Battery amp meter.

[IOWNATRUCK]

My battery amp meter suddenly stopped functioning and now it stays on 8 amps all the time, any common problems these things have or anything to check for?
thanks

[INSIDIOUS '86]

I would suggest changeing from an ammeter to a voltameter

[Corts60]

Not to offend your intelligence but did you check to see that the alternator is still charging?

[IOWNATRUCK]

Haha, yes, the alternator is charging , its brand new as is the battery, it starts and runs no problem, even if the alternator wasnt charging, the ammeter would still vary a little from just when the battery is on from the key, and when it was starting and running and shut off.

[Goldfinger]

I'd be cautious with that ammeter as i recall I've heard them catch on fire because they were shorts waiting to happen. That could be a possible reason for it not working correctly its losing power over the wires somehow. have you added extra's to the battery cable?

[IOWNATRUCK]

no nothing extra, thanks for the heads up, il tear it out tomorrow and check all the wires and stuff. thanks everyone.

[Gregski]

I know this is a three year old thread but really hate abandoned threads and don't need to start yet another one on the same exact topic

[ray_mcavoy]

Originally Posted by Gregski

I know this is a three year old thread but really hate abandoned threads and don't need to start yet another one on the same exact topic

In reading through the old posts, it appears as though there may have been some confusion as to what type of gauge was in IOWNATRUCK's truck.

The ammeters used in 73-75 trucks are marked D for discharge and C for charge ... no numbers on the face. Unlike the full-flow type ammeters used in some other vehicles, these ammeters are no more of a fire hazard than a voltmeter. That's because they allow nearly all of the current to flow through the charging system wiring (same as it would in a truck without gauges or with a voltmeter). Only a small fraction of the current flows through the ammeter itself. And the meter & wiring are protected by a couple of inline 4 amp fuses. Those fuses (located in the engine compartment along the firewall) are the first things to check if you have a 73-75 truck with an inoperative ammeter.

The voltmeters used in 76+ trucks are marked off at 8, 13, and 18 Volt increments. When IOWNATRUCK mentioned his being stuck on 8, that leads me to believe his truck actually has a voltmeter instead of an ammeter.

To test the voltmeter, remove the dash bezel, the clear lens, and "front tin" from the instrument cluster. Then remove the 2 screws holding the voltmeter in place and pull it straight out of the cluster.

With the voltmeter removed, use a test light or multimeter to check the power & ground terminals in the cluster. The upper terminal/clip is the voltmeter's ground and the terminal/clip that's down and off to the side a little is the voltmeter's power. No power and/or ground here is usually the result of a bad connection. The most likely places to start looking for bad connections are where these terminal/clips make contact with the printed circuit on the back of the cluster and where the cluster plug connects to the printed circuit.

If the power & ground terminals in the cluster check out okay, move on to testing the voltmeter itself.



Use a multimeter (set to Ohms) to measure the resistance between the studs marked A and C. You should get a reading somewhere around 127Ω. If you get an open-circuit reading, the resistor (white ceramic piece) is either bad or not making good contact with the nuts that secure it in place (be careful not to over-tighten the nuts).

Next, measure the resistance between the studs marked B and C. This should be somewhere around 122Ω. An open circuit here means the voltmeter's internal coil is bad and you'll have to replace the meter assembly.

[Gregski]

Originally Posted by ray_mcavoy

The ammeters used in 73-75 trucks are marked D for discharge and C for charge ...

man I could stop reading here and die a happy man having to learned something already, I bet many of us would have bet the farm believing the D stood for Direct and the C for Current at least or maybe that the trucks were customized for Malcolm Young and Phil Rudd, jk

Originally Posted by ray_mcavoy

Unlike the full-flow type ammeters used in some other vehicles, these ammeters are no more of a fire hazard than a voltmeter. That's because they allow nearly all of the current to flow through the charging system wiring (same as it would in a truck without gauges or with a voltmeter). Only a small fraction of the current flows through the ammeter itself. And the meter & wiring are protected by a couple of inline 4 amp fuses.

Thank you, all you have to do is look at how the ammmeter is connected in the cluster and you would realize that there is absolutely no way it could rely on those skinny circuit board connections to carry the full current of the vehicle, which is how ammeters usually work, a fat wire runs to them, in our case these connect to a circuit board which in turn is connected to a skinny multi fused black wire

thanks once again, now it's time for me to get it to work knowing it's NOT going to catch my '74 rig on fire, Myth Busted!!!

