Fuel gauge power source?

[CURSED GEARS]

Hello. Just finishing a 7 year rebuild on my 1970 C10. I got it running and I'm trying to button up the gremlins. While test driving it, a new fusible link blew, stranding me on the highway. Prior to the link blowing, I noticed the fuel gauge had stopped working. My mechanic told me I used to small of a wire for the fusible link, and that is why it blew.
Once we hooked up the correct size wire, I drove home and went to see why the fuel gauge wasn't working. On the fuse block where the wire to the tank connects, has no power, where it did before. I checked the pink wire that plugs into the dash panel, and it has power.
I guess my question is two fold. On the back of the fuse panel where the gas tank wire goes, is a brown wire. No power on the backside either. The questions are, where does the brown wire and the pink wire meet? Where is the end of the brown wire go? The end of course, is in the fuse block. Why does the fuse block no longer have power at the fuel connecter?
I completely went through the wiring harness while the truck was apart, fixing wires that was questionable, or hacked from previous owners. I also switched from idiot lights to gauges. I got 99 percent of my information from this site. Repinned everything. It all works with the exception of the alternator, as I'm waiting to purchase o shunt from Stewart Warner next month when they start selling them again.
Any help would be appreciated.

[Tomchev]

The wire from the fuel sending unit goes to one side of fuel gauge. 12v goes to the other side of gauge. The fusible link are rate by color of wire.

[CURSED GEARS]

Thanks for the reply Tomchev. I may have muddied the waters with the fusible link side of the story. To be clear, gauge was working, then it wasn't. I wired the new Boyd tank as follows, Sender unit was grounded, new wire from the positive side of the sender, to the fuse block, where the positive wire for the tank is located. My question was (maybe unclear with all of my babbble) is the terminal for the tank on the fuse block had electricity, now it does not. Does the pink wire feed the terminal at the fuse block some how? I hooked the wire from the tank to the pink wire, and the tank worked again, but I want it wired correctly, but not sure how the power to the fuse block is fed.

[BigBird05]

Unplug and ground the tan wire at the fuel tank. The gauge should either go to fuel or empty. ( forget which ) If it does either your sender is bad or the ground at your tank is bad. Run a ground from your sender to the - side of your battery. If the sender works you need to fix the ground wire.

[dmjlambert]

I suggest post pictures of what you have and ask questions. Hooking up a pink wire to the fuel tank and having that result in the tank worked again is confusing to me. I don't recommend hooking up a wire with power on it to the wire going to the fuel tank.

I think maybe in the far past color of fusible link wire may have determined how big it was or how much current it passed, but the various size fusible link I see in O'Reilly is grey. You need to pick the size fusible link that is 2 wire sizes below (which is 4 wire numbers higher) than the wire in your truck you want to protect. For stock wiring, the unfused wiring going between the battery and the cab is 12 gauge, so you would put 16 gauge fusible link at the battery to protect all that 12 gauge wire.

Normally there is a 20 gauge pink wire that goes from the fuse block to the instrument cluster. That is fused power, so if it is not powering the instrument cluster temp and fuel gauges and brake light, check the fuse. Then coming out from the instrument cluster is the 20 gauge tan wire that goes the fuse block to the "fuel ga." terminal. This terminal does not really belong on the fuse panel as it does not supply "power" to anything. It is simply a convenient place to plug in the wire going to the fuel sender on the fuel tank. Some engineer in 1967 thought it was a good idea, though. Anyway it is there. it does not "power" your fuel tank. The wire you plug in there goes to a variable resistor in the tank and gives a variable amount of resistance to ground to make the gauge move. When the tank is almost empty it has very little resistance to ground. When the tank is full, it gives 90 ohms resistance to ground. The sender is normally a 0-90 ohm unit in order to work with the stock fuel gauge.

[CURSED GEARS]

Originally Posted by dmjlambert

I suggest post pictures of what you have and ask questions. Hooking up a pink wire to the fuel tank and having that result in the tank worked again is confusing to me. I don't recommend hooking up a wire with power on it to the wire going to the fuel tank.

I think maybe in the far past color of fusible link wire may have determined how big it was or how much current it passed, but the various size fusible link I see in O'Reilly is grey. You need to pick the size fusible link that is 2 wire sizes below (which is 4 wire numbers higher) than the wire in your truck you want to protect. For stock wiring, the unfused wiring going between the battery and the cab is 12 gauge, so you would put 16 gauge fusible link at the battery to protect all that 12 gauge wire.

