Electrical: ammeter behavior

[ratskins]

I have a 1951 Starlight Coupe. All electrical items have been rebuilt or replaced except dash gauges. Car is 6 volt positive ground. I'm using a good condition Optima AGM 6 volt battery.

In the old days the ammeter used to show a discharge when operating light or heater, but swung to positive when rpms increased. Years later it would still show discharge, but stayed in the middle when the rpms were increased. Then when you would go over a bump then needle would swing to a significant charge. Lately, it shows a discharge, but doesn't appear to charge at all, or if it does it appears very minimal on the ammeter. Going over a bump doesn't cause a change anymore.

While the engine was running I measured the voltage across the battery terminals with two separate voltmeters. They both showed 7.2 volts. With the car not running the voltage across the battery was 6.6 volts, which seems high, but that is due to the excellent smart charger/desulfator that I keep connected all the time. As an aside, the one I use is made by BatteryMinder and it does a good job of restoring batteries that are sulfated if they are not too far gone. Good at preventing sulfation as well. Note: charger was not attached while measuring battery voltage.

One other thing. I just replaced the water pump and at the time I did not tighten the fan belt very much for fear of damaging the new water pump bearing. Maybe I should tighten that a little more?

One more thing: years ago I had a short that partially melted the plastic insulation on the fat red wire connected to the ammeter. However, car seemed to run OK after that. Is it possible the gauge got damaged?

Any ideas?

[Bud]

If you are showing 7.2 volts at the battery with the engine running above a fast idle, then I suspect your ammeter has had it. Bud

[karterfred88]

I think you're mixing apples and oranges. The gauge, I believe is an AMP gauge not a volt gauge. You seem to be measuring the generator output voltage by your test. Not sure of the best way to test amps but I believe you'll need to put it (the test meter) in line (series) between the generator and the battery, not parallel as you are doing.

[ratskins]

If you are showing 7.2 volts at the battery with the engine running above a fast idle, then I suspect your ammeter has had it. Bud

You raise a good point. I was at a regular idle . . . not a fast idle. Maybe I should redo that test.

UPDATE: redid the test at a faster idle. Voltage was 7.28. Factory manual says regulator output is between 7.2 and 7.6 so we are OK there.

[ratskins]

I think you're mixing apples and oranges. The gauge, I believe is an AMP gauge not a volt gauge. You seem to be measuring the generator output voltage by your test. Not sure of the best way to test amps but I believe you'll need to put it (the test meter) in line (series) between the generator and the battery, not parallel as you are doing.

My thought with the voltage test was that if no charging was happening then the voltage would be the same as the battery voltage when the car was not running. But the voltage was higher, so it theoretically it was possible to pump some current back into the battery.

[ratskins]

UPDATE:
Now ammeter does not show movement in either direction. Car still starts, runs, and drives great. Voltage across battery still looks good. Looked inside regulator and did not see any problems with pitting or burned points or discolored coils. Leaning towards bad ammeter. Your thoughts?

[TWChamp]

Check the battery voltage with the engine off, then again with the engine running about 1000 RPM, then a third time with the lights on and engine at 1000 RPM.

If the second two checks show more voltage than the first check of battery voltage, then you are good to go.

The ammeter needle won't move much for a fully charged battery, so if you are wondering about the meter, then crank the engine with the key off for 10 seconds and repeat the above checks. Or you can turn on the headlights for a minute to drain the battery some and repeat the above checks.

[ratskins]

UPDATE!
I drove the car to a battery shop and they tested the charging system. They tell me the charging system is working "great". The gauge is now completely non-functional. My only question now is whether there is possibly some kind of shunt resistor that could have gone bad. If one of those isn't used with this car then it has to be a bad gauge.

[52-fan]

I don't think you will find any kind of resistor in the system. The ammeter is wired direct to the source.

[karterfred88]

UPDATE!
I drove the car to a battery shop and they tested the charging system. They tell me the charging system is working "great". The gauge is now completely non-functional. My only question now is whether there is possibly some kind of shunt resistor that could have gone bad. If one of those isn't used with this car then it has to be a bad gauge.

Turn on all your electrical items, pull the coil wire and crank it as you do watch the gauge, it should drop below zero and indicate amp draw. If it don't move you will have a better idea-check wires for a break, or replace the gauge. Disconnect the wires to the gauge, and use you meter to test for amps there as you crank, doesn't show anything-wiring problem, shows on meter, gauge shot. The coils on the internal magnets may be shot. As long as the current goes through it, your car should run fine.

[52-fan]

You shouldn't have to crank the engine. Any electrical device turned on should be drawing through the ammeter. The cigar lighter is the only exception I can think of off hand.

[TWChamp]

Just turn the lights on with the engine off and see if the ammeter shows about a 15 amp drain. The starter current does not go through the meter.

[Bud]

There is a shunt inside of the ammeter which is still good or nothing electrical in the car will work as everything except the starter, horn and the cigar lighter work through the ammeter. An ammeter is really a volt meter as the meter reads the voltage drop across the internal shunt in the ammeter. Like I said in my first post, if the charging system is working normally, then the ammeter is bad and needs to be replaced. Bud

[ratskins]

There is a shunt inside of the ammeter which is still good or nothing electrical in the car will work as everything except the starter, horn and the cigar lighter work through the ammeter. An ammeter is really a volt meter as the meter reads the voltage drop across the internal shunt in the ammeter. Like I said in my first post, if the charging system is working normally, then the ammeter is bad and needs to be replaced. Bud

This makes the most sense to me. I have an ammeter on order and once installed I'll post a followup, which will hopefully be helpful to others with this issue.

[ratskins]

UPDATE:
I put an NOS ammeter in and everything works fine now. I had an auto electric shop do it along with some other misc things.

Fan belt tension was OK. I set it with a spring scale used for weighing suitcases that I picked up as Ross for $6. Fourteen lbs at the center of the fan belt should produce around a 1/2 inch deflection, according to the original Stude service manual.