the 7th entry of the Diagnostic and Repair Notes at the bottom of the page in Functional Tests of Factory GM Electric Gauges for more detail on servicing gauge connections.

[darmentle]

Rich yes i was doing it the correct way and i had a ground problem in the pass with the truck not wanting to start so i went threw and replaced most of the grounds on the truck.  I did not see a diode on the back of the gauge just 3 little studs. I will finish up the other test later today thanks.

[rich weyand]

It's not a diode, it's a resistor.  The gauge works by comparing this resistor to the sender unit resistor.

This one will work.
http://www.classicmuscle.com/p-6237-fuel-gauge-resistor.aspx

[darmentle]

is that what i need to order to make it work?

[rich weyand]

You may have another problem as well, but it will never work without the resistor in place.