View Full Version : Ammeter Gauge

06-30-2016, 08:37 AM
The ammeter gauge was smoking with the key ON, engine not running. Removed it to find the winding burned out.

For those wanting to repair the ammeter gauge winding on a '66 Thunderbird, it is 60 turns of 24 AWG insulated wire. Rather than turn a new winding, I had on-hand a spare component that would suffice and soldered it to the gauge assembly. (See photo.) A bench test of the repaired gauge was good.


However, still need to determine the cause of the winding burn-out. It's possible that, given the age of the car, the winding insulation broke down and shorted it out. But if there's another cause, that needs to be addressed before re-installing the repaired gauge.

Researching the problem online, a lot of the comments mistakenly assume that the entire current of the system is routed through the gauge. There are two reasons that couldn't possibly be correct: 1) the 24 AWG gauge winding (about 0.3ÁH) is virtually a short circuit, that will only support a relatively small current and, 2) if the gauge is removed from the car (i.e., an open circuit) the car still runs.

Note that the car gauge is not an ammeter in the usual sense. A bench ammeter is connected in series (plus to reference) to measure the amount of current flow. However, the car ammeter is connected in parallel between the alternator + voltage and the battery + voltage to detect the direction of current flow between the two voltages. The direction of current flowing through the winding varies with the car's electrical load and the output of the alternator.

(For the purpose of this discussion, we'll assume that current flows from positive to less-positive.) This is my understanding of how the gauge works:
If the alternator voltage is higher than the battery voltage, the alternator is providing enough current to run the car's electrical load. There will be current flow through the gauge winding toward the battery. The winding will generate a magnetic field that will move the gauge needle toward the C (charge) position.
If the battery voltage is higher than the alternator voltage, it means that the electrical load is greater than the alternator's current output, causing the alternator voltage to drop. The electrical load is now being partially supported by the battery. The current flow in the winding reverses, causing the gauge needle to move to the D (discharge) position. To test that theory, I'll connect a low ohm, high wattage resistor in series with the car's gauge wires and measure voltage across the resistor and current through the resistor with the key ON (engine off) and with the car running. That will determine if the gauge burn-out was due to excessive current.

Any advice is, of course, appreciated. Thanks.


06-30-2016, 12:35 PM
My '64 in stock form, had the heavy gauge wire directly from the alternator feeding a shunt inside the actual gauge. That in turn fed the fuse panel. I believe that the ignition circuit was unfused off the battery so an interruption in the ammeter circuit wouldn't keep the car from running. The indicator on the gauge is reading the magnetic field developed.

Being a later model year yours is probably a remote shunt, mounted somewhere safer and the gauge is actually reading + or - volts, displayed as amperage.

Manufacturers eventually gave up on ammeters altogether and gave us volt meters instead.

06-30-2016, 03:01 PM
Richard's post is very good and informative. It's important to note that in his meter rebuild, 'coil wire' must be used because it is varnished (insulated). Regular copper wire would short straight across the windings and produce no magnetism. We would normally say his meter is internally shunted but it really uses magnetism to directly move his meter.

When current flows through a wire it creates magnetism around that wire. We can multiply that magnetism by winding the wire in a coil of turns. Loudspeakers also use this technology to move a cone.

All ammeters are really volt meters. There is no way heavy current can possibly power a small meter movement. Besides, why run 100amps into the passenger compartment, then back out into the engine bay? When generators produced a measly 30amps, #10 wire was possible but today's 100amp systems require at least #4awg which is too heavy.

So, how do we read amps from voltage? We use a 'shunt resistor' with low resistance, usually mounted on the fender apron or close to the starter solenoid. The meter must match the shunt resistor in a specific ratio.

Here is a typical shunt resistor designed to measure 100 amps:

Major current flows through both ends (large connection holes) but notice those two small screws? They connect to both meter wires, so a ratio of power feeds the meter. This shunt resistor (http://www.rammeter.com/ram-meter-inc-f100a50-100-amp-50-dcmv-dc-current-shunt.php) is designed to pair with a 50-millivolt (0.05v) meter. Yes, whether the meter is connected or not makes no difference to the shunt resistor. Major current still flows through.

Ford did exactly the same method, using a shunt resistor to save money.

Another method using a Current Transformer can be used. A C.T. is simply turns of wire around the battery cable, directly feeding an ammeter. Again, the number of turns will determine power to the meter. This is how we accurately measure extremely large current, like starter motor draw using no resistance.

Modern cars use a simple volt meter, connected anywhere including the cigarette lighter. The volt meter reports the battery's charge. If your charging system isn't working, battery voltage will be low. If the battery has shorted cells, the charging system will still feed 14volts but the battery will only hold two volts per working cell. Can a car start on 10volts? Yep, some folks never know that their new battery came with a dead cell.