'68 fuel guage troubleshooting...HELP!
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Pro
Joined: Oct 2003
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From: Storm lake Iowa
'68 fuel guage troubleshooting...HELP!
Sorry, I don't even know how to troubleshoot this one. When I purchased the car 9 months ago, the fuel guage "sorta" worked, but never came up to full, and read empty long before the tank was actually near empty. I had the whole interior out this winter, and when I reassembled the fuel guage no longer worked at all. I pulled the instrument cluster yesterday, and can't see anything wrong with the connections to the back. With the car running, I don't read any current whatsoever (with a voltmeter) at the electical contacts on the back of the guage. The only two wires attatched are black and brown; The same black and brown wires that appear to leave the sending unit back below the gas tank. There is no detectable current at the contacts back there either.
How should the system work? Where should the power for the guage originate? I did notice a third post near the back of the fuel guage with nothing attatched, and there is what appears to be some sort of resistor dangling behind the instrument cluster that looks like it should be attatched to an electrical post somewhere, but temporarily fastening it to the stud next to the fuel guage didn't change anything.
where do I go from here?
-Roy
How should the system work? Where should the power for the guage originate? I did notice a third post near the back of the fuel guage with nothing attatched, and there is what appears to be some sort of resistor dangling behind the instrument cluster that looks like it should be attatched to an electrical post somewhere, but temporarily fastening it to the stud next to the fuel guage didn't change anything.
where do I go from here?
-Roy
Team Owner
Joined: Jul 1999
Posts: 26,545
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From: Huntersville NC
I should be able to help you because I have been thru the whole fuel guage thing, but my mind is like a sieve... 
I know the sending unit in the tank changes the resistance as it goes up and down. At the sending unit under the tank the black wire should go directly to a good ground (important!)
I know with mine the wiring harness was so fubared we just ran a new wire from the fuel guage back to the sending unit. I also remember testing the guage by sticking a wooden dowel into the tank, pushing down on the sending unit and watching the resistance change on my ohm-meter. Hopefully someone else can fill in the many blanks.
I know the sending unit in the tank changes the resistance as it goes up and down. At the sending unit under the tank the black wire should go directly to a good ground (important!) I know with mine the wiring harness was so fubared we just ran a new wire from the fuel guage back to the sending unit. I also remember testing the guage by sticking a wooden dowel into the tank, pushing down on the sending unit and watching the resistance change on my ohm-meter. Hopefully someone else can fill in the many blanks.
Drifting
Joined: May 2003
Posts: 1,634
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From: Melbourne, Victoria, Australia, the World
I am pretty sure GM used 90 Ohm senders for the fuel gauge. Resistance varies between 0 and 90 ohms for the full travel of the sender float. One side of the variable resistor is grounded (black wire) and make sure that is a good ground as mentioned before.
The TAN (brown) wire goes directly to the gauge through a multi pin (12 pins) connector behind the dash. On the back of the gauge there is a 2 pin connector with the TAN/brown and a pink wire. The pink has the volts when ignition is switched on at the key. the pink comes from the fuse box.
I think the resistor thingy mentioned is a trim resistor to give exactly a full reading when full.
See if the TAN/brown wire reads between 0 and 90 ohms to ground approximating the level of fuel in the tank. I think the black wire at the gauge is either Bubba at work or someone has accidently put the ground for the cluster on the wrong tag.
Have fun getting all the bulbs back in the cluster!
The TAN (brown) wire goes directly to the gauge through a multi pin (12 pins) connector behind the dash. On the back of the gauge there is a 2 pin connector with the TAN/brown and a pink wire. The pink has the volts when ignition is switched on at the key. the pink comes from the fuse box.
I think the resistor thingy mentioned is a trim resistor to give exactly a full reading when full.
See if the TAN/brown wire reads between 0 and 90 ohms to ground approximating the level of fuel in the tank. I think the black wire at the gauge is either Bubba at work or someone has accidently put the ground for the cluster on the wrong tag.
Have fun getting all the bulbs back in the cluster!
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Pro
Joined: Oct 2003
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From: Storm lake Iowa
Hmmm...
Thanks guys, this is good information...unfortunately it does not seem to match well with what I'm seeing here...maybe, like so many thinks on this car, Bubba'd Beyond Recognition? There is definitely no pink wire showing at the guage. I haven't checked resistance yet...I was hoping to find some voltage somewhere first, on the assumption that there needs to be some juice entering the circuit someplace before resistance is going to make any difference.
I'll check the black wire at the back to be sure it's grounded.
The funny thing is, there is a two prong female connector that hooks to the back of the fuel guage, with BLACK and BROWN wires running away, that appears to be factory made, not Bubba, and the wires are identical in color to the two that leave the sending unit. Where's the pink???
I'll check the black wire at the back to be sure it's grounded.
The funny thing is, there is a two prong female connector that hooks to the back of the fuel guage, with BLACK and BROWN wires running away, that appears to be factory made, not Bubba, and the wires are identical in color to the two that leave the sending unit. Where's the pink???
Drifting
Joined: May 2003
Posts: 1,634
Likes: 2
From: Melbourne, Victoria, Australia, the World
The info I posted was from the AIM and internal memory. I probably need to back that up soon 
I was thinking there might also be pressed into the sheetmetal housing coding which indicates the colors of wires and where they are connected. I am sure they did that on the speedo and Tach. I think the connectors were different so you could not put the wrong one in the right place. Anti bubba at the factory you might say.
Look for the resistance first to confirm you have continuity between the sender and the gauge. Check to see if the black on the gauge is ground as well.
I was thinking there might also be pressed into the sheetmetal housing coding which indicates the colors of wires and where they are connected. I am sure they did that on the speedo and Tach. I think the connectors were different so you could not put the wrong one in the right place. Anti bubba at the factory you might say.
Look for the resistance first to confirm you have continuity between the sender and the gauge. Check to see if the black on the gauge is ground as well.
