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Ammeter vs Voltmeter: How They Work, and Which One Is Right for Your Car

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Upgrade to a Volt Gauge or Stick with The Original Amp Gauge?

Most vehicles pre-1960s used a generator charging system to maintain the vehicle’s 6-volt or 12-volt electrical needs and recharge the onboard battery as needed. These systems were usually monitored by a simple charging/warning lamp on the dash (an “idiot light” as many have called it endearingly), though higher trim levels and sportier models often had additional instrumentation, including an ammeter that replaced the warning lamp.

The Pontiac GTO, even though it was a sporty car, used a basic dash warning light for the charging system (under the fuel gauge on the far left). Also notice the 3-gauge panel with voltmeter!

These early generator-equipped vehicles typically had very little in the way of electrical accessories. Lighting, ignition, and the starting/charging circuits were pretty much what a car would have for electrical needs. As such, generators often were regulated to 25-30 amps and used a current limiter. When alternators became common in the mid-1960s they too were often regulated to 35-40 amps for the vehicle’s charging needs but their regulator limits voltage, allowing more current (amps) to be utilized if needed. These relatively low amperage requirements allowed for ammeters to become a standard instrument feature in the dash of most 1960s vehicles, which means that your muscle car or classic truck most likely has one. Whether it is working or not is an entirely different story though!

The Ford Mustang, also considered a sporty car, in fact used a standard ammeter in the 1965 GT (inductive loop style) and in the 1966 and up models (shunt style).
What Does an Ammeter Measure?

The car ammeter measures electrical “flow” in amps into or out of the battery (charge or discharge). Typical ammeter connections are made using a shunt setup on the main wire from the generator or alternator to the battery. There is a loop wiring circuit from the shunt to the gauge to “sample” the amperage of the main wire. Some cars used inductive loop ammeters that simply had the charge wire pass through a loop on the back of the gauge. The shunt style wiring is much safer than the inductive loop, as the entire charging system wire must travel through the dash to the ammeter’s inductive loop to be sampled.

How To Read an Ammeter
The typical ammeter gauge face reads discharge or charge in amps.

An ammeter samples the amperage flowing through the main charging wire. This sample is then indicated on the gauge face via “charge” or “discharge” area of the instrument face that the needle points to. Some instruments will only show a “C” and a “D” or even a simple “+” and “-“ indicator. This is often incorrectly assumed by the vehicle operator as the charging system is working properly since the car ammeter is in the “C” range. In fact, the ammeter is simply measuring current (amperage) flow and which direction it is flowing.

A heavily discharged battery will move the needle to “C” when the charging system output is recharging the battery, while the needle will move to “D” when the charging system cannot keep up and the battery is tapped into for additional power needs. Due to the differences in how generators and alternators work, the car ammeter in a generator charging system is often more “active,” while in an alternator charging system the needle will barely move. In these early generator style charging systems, the battery was used more often to “help” the system power the lights and other accessories.

What Does a Voltmeter Measure?
A voltmeter gauge will read volts directly from the charging system. Some gauges offer color coded zones to show under or over charging conditions.

As alternators came on to the scene the vehicle’s charging system was much more robust and reliable. As such, the ammeter and its special wiring needs were replaced with a voltmeter. The voltmeter circuit simply measures the voltage, the “pressure” if you will, to the amp’s “flow” in an electrical system. Voltmeters simply read the charging system’s voltage output, as alternator charging system amperage is not limited like in a generator system. A properly charging alternator will output 13.5-14.5 volts, which is easy to confirm with a quick glance of a voltmeter.

Can You Use an Ammeter with A High Output Alternator?

Today’s classic hot rod or muscle car with modern wiring harness and electrical needs that include high amperage electric cooling fans, big stereo systems, electric fuel pumps, and other power hungry accessories create quite the demand on the charging system. While people say the shunt style ammeter can safely be used in higher amp applications (the loop style ammeter should not be used at all), there is a valid concern for damaging the gauge or original dash wiring or worse.

A single gauge panel is a simple solution to mounting a voltmeter on the leading edge of your lower dash.

When it comes to amp gauge vs volt gauge, our opinion is that the ammeter should be replaced with a voltmeter if you’re using a 75 amp or higher alternator in your build. There are gauge companies that can convert your factory ammeter to a voltmeter if you’re wishing to retain the stock instrument panel. However, there are plenty of great choices for voltmeters that will work in standalone gauge panels or gauge cups or will match your existing aftermarket gauges. This is a much safer option for those with high output alternator charging systems. If you do wish to stick with an ammeter, be sure to use 10 gauge wiring at a minimum when wiring your new ammeter into your vehicle.

Speedway Motors offers this trick digital 3 in 1 gauge that combines a voltmeter, water temperature, and oil pressure gauge in one display.
How To Wire Up a Voltmeter Gauge
A voltmeter is much easier to install. The marked terminals show + for your 12-volt switched wire and - for the ground wire.

Unlike the ammeter, a voltmeter does not require a special shunt or other specially sized wiring to work. The voltmeter circuit simply needs a switched 12-volt power wire and a ground wire connected to the gauge to read the charging system voltage the alternator is outputting at a given rpm. For most installations the switched 12-volt power feed to the instrument panel is sufficient to also power the voltmeter and provide a correct reading.

This power wire is usually wired in parallel, with the power lead “daisy chained” from one instrument to another. If you’re adding a standalone voltmeter, you can simply add on from this power feed to your voltmeter and then run your ground to a solid ground location (steel dash frame, firewall, steering column, etc.). So, when comparing ammeter vs voltmeter for car wiring it is easy to see why voltmeters are so much easier to wire correctly.

Ammeter to Voltmeter Conversion - What You Need to Know

Most factory ammeters are part of a gauge panel and not an individual freestanding instrument. As such, short of having the ammeter converted to a voltmeter by a gauge restoration outfit, your best bet is going to be simply leaving the factory ammeter in place and disconnect and cap/seal the factory wiring terminals that went to the ammeter. Then, as we previously mentioned, install a new voltmeter using a gauge panel or cup on your dash, steering column, or console using new wiring of 18 gauge or larger (numerically smaller). Remember to use a switched 12-volt circuit like your existing 12-volt gauge wiring so that the voltmeter is not powered all the time. For example, do not wire it to radio accessory or your cigar lighter/power port wiring connections of your wiring harness.

Whether you decide to stick with an ammeter or upgrade to a voltmeter for your hot rod, muscle car, or classic pickup project, the important thing to note is that the gauge is properly wired to prevent any electrical hazards. Follow all instrument instructions for wiring gauge size, where to connect the wiring to your vehicle, and so on. Whatever your choice in instruments, having something other than a battery or alternator warning light will go a long way in ensuring you're not left on the side of the road with a charging system issue.

One solution to adding a voltmeter is to update your entire instrument cluster with a direct fit replacement that features updated instruments, like this Dakota Digital cluster for 55-59 Chevy trucks.

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