Normally there is a 20 gauge pink wire that goes from the fuse block to the instrument cluster. That is fused power, so if it is not powering the instrument cluster temp and fuel gauges and brake light, check the fuse. Then coming out from the instrument cluster is the 20 gauge tan wire that goes the fuse block to the "fuel ga." terminal. This terminal does not really belong on the fuse panel as it does not supply "power" to anything. It is simply a convenient place to plug in the wire going to the fuel sender on the fuel tank. Some engineer in 1967 thought it was a good idea, though. Anyway it is there. it does not "power" your fuel tank. The wire you plug in there goes to a variable resistor in the tank and gives a variable amount of resistance to ground to make the gauge move. When the tank is almost empty it has very little resistance to ground. When the tank is full, it gives 90 ohms resistance to ground. The sender is normally a 0-90 ohm unit in order to work with the stock fuel gauge.

I'll have to study on this and let it sink in. I'm not sure what I'd take pictures of that you guys haven't already seen in your own trucks. A fuse block, a plug in for the gauge cluster, ETC. I owned this truck for about twenty years and the fuel gauge always worked. I wired it back the same way it was before I tore the whole truck apart. the only difference this time is
I now have a new fuel gauge sending unit in a new tank, and a different fuel gauge. It is not working now after working before.
Is there a schematic that shows the wiring in its entirety that I could look at? i guess like so many others, electricity is my kryptonite.
I really appreciate all you folks trying to help.
One last thing. I will say that before, when I had the stock gas tank in the cab, I had a tan, or brown wire running to the fuse block to the place where the wire is suppose to go. After the rebuild, I bought a Boyd tank and put it in the back of the frame. I then made a new wire out of the same gauge, and ran it from the fuse block, to the new sending unit. At first, it didn't work, and I brought out the trouble shooting light to find the problem. I tested the fuse block, and It had power to the connection, though the light in the trouble light was dimmer then the other connections. I asked my go to mechanic, who said that was normal. Later, while researching this problem here, ran across a post where the poster mentioned the same result when he took a trouble light to his fuse block. Later, while retesting after it stopped working, the connecter on the fuse block no longer had power.
That is about the extent that I know about my problem. To my way of thinking in my non-electrical mind, is the gauge is now no longer working because the fuse block where the gas tank wire attaches, is no longer charged. May be thinking wrong, but there it is.
Thanks again.

[dmjlambert]

Here is the wiring diagram
http://67-72chevytrucks.com/vboard/s...27#post9186727
Instrument cluster diagram
http://67-72chevytrucks.com/vboard/s...d.php?t=709847
Fuel gauge testing/troubleshooting
https://67-72chevytrucks.com/vboard/...d.php?t=640615

[Sheepdip]

The wire going to your tank sending unit is a ground wire. It will hook to the (-) side of the gauge.

The mounting flange of the sending unit that bolts to the top of your gas tank should also have a good grounding wire to the frame. A wire from one of those little 5+/- mounting screws to the frame is how it is usually done. If it's the stock tank it's grounded thru the mounting bolts already.

The sending unit in your tank is nothing more than a rheostat that adjust/scales grounding ohms to the gauge to go from empty to full and everything in between.

Your gauge and sender must be a match for ohms of resistance.

Hope this helps

[CURSED GEARS]

Thanks for all of the post guys. When its all hooked up it reads quarter of a tank, and 3 quarters when the sending unit wire is off. I know the answer is in here in this thread, but I'm going to work on something else for a while and get some distance from the problem for a while. There is a show in two weeks and I was hoping to make it. Still got a lot to do before its show ready, (At least for me).
Thanks again. And I appreciate the schematics. Most of the others I seen were blurry, and these are nice and large and clear.

[MARKDTN]

Originally Posted by Sheepdip

The wire going to your tank sending unit is a ground wire. It will hook to the (-) side of the gauge.

The mounting flange of the sending unit that bolts to the top of your gas tank should also have a good grounding wire to the frame. A wire from one of those little 5+/- mounting screws to the frame is how it is usually done. If it's the stock tank it's grounded thru the mounting bolts already.

The sending unit in your tank is nothing more than a rheostat that adjust/scales grounding ohms to the gauge to go from empty to full and everything in between.

Your gauge and sender must be a match for ohms of resistance.

Hope this helps

This! In GM wiring, Pink is usually ignition switched. So if you are using aftermarket gauges you will have pink (ign switched) to the + side of the gauge and from the sending unit to the - side of the gauge. The sending unit should also have a chassis ground. The only reason to have hot to the sending unit would be if you had a fuel pump in the tank.

[Tomchev]

Terminal 3 purple wire is 12 volts to instrument panel even though is labeled fuel gauge. Terminal 4 tan wire is from sending unit it is not fused that goes to one side of fuel gauge.

[dmjlambert]

Yes, except 20P on terminal 3, the P means pink not purple.