Battery/Amp Gauge

[gschroeder]

I have a 1970 CST10 with a stock battery/amp gauge. The gauge needle is basically always right in the middle (between the D and C). If I rev the engine, the needle barely moves toward the C (and I mean barely moves). Is this normal, or do I need to replace my gauge? I would think the needle would move more when the engine revs, lights turned on/off,... I did test the battery and alternator, and both are good. Battery is at 12.5 volts when engine off, and jumps to 14.5 volts when engine/alternator is running. Also, I did confirm the gauge does have power to it.

Thank you

[Accelo]

The gauge actually reads voltage. Millivolts to be exact. A millivolt is one thousandth of a volt. It's a volt gauge that is calibrated and marked in amps. It is in what is called a shun circuit. All the connections have to be perfect for this to work. Both ends of the gauge are hot and both terminate at the same wire just in a different spot on the wire. Confusing isn't it? This is the reason gm with to a volt meter with a higher range actually marked "volts".
None of this matters. All you need to know is the terminals have to be really clean.
One side terminates at the little black connector next to the battery. Pull out the wires and clean them with sandpaper and clean the nut and stud. Cover with grease or something so the corrosion will not come back.
Check the two inline fuses. The connections for this connection have to be spotless. That is why the fuse is fully enclosed from the factory. There are two of them one in each front corner next to the fender and core support. Confirm the fuse is there and clean them till perfect. Then do not run the motor just turn on the high beams and watch the gauge. Most the time you can get them to operate with these simple fixes.

[franken]

The gauge reads a representation of the voltage difference between a spot close to the the battery vs one close to the alternator. If the battery side is higher than the alt side, it shows toward the discharge side. By representation I mean that the gauge is set to read from the center and move left or right depending on which side is more positive. Note that the gauge has no units (other than C and D) and is calibrated not at all.
I'd test by measuring each side to ground with a DVM.
Turn on all electrical items such as lights, heater blower, wiper, dome light, etc. with the engine running and see if the gauge moves.

[gschroeder]

Accelo, thank you for the advice. I confirmed both inline fuses are clean and delivering power to the amp gauge. Only thing odd I noticed is the inline fuse near the battery is 3 amp, and the inline fuse near the alternator is 4 amp. Could this be the problem?

Also, for the heck of it, I disconnected the inline fuse near the battery to see what the amp gauge would do. It did absolutely nothing. Needle still remined perfectly in the middle of D and C and did not move. Shouldn't the amp gauge move when one of the inline fuses is disconnected? Thanks again for your help.

Thank you

[Accelo]

The amperage difference makes no difference. Disconnecting one side doesn't deflect the needle at all.
I would start by confirming there is 12 volts (to ground) on both terminals on the back of the gauge.

[gschroeder]

Accelo, I confirmed there is exactly 12.4 volts (to ground) on both terminals on the back of the amp gauge. Is there anything else I can do to make the needle move? I just want to make sure the amp gauge really works (or is it broke and needs replaced). I tried turning on headlights, blower on high,... Nothing seems to make the needle move. Thanks again for your help. Thank you

[Accelo]

You can take a used 1.5V battery and put it between the terminals. If the gauge deflects it's a good gauge. Remember the gauge is measuring mil-volts and reading in Amps.

I have added fuses to the circuit before and used the old-style fuses with snap together cases.
I had the circuit working and it quit after adding the fuse. I pulled apart the new fuse holder and sanded the brass and the fuse and then the circuit began working again.
To be honest I was about to solder the wire to the fuse to be sure I got a good connection.

GM added fuses to the circuit and they were encased in small rubber holders that fit tight enough to work underwater. I would purchase, and use them, if I knew where to get new ones.

This circuit is tough as most do not appreciate how little resistance it takes to disable this circuit. Testing the circuit like you did was perfect. Will give you a high draw to work with.
If you haven't cleaned the terminals at the horn relay it's a good time to do it. You may even have to replace the terminal at the black connector by the battery. Seems to be a problematic area.
The connections are made as you have the 12V at each terminal. Resistance isn't your friend in this circuit and it's so low it can't be easily measured with a ohm meter.
The diagram is included to show how the circuit works. The connection on our C10's is made at the battery not the starter.

[gschroeder]

Accelo,

Thank you for your reply. I will clean the connections at the horn rely. I will also replace the 4 amp fuse holder near the battery. That fuse holder looks like it was apart at one point, and the clamp on one end that holds the fuse is bent up a little.

For the 1.5V battery test, I want to make sure I understand you correctly. I should remove both wires on the back of the amp gauge, then connect a used 1.5V battery (like a AA battery), to both terminals and look for deflection. If the needle moves, the amp gauge is good. If the needle does not move, the amp gauge is bad. Is my understanding on how to do this correct?

Thank you

[Accelo]

For the 1.5V battery test, I want to make sure I understand you correctly. I should remove both wires on the back of the amp gauge, then connect a used 1.5V battery (like a AA battery), to both terminals and look for deflection. If the needle moves, the amp gauge is good. If the needle does not move, the amp gauge is bad. Is my understanding on how to do this correct?

That is exactly correct. A used battery works better

[gschroeder]

Accelo,

I tested the amp gauge with a AA battery, and confirmed it works OK. I then replaced the 4 amp in-line fuse holder near the battery, and that fixed the problem. The amp gauge now moves back and forth from D to C as it should. Apparently the problem was a poor connection on the fuse holder near the battery. It looked like someone at some point damaged/bent the fuse holder connections which was causing a poor connection. Thank you again for all your help. Thank you

[Accelo]

Everyone loves a success story.

It is really tough to do electrical troubleshooting over the internet. Good Job.

You have a very rare truck. One with a working Amp Gauge!