58 chevy truck amp gauge wiring help needed

[TA1754]

I've got the entire gauge cluster out of my '58 chevy stepside & just installed a new fuel gauge.
When I removed the 2 wires on back of the amp gauge, they weren't marked.
I connected power back to the battery, checked each wire, & both have constant 12 v. to ground.
I called the shop that rebuilt my speedometer couple yrs ago, he told me only one should be constant hot, 2nd wire should be load going to lights etc.
Running a 77 350 engine, & complete drive train is from a 77 malibu.
Can't recall ever seeing the gauge needle reading wrong when it was hooked up.
Need some advise.

[VetteVet]

Originally Posted by TA1754

I've got the entire gauge cluster out of my '58 chevy stepside & just installed a new fuel gauge.
When I removed the 2 wires on back of the amp gauge, they weren't marked.
I connected power back to the battery, checked each wire, & both have constant 12 v. to ground.
I called the shop that rebuilt my speedometer couple yrs ago, he told me only one should be constant hot, 2nd wire should be load going to lights etc.
Running a 77 350 engine, & complete drive train is from a 77 malibu.
Can't recall ever seeing the gauge needle reading wrong when it was hooked up.
Need some advise.

Both wires are positive since it measures the amperage going through the positive side of the circuit. You are reading the normal voltage potential between positive and ground, and stating that they are constant hot to ground sounds like they are shorted to ground, which is not what you meant.

Prior to 1963 all the positive current ran through the Amp meter which only read up to 60 amps. GM decided to go to a battery gauge which only measures a tiny current that shows whether the alternator is charging or not.
It also reduces the fire hazard from a short with this type meter, since it is fused only with a fusible link at the battery.

This is your wiring diagram and not knowing which wire goes to which side of the ampmeter only means that the meter will read backwards showing negative amperage (discharge) when it is really charging. You may be able to read the diagram and figure out which of your wires goes where.



You probably have a generator instead of an alternator as shown in the diagram but the general wiring is the same.

[TA1754]

Originally Posted by VetteVet

Both wires are positive since it measures the amperage going through the positive side of the circuit. You are reading the normal voltage potential between positive and ground, and stating that they are constant hot to ground sounds like they are shorted to ground, which is not what you meant.

Prior to 1963 all the positive current ran through the Amp meter which only read up to 60 amps. GM decided to go to a battery gauge which only measures a tiny current that shows whether the alternator is charging or not.
It also reduces the fire hazard from a short with this type meter, since it is fused only with a fusible link at the battery.

This is your wiring diagram and not knowing which wire goes to which side of the ampmeter only means that the meter will read backwards showing negative amperage (discharge) when it is really charging. You may be able to read the diagram and figure out which of your wires goes where.

Attachment 1669754

You probably have a generator instead of an alternator as shown in the diagram but the general wiring is the same.

I appreciate your input, I have an alternator, as previously stated when I started the post, my entire drive train is from a 77 Chevy malibu.
The 2 wires from back of the gauge are not shorted to ground.
My test leads one on ground, other on a wire is reading 12 volts.
I was told that if I had both wires off the back of the amp gauge that only one wire should have constant 12 volt reading, but I'm reading 12 volts on either wire.
Yesterday I removed the wires at the alternator, & was still reading 12 volts on either wire.

[VetteVet]

Originally Posted by TA1754

I appreciate your input, I have an alternator, as previously stated when I started the post, my entire drive train is from a 77 Chevy malibu.
The 2 wires from back of the gauge are not shorted to ground.
My test leads one on ground, other on a wire is reading 12 volts.
I was told that if I had both wires off the back of the amp gauge that only one wire should have constant 12 volt reading, but I'm reading 12 volts on either wire.
Yesterday I removed the wires at the alternator, & was still reading 12 volts on either wire.

It's quite possible that you have a wire from the battery side (Red) connected to something on the black side without going through the amp meter. Example would be the radio, a cigarette lighter or some light. It wouldn't create a problem unless it was drawing current all the time, in which case it would discharge the battery.
If you have the ampmeter wires off can you get anything to work like the loghts or horn or heater fan. This would help find the current crossover.

Do you have any mix and match from the malibu drive train concerning electrics, like an external voltage regulator, or alternator that would need to be hot all the time with key off?

[TA1754]

Originally Posted by VetteVet

It's quite possible that you have a wire from the battery side (Red) connected to something on the black side without going through the amp meter. Example would be the radio, a cigarette lighter or some light. It wouldn't create a problem unless it was drawing current all the time, in which case it would discharge the battery.
If you have the ampmeter wires off can you get anything to work like the loghts or horn or heater fan. This would help find the current crossover.

Do you have any mix and match from the malibu drive train concerning electrics, like an external voltage regulator, or alternator that would need to be hot all the time with key off?

Checked wiring on back of alternator, constant 12 v. there with key off.
Replaced my fuel gauge, put everything else back together, cranked the engine, amp gauge just stays in the middle, no matter if key is off, or engine running.
I paid somebody to rewire the entire truck over 20 yrs ago when the front clip was done.
Never really paid much attention to the amp gauge or the wiring until now.
Don't think it was ever wired correctly.
Trying decide if I should buy a replacement amp gauge, or find an auto electric specialist to help me out.
Don't think there should always be 12 v constant power @ each of the 2 terminals on back of the amp meter.

[VetteVet]

Originally Posted by TA1754

Checked wiring on back of alternator, constant 12 v. there with key off.
Replaced my fuel gauge, put everything else back together, cranked the engine, amp gauge just stays in the middle, no matter if key is off, or engine running.
I paid somebody to rewire the entire truck over 20 yrs ago when the front clip was done.
Never really paid much attention to the amp gauge or the wiring until now.
Don't think it was ever wired correctly.
Trying decide if I should buy a replacement amp gauge, or find an auto electric specialist to help me out.
Don't think there should always be 12 v constant power @ each of the 2 terminals on back of the amp meter.

You will read 12 volts on each side of the gauge as long as the battery side wire is connected because the gauge terminals are connected together and they read the amperage that flows through it. You should see positive amperage when the alternator is charging and negative amperage with the key off and a draw on the battery, like turning on the headlights or starting the engine. If the gauge isn't reading either way then the circuit current must be bypassing it through another path.