# Fuel gauge and wiring problem. Help!



## wishihaditback (Jun 9, 2011)

The gas gauge WAS working in my '68. Then one day i noticed the gauge was pegged above "full". I've check the ground from the sending unit....good. I've read GM forums and the testing and terminology are a bit beyond me. I know the ground is good and I've found the "power" wire from the sending unit in a connector in the trunk with the license plate light. With the key on, is this wire (brown i believe) supposed to be hot? If so, mine is not. The 'instrument" fuse in the fuse block is good. Should there be a terminal on the back of the gauge that is supposed to be hot? What next if it is, OR is not? Thanks.


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## rlslavik (Jun 21, 2014)

I'm doing the exact same thing right now although my issue is a little different.
Here's what I DO know.

With the gauge hooked up under the dash with the big multi plug
connector, if you disconnect the tan wire in the trunk you will get
a FULL reading.
Ground that wire to the frame and you should get EMPTY.

SUPPOSEDLY this would mean the gauge/wiring is good and the problem is in the sending unit or the ground at the tank.
Some folks say they have received a 30 ohm sending unit from parts houses recently when you need a 90 ohm.
You haven't replace your sender and that is not your issue but bear with me for a moment.
A Multi meter at the hot and ground of the sender should read zero ohms when you pull the float to "empty" and around 90 ohms when pulled to "full".
If it reads 30 or anything NOT near 90 your sender is the issue.

MY issue is that all that checks GOOD.
Yours doesn't so bear with me.

My problem is NOT the sender, or the ground OR the wiring to the trunk.
Yours is Forward of the trunk

I have "decided" my issue has to be the RESISTOR that runs across the backside of the gauge.

Your issue somewhere forward of the trunk, possibly a broke brown/tan wire.

Disassembling the entire dash to REMOVE the fuel gauge / resistor 
and checking my RESISTOR in hand with a DMM reads anywhere from ZERO to 300 ohms and it jumps all over constantly. I can't get a constant reading. Obviously a problem there.

The "similar" resistors on the back of the temp and oil gauges reads around 60 ohms each - if I remember right.

When I replaced the original resistor with a new style 100 ohm resistor it read 125% percent full.
Too much.
A 82 ohm resistor reads 4/5 full.

I have been researching what the original resistance is supposed to be and have had no luck. Year One, and OPGI have no clue.
I am ASSUMING 90 oms only because of the sending unit ohms
but I don't have a 90 ohm resistor yet to test it.

And if you happen to go as far as pulling off your resistor please let me know what reading you get.


AND YES
With your key ON, the tan wire should be hot at the trunk.
You have no power in the tan wire.
It WOULD read full with it disconnected somewhere aft of the gauge.
Your gauge is MOVING and I assume the other gauges work.
Since they all share the same power IN to the circuit board, you have power at the FRONT side of the gauge at LEAST.

Power goes from the PINK wire in the cluster connector on the back of the dash through the printed circuit board through the gauge to the brown wire, to the sending unit and finally grounds at the front side of the trunk. Small black wire.

Again, With the gauge hooked up under the dash with the big multi plug connector, if you disconnect the tan wire in the trunk you will get a FULL reading.
Ground that wire to the frame and you should get EMPTY.
A 90 ohm resistor from the tan wire to the frame in the trunk should read full.

Sounds like YOUR going to get full even when grounded.
Bad wire or somewhere in the gauge cluster.

You should show voltage across the gauge/resistor and current coming OUT

Hope all that gets you somewhere.
And again, if you check your resistor on the back of the gauge "in hand" disconnected from the gauge let me know what you get.

Good luck


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## rlslavik (Jun 21, 2014)

Quick update.
I just wired a 26 and 64 ohm resistor together, hooked them across the back of the fuel gauges with the original one removed and the gauge reads perfect FULL with my sending unit pulled to the top.
Empty when pulled to the bottom.
Yea,
Success


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## wishihaditback (Jun 9, 2011)

Thanks for your reply. Due to other issues i'm in the process of replacing the tank and of course will install a new sending unit and see what happens. Probably nothing as my problem may be forward of that, if i'm reading your response correctly. I'll post that result when i've changed the tank and sender out. Also the car only has the gas gauge....idiot lights for the others. Thanks again.


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## rlslavik (Jun 21, 2014)

If you have power to ANY gauges or idiot lights, you have power to ALL.

Not sure if the GEN light is included in that - don't remember.
Probably IS.


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