I have a '69 with a factory ammeter on the console. I've noticed it never reads more than about 5 amps charge or discharger. I just installed a new 100amp alternator, and was a little concerned by because it only reads +/- 40, and what implications that might have.

So I let the battery discharge quit a bit, then fired it up with the new alternator, and again, 5 amps charge rate.

Now, I always imagined that the factory ammeters were kind of useless, but are they really THAT useless?

Will they ever read higher charge and discharge rates if they're really happening, or is something wrong with mine?

I "think" they are pretty useless.

Try this one: Unplugs the regulator and fire the car. Check the meter. Start turning on electrical loads while looking at the meter. Can you make it read more than -5 amps?

Your never going to send 100 amps to the battery unless you can charge at about 50 volts. The voltage setting (14.5V) and the batteries internal resistance determines the charge rate (unless you have an external charger which typically will put out more than 14.5V). The factory ammeter only measures the current flowing to or from the battery thur the no. 12 awg red chargine wire running from the horn relay to the battery. It does not measure the ammeter output. If you change the red charging wire to a larger gauge you will also screw up the gauges calibration.

Actually, the factory ammeter seemes to do some voltage-based "calculation" since it uses the same line as the ALT idiot light in non gauge cars. I think it measures a voltage delta and displays that as amps charge rate.

"Real" ammeters go in line and actually measure current flow, but I'm pretty sure the factory one doesn't.

It does, but it doesn't use any part of the idiot light circuit. Its a millivolt meter, -0.5V to .5V with and indicator scale in Amps. Higher voltage at the horn relay end of the charge wire shows up as charge, higher voltage at the battery shows up as discharge. It uses the charge wire as an external shunt to determine the range of the gauge. If you put this one inline with the ammeter, or battery you'll be buying a new one real soon.

If it were a series device (reading Amps) the 100 Amp alternator would toast it letting the sacred smoke out.

You have to remember the biggest alternator sold back in '69 was on the Cadillac and put out 72 big ones at full throttle with clean cable connections. The highest alternator I can remember on the Chevy was 64 Amps on the Caprice and about 37 for most Camaro's, and Chevy II's which only came with lights, a blower for the defroster, and a radio. Turn on the wipers in the rain and your lights went yellow.

Most likely if it is reading amps (not volts, which is read in parallel) it is reading inductance off a sensing coil (isolated from the circuit) instead of running all the current through the meter. As an inductance meter it's sensitivity would be very dependant upon the design and construction of the sensing coil (were not talking digital Fluke sensitivity here).

Which is what I would use to measure your actual amperage with a clamp on low voltage amp meter. (Remember the average Fluke can only handle 10 Amps directly through the meter and the obligatory fast acting fuse).


Larger Dave

Mark's correct, of course.
The factory ammeter connects to the horn relay and the junction block behond the battery, none of the idiot light circuitry is used.

With all of the "aftermarket" parts for upgrading our cars, you would figure someone would have come up with a factory looking volt gauge to retrofit into the 68-9 console gauge pod. It would be a good selling item I think.

With all of the "aftermarket" parts for upgrading our cars, you would figure someone would have come up with a factory looking volt gauge to retrofit into the 68-9 console gauge pod. It would be a good selling item I think.
That simple fact of aftermarket neglect is one of the main reasons I went with autometer instead of factory style guages.

oops, i hate it when i sometimes hit my edit button instead of the quote button. I didn't intend to, and didn't change your post.

Years ago when I was a teenager, I had a real ammeter in one of my cars (the series type). Unfortunately, it was a 30 amp gauge and a 60 amp alternator. Even had it been large enough, running the entire charging circuit through your dashboard is still a "bad idea" (tm).

Anyway, at some point I was crusing through the park, and could smell Chinese takeout food. It was the ammeter melting down.

I have no idea why the internals of an ammeter smell like Chinese food, but to this day the smell of egg rolls makes me nervous about an eletrical fire.

That's too funny davepl. :D