[ray_mcavoy]

You're welcome Gregski! Sorry, it wasn't a rare customization made for AC/DC

You're right, just looking at the printed circuit it should be pretty obvious that the meter doesn't handle the full charging system current. It's basically an external shunt ammeter but instead of using an actual resistor for the shunt, they simply use the inherent resistance in the charging system wiring.

GM started using this external shunt ammeter design in trucks back in the early 60's. Some years did have issues with melted/burnt wiring simply because the ammeter circuit was left un-fused. The fuses became standard issue in 1967 & newer trucks with factory gauge clusters and they protect the wiring, printed circuit, and meter from two things: (1) A short to ground on either leg of the circuit, and (2) an open circuit in the charging system wire that serves as the shunt. An open circuit in the shunt wire would basically put the meter in a full-flow configuration but the fuses will blow before a damaging amount of current can flow through the circuit.

So the factory ammeters are safe as long as the 4 amp fuses are present in the original locations.

On a somewhat related note, the 73-75 ammeter clusters can be dangerous if one is accidentally plugged into a 76+ truck that is wired for a voltmeter. That's because the ammeter (which has a very low internal resistance) basically ends up shorting together the +12V and ground terminals intended for the voltmeter.


In my earlier reply, I didn't go into very much detail on troubleshooting the 73-75 ammeter other than mentioning to check the fuses. The procedure is similar to what I posted for the 76+ voltmeter except for the following differences:

(1) When you remove the meter test the terminals in the cluster, both ammeter terminals should show full system voltage.

(2) The ammeter just has 2 studs on the back



and unlike the voltmeter, the ceramic block is just an insulator ... not a resistor.
The ammeter's internal coil should measure very low resistance ... only about 0.1 to 0.2Ω.

[MikeB]

Yes, it's a voltmeter with zero volts in the center. It responds to battery charge and discharge current, hence the + and - amps. As I recall, it's connected across a shunt resistor (OK, the actual wiring resistance). The higher the current, the more the voltage drop, and the more the pointer moves in one direction or the other. The wiring diagram in a Haynes manual will show the circuit.

[enaberif]

Question regardin the ammeter. Could I pull it out and pull the fuses from the 2 wires running along the firewall and everything still work ok? I want to rip out all the wiring and everything associated with the ammeter and go to volt meter.

[ray_mcavoy]

Originally Posted by enaberif

Question regardin the ammeter. Could I pull it out and pull the fuses from the 2 wires running along the firewall and everything still work ok? I want to rip out all the wiring and everything associated with the ammeter and go to volt meter.

Yes, you could pull both fuses for the ammeter and all of the other gauges in the cluster will continue to operate normally.

To change over to a voltmeter you'll have to change the connections to pins #3 and #4 of your cluster plug. The voltmeter needs a ground on pin #3 and system voltage on pin #4.

A voltmeter won't be a direct fit into a cluster that originally had an ammeter ... the stud/terminal layout on the back is slightly different. I think it could be made to work with some minor modifications though. The other alternative would be to swap to a 76+ cluster that already has a voltmeter. Just be aware that you might have to make additional changes to the pin-out of the cluster connector (especially if you swap to a 78+ cluster with an electric oil pressure gauge).

[Gregski]

Originally Posted by ray_mcavoy

In my earlier reply, I didn't go into very much detail on troubleshooting the 73-75 ammeter other than mentioning to check the fuses. The procedure is similar to what I posted for the 76+ voltmeter except for the following differences:

(1) When you remove the meter test the terminals in the cluster, both ammeter terminals should show full system voltage.

(2) The ammeter just has 2 studs on the back



and unlike the voltmeter, the ceramic block is just an insulator ... not a resistor.
The ammeter's internal coil should measure very low resistance ... only about 0.1 to 0.2Ω.

Ray do you realize how helpful you are being right now, you are charting on the Gold Star territory. Also your photobucket rocks!

[enaberif]

Originally Posted by ray_mcavoy

Yes, you could pull both fuses for the ammeter and all of the other gauges in the cluster will continue to operate normally.

To change over to a voltmeter you'll have to change the connections to pins #3 and #4 of your cluster plug. The voltmeter needs a ground on pin #3 and system voltage on pin #4.

A voltmeter won't be a direct fit into a cluster that originally had an ammeter ... the stud/terminal layout on the back is slightly different. I think it could be made to work with some minor modifications though. The other alternative would be to swap to a 76+ cluster that already has a voltmeter. Just be aware that you might have to make additional changes to the pin-out of the cluster connector (especially if you swap to a 78+ cluster with an electric oil pressure gauge).

Well I know 2 more wires I can pull out of the bulkhead connector Thanks