Need help with gas gauge

[Chevyguy111786]

i was looking for the brown wire and havent been able to find it. but then...out of nowhere appears a light green or very very light brown wire coming from behind my seat. it's connected to something behind my seat but nothing in front of my seat. what's this for? i'm gonna take my seat out tomorrow to trace it. but if anyone has any ideas let me know...... cory

[ddsmith]

I'll just throw this out for discussion. The sending unit could be the type which has higher resistance. The aftermarket gage could be designed to work with the sending unit. An example is the the stock sending unit on my 66 C30 has 0 Ohms when empty and indicates about 35 Ohms when the float is against the upper stop. Aftermarket sending units frequently have 45 or 90 Ohms resistance when at the full stop. If you assume a linear resistance throughout the float travel and you have a 90 Ohm sending unit your gas gage will indicate full until you are about 1/3 of a tank of gas left and it will drop like a rock after that. It's all a matter of current flow through the windings on the gage. Power comes to the gage from the fuse block and leaves the gage running through the brown wire under the driver side sill area and connects to the sending unit. In the sending unit (which is a variable resistor) the float level determines resistance of the variable resistor in the circuit. Then the current flows to ground via the sending unit ground wire. If the wire from the gage to the sending unit is shorted to ground the gas gage will indicate empty because it would be the same as have 0 Ohms resistance in the sending unit. If the brown wire is broken the gage will indicate full all the time. If the sending unit is incorrect resistance value the gage will stay on full for a long time and then drop like a rock when the level in the tank is 1/3 full if the sending unit is a 90 Ohm unit or a 45 Ohm unit will cause full indication until the tank level is about 2/3rds of a tank and will go down faster than normal. No power to the gage should cause a full indication because of no current flow. An easy way to determine what is the problem would be to disconnect the brown wire to the sending unit. Take this lead and ground it to a known good ground. The gas gage should indicate Empty. Unground the wire and the gage should show Full on the gage. If the gage does not respond then the problem is upstream. You can rule out the brown wire by disconnecting the wire from the gage and connect a jumper wire from the connector on the gage to ground. Does the gage indicate Empty? If you get no response then it is either gage or power feed. The gage can be checked with an Ohm meter. It should show almost 0 Ohms resistance. If resistance is infinite the gage is shot. I'm pretty sure this is not to hard to repair. I don't know if it's worth the time but I've repaired similar gage problems in the past. As a matter of fact this is also how the gage could be modified to work with a different resistance value sending units. Someone also posted about using a resistor in parallel to provide proper indication. Sorry this was so long winded and anyone knowing anything different please post it. I'm just putting out what I've learned from many years of working on GM products and not specifically 67-72 Chevy or GMC trucks. Good luck.

Don

[9teen69]

ddsmith- Thanks for the help, there's a lot of good info there.

I can't get any kind of response from my gauge -its sitting way past empty. All the other gauges I tried didn't move either. Unfortunetly I don't have the other gauges handy now to test grounding the brown wire.

I don't know if this has anything to do with it. I found a black wire with a fuse inside a black rubber boot near the battery. Any idea what this is for?

[68 Stepside]

The black wire with the fuse is for your factory amp meter. There are actually 2 fuses. On the drivers side, tucked up under there's one as well.

Sorry I can't help with the gas gauge.

TJ

[72LongHorn402]

This may or may not help. But My gas gauge, and ampmeter were not working and one day while trying to locate a short going to the starter i found two small fuses along the radiator support. One in each corner, ( if I had a digital camera I would post pic), about a half inch long in a water proof cover. I replaced those two and gauges started working! Hope this helps.

[gmc67swb]

you could connect your gauge direct and eliminate any possible problems with the printed circuit. there are two nuts on the back of the gauge you could connect to. mine is that way right now. i did it when i was trouble shooting mine and never moved it back.

[Jim68]

sometimes it can be something as simple as the inst cluster feed fuse, that fixed my guage problems.

[9teen69]

I fixed it! You were right Jim68! It was the fuse - it looked good but the broken wire was hidden behind a purple band on the fuse. Uuuugh!

Thanks for the all the help from everyone!

[Chevyguy111786]

I FIXED MINE TOO! it was the brown wire...it had come disconnected somewhere....now i have an 1/8th tank...better go fill up.