Fuel Gauge and Voltage Wiring

[SheHlpd]

Looking for some help. I have a '49 3600 with the 235ci and it's been converted to a 12v system. I am using the painless 21 Circuit wiring harness with stock gauges. I have burnt up two wires to the fuel gauge. I thought it was because of the size wire I used but then went from an 18ga wire to a 14ga and that burnt up too. Almost started a fire.
I have the negative side of the fuel gauge (the top nut) running to the sending unit and the sending unit is grounded. I connected the right nut (as you're looking at it from the back of the gauge) to the ignition with a jumper wire to the voltage meter. I connected the battery (ignition is off) and the only wire that fries was the one coming from the ignition to the fuel gauge positive terminal.
I have since disconnected the battery and removed the wire.
What am I doing wrong or what am missing?

 

[Bill Hanlon]

The original gauge and wiring was connected like this (using a wiring diagram from this '50 Chevy truck https://chevy.oldcarmanualproject.com/electrical/wiring/pdf/50truck.pdf )

The terminal on the back near the "E" end of the gauge was connected to the "on" terminal of the ignition switch.
The terminal on the back near the "F" end of the gauge was connected to the sending unit on top of the gas tank.
The original wiring was 16ga and was likely much larger than actually needed for the amount of current in the circuit.

Some people have posted that you need a 12v to 6v reducer for the gas gauge circuit between power and the "E" end of the gauge. Our host offers https://classicparts.com/1947-54-gas-gauge-voltage-reducer Others say you can use the gas gauge on 12 volts without the reducer, but once used on 12 volts it will never work properly on 6 volts again. I've never tried either. By the way, the reducer won't fix your smoking problem.

I'd:
1. remove the wire from the "E" end of the gauge, keeping it near to its original position.
2. turn on the key for a few seconds.
If smoke is detected the problem is not with the gauge, gauge to sending unit wiring or the sending unit.

Post a wiring diagram of the Painless kit and I might be able to give further troubleshooting advice.

Question: You said "I have burnt up two wires to the fuel gauge." Where did the wires "burn"?

 

[Bill Hanlon]

I should have looked more carefully at your picture.

It looks to me like
1. your gas gauge gets its power from the + side of the ammeter via the "voltage jumper"
2. the - side of your ammeter is connected to ground (WRONG!, would cause smoke if the + side had 12v as it should, but ..... )
3. .... there is no 12 volt source for the + side of the ammeter.

 

[SheHlpd]

The original gauge and wiring was connected like this (using a wiring diagram from this '50 Chevy truck https://chevy.oldcarmanualproject.com/electrical/wiring/pdf/50truck.pdf )

The terminal on the back near the "E" end of the gauge was connected to the "on" terminal of the ignition switch.
The terminal on the back near the "F" end of the gauge was connected to the sending unit on top of the gas tank.
The original wiring was 16ga and was likely much larger than actually needed for the amount of current in the circuit.

Some people have posted that you need a 12v to 6v reducer for the gas gauge circuit between power and the "E" end of the gauge. Our host offers https://classicparts.com/1947-54-gas-gauge-voltage-reducer Others say you can use the gas gauge on 12 volts without the reducer, but once used on 12 volts it will never work properly on 6 volts again. I've never tried either. By the way, the reducer won't fix your smoking problem.

I'd:
1. remove the wire from the "E" end of the gauge, keeping it near to its original position.
2. turn on the key for a few seconds.
If smoke is detected the problem is not with the gauge, gauge to sending unit wiring or the sending unit.

Post a wiring diagram of the Painless kit and I might be able to give further troubleshooting advice.

Question: You said "I have burnt up two wires to the fuel gauge." Where did the wires "burn"?

The wire burnt all the way from the ignition to the fuel gauge.
The second time it arced and took out a gauge light.
The photo is how it looks now with no wire from the ignition to the gauge.

 

[SheHlpd]

I should have looked more carefully at you picture.

It looks to me like
1. your gas gauge gets its power from the + side of the ammeter via the "voltage jumper"
2. the - side of your ammeter is connected to ground (WRONG!, would cause smoke if the + side had 12v as it should, but ..... )
3. .... there is no 12 volt source for the + side of the ammeter.

When you say "2. the - side of your ammeter is connected to ground (WRONG!, would cause smoke if the + side had 12v as it should, but ..... )"
Where should the (-) Negative wire go? Shouldn't it connect to ground?

 

[Bill Hanlon]

When you say "2. the - side of your ammeter is connected to ground (WRONG!, would cause smoke if the + side had 12v as it should, but ..... )"
Where should the (-) Negative wire go? Shouldn't it connect to ground?

NO, not to ground.

It goes to supply anything that should be measured by the ammeter. In the original wiring diagram (see link above) it feeds voltage to everything except the horn and starter. Having this wire grounded WILL CAUSE SMOKE.

Post your Painless wiring diagram (or a link to it) and I'll try to see where the - terminal of the ammeter should go.

 

[SheHlpd]

NO, not to ground.

It goes to supply anything that should be measured by the ammeter. In the original wiring diagram (see link above) it feeds voltage to everything except the horn and starter. Having this wire grounded WILL CAUSE SMOKE.

Post your Painless wiring diagram (or a link to it) and I'll try to see where the - terminal of the ammeter should go.

Thank you for the all of the help. Here is the link to the Painless wiring kit that I am using.
https://painlessperformance.com/wp-content/uploads/2024/09/90625.pdf

I am trying to get power to the fuel gauge without burning up anymore wires or worse, set fire to the truck and the garage.

Should I remove the jumper from the (+) fuel gauge to the (+) ammeter?

You said the What should I put to the (-) negative post of the ammeter? Maybe the headlight switch?

 

[Bill Hanlon]

By grounding the - side of the ammeter you have a lot bigger problem than just with the fuel gauge. The reason the fuel gauge wire burned was because it was the smallest gauge wire in the un-fused offending circuit.

Too bad that Painless does not have an actual wiring diagram for me to look at. If so, I could probably tell you how to wire in the ammeter. Way down in the fine print of their instructions I found the following: "This harness will not support the use of a factory or aftermarket Ammeter."

 

[SheHlpd]

By grounding the - side of the ammeter you have a lot bigger problem than just with the fuel gauge. The reason the fuel gauge wire burned was because it was the smallest gauge wire in the un-fused offending circuit.

Too bad that Painless does not have an actual wiring diagram for me to look at. If so, I could probably tell you how to wire in the ammeter. Way down in the fine print of their instructions I found the following: "This harness will not support the use of a factory or aftermarket Ammeter."

Thank you for the help. I called Painless and they directed me to the proper wiring process (gauge power from the fuse box through fused wire 935) and I connected the (-) negative to the headlight switch ammeter spot. The only thing left is to figure out what's up with fuel gauge. The ammeter moves when the key is turned on but the fuel gauge still doesn't work.

As with any project, onto the next issue.

 

[Bill Hanlon]

I don't have a wiring diagram of your setup, but I'll bet your ammeter will only show discharge where it is hooked into the circuit.

A very basic drawing of an ammeter should look like this:

Clipboard04

• Bill Hanlon
• Apr 22, 2025

My "unswitched load" would be equivalent to the Painless fuse box "Battery Constant Power" section.
My "switched load" would be equivalent to the Painless fuse box "Ignition Switched Power" section.