Car Ammeter Guide

An ammeter (from Ampere Meter) is a measuring instrument used in measuring the current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere/microampere range, are designated as milliammeters/microammeters. Earlier, ammeters were laboratory instruments which relied on the Earth’s magnetic field for operation. By the late 19th century, improved instruments were made which could be mounted in any position and allowed accurate measurements in electric power systems. It is usually represented by letter ‘A’ in a circle. Ammeters have quite low resistance and are always connected in series in any circuit.


Majorly, all ammeters are either connected in series with the circuit carrying the current to be measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In either case, the current passes through the meter or (mostly) across its shunt. Ammeters should not be connected directly across a voltage source since their internal resistance is very low and excess current would flow. Ammeters are made for a low voltage drop across their terminals, much less than one volt; the extra circuit losses which are produced by the ammeter are known as its “burden” on the measured circuit.

Principle of Ayrton on shunt switching

Ordinary Weston-type meter movements can measure at most milliamperes only, because the springs and practical coils can carry only limited currents. To measure larger currents, a resistor known as shunt is placed in parallel with the meter. The resistances of shunts is found in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small amount flows through the meter. This helps the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale deflection (FSD) of 50 mV, so shunts are usually designed to produce a voltage drop of 50 mV when carrying their full rated current.

Principle of Ayrton on shunt switching

To make a multi-range ammeter, a selector switch can be used for connecting one of a number of shunts across the meter. It must be a make-before-break switch to stop damaging current surges through the meter movement when switching ranges.

A better arrangement is the Ayrton shunt or universal shunt, which is invented by William E. Ayrton, which does not require a make-before-break switch. It also helps to avoid any inaccuracy because of contact resistance. In the figure, which can be assumed for example, a movement with a full-scale voltage of 50 mV and desired current ranges of 10 mA, 100 mA, and 1 A, the resistance values would be: R1=4.5 ohms, R2=0.45 ohm, R3=0.05 ohm. And if the movement resistance is 1000 ohms, for instance, R1 must be adjusted to 4.525 ohms.

Switched shunts are occasionally used for currents above 10 amperes.

Zero-center ammeters are used for applications needing current to be measured with both polarities, common in scientific and industrial equipment. Zero-center ammeters are commonly placed in series with a battery too. In this application, the charging of the battery deflects the needle towards one side of the scale (commonly, the right side) and the discharging of the battery deflects the needle towards the other side. A special type of zero-center ammeter for testing high currents in cars as well as trucks has a pivoted bar magnet that moves the pointer, and a fixed bar magnet to keep the pointer centered having no current. The magnetic field around the wire carrying current which is to be measured deflects the moving magnet.

Since the ammeter shunt has quite very low resistance, mistakenly wiring the ammeter in parallel with a voltage source will lead to a short circuit, at best blowing a fuse, possibly damaging the instrument and wiring, and making an observer prone to injury.

In AC circuits, a current transformer converts the magnetic field which is around a conductor into a small AC current, typically either 1 A or 5 A at full rated current, that can easily be read by a meter. In a similar way, accurate AC/DC non-contact ammeters have been constructed with Hall Effect magnetic field sensors. A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial as well as commercial electrical equipment, which is temporarily clipped over a wire to measure current. Some recent types have a parallel pair of magnetically soft probes that are present on either side of the conductor.

How to Wire an Ammeter into a Car

Whether you have a performance sports car or a motorcycle, an ammeter can be a very handy gauge to have. This tool will help you in determining whether or not your alternator and battery are working in harmony. These meters are used in a variety of cars where there are multiple pieces of equipment that are dependent on the battery. Although some cars have simple ammeter already installed, for those that make additions to their car or motorcycle, it is recommended that you wire up a more professional style of the meter to make sure that everything is functioning properly and your battery is staying charged. For some components, it is very necessary that they do not lose power, as items that incorporate the use of computers may reset to default settings if they lose power entirely, which could harm the performance of your motorcycle or car. By installing an ammeter, you can put your mind at an ease as you are on top of your voltage needs.

Step 1 – Gather Materials

Before starting your project, you should make sure that you have everything that you need so you will not have to stop what you are doing to look for a tool. You should have a pair of safety goggles, gloves that permit hand dexterity, wire strippers, a red 14-gauge wire that comes equipped with eyelet terminals, a socket wrench, and a screwdriver. After you have gathered all of your tools, you can now start to begin installing your ammeter.

Step 2 – Disconnect the Battery

Having your materials in a place where all tools can be reached easily, you should then locate the battery terminal towards the positive side. The positive terminal is generally red and is indicated by a plus sign. Once you have located the terminal, you then need to take either your socket wrench or screwdriver and disconnect your positive terminal from the battery post. This will help you to ensure that you don’t shock yourself if any wires cross that are not supposed to.

Step 3 – Connect the Wire

Once you have your positive terminal which are disconnected from the battery post, you will then connect one end of the red 14-gauge wire to the positive battery post. Ensure that the eyelet fits snug over the post and will make proper contact once the positive battery wire is reconnected. After connecting your red 14-gauge wire to your battery post, take the other end and connect it to your ammeter terminal. Remember to stay consistent with your connections as you don’t want your wires to cross. After you have connected both of the wires, you should hand tighten the nuts just in case you have to repeat any steps of the later on.

Step 4 – Connect the Ammeter to the Alternator

With your positive battery terminal now being connected to the ammeter, you are ready to connect the next wire. Take the other piece of red 14-gauge wire and connect it to the other terminal of the ammeter. Once you have connected it to the ammeter, you will then have to connect it to the alternator terminal post. After this is done, you can tighten all of your fittings and your project should be complete.

How to Install a Car Volt Amp Gauge

When you are thinking about the number of sensors that your engine has, it seems like there is an endless amount of gauges to be installed to monitor their readings. Some of these readings are important, but many of them are just simple inputs to the vehicle’s computer. Most of the gauges on today’s vehicles are the speedometer, tachometer, fuel gauge, and temperature gauge. Besides having these gauges, your car needs a number of warning lights that will illuminate if there are problems with those systems. One gauge that is usually missing in most vehicles is the charging, or voltage, gauge. With a little information you can add a voltage gauge to your vehicle quite easily.

Part 1 of 2: The purpose of a voltage gauge

Today, most vehicles built come with a warning light on the dash that looks like a battery. When that light illuminates, it just means that there is not enough voltage in the vehicle’s electrical system. Mostly, this is due to a failure in your vehicle’s alternator. The downfall of this warning light is that, when it comes on, the voltage in the system is quite low – and if the battery becomes low enough, the vehicle will end up stalling.

Installing a voltage gauge will help you to see changes in the charging system well before it becomes a serious problem. Having this gauge will make it a lot simpler to decide whether it is time to pull your vehicle off of the road, or if you can make it to where you are going.

Part 2 of 2: Installing the gauge

Materials Required

  • Fusible wire link (needs to match amperage rating of gauge)
  • Pliers (wire strippers/crimpers)
  • Memory saver
  • Voltage gauge assembly
  • Wire (at least 10 feet with the same gauge rating as the voltage gauge wiring)
  • Wire loom
  • Wiring connectors (assorted connectors and a 3 way connector)
  • Wiring schematic (specific to your vehicle)
  • Wrenches (assorted sizes)

Step 1: Park your vehicle and apply the parking brake. Your parking brake needs to be a pedal or a hand brake. If it’s a pedal, then push it down until you feel the brake engage. If it’s a hand brake, then push the button in and pull up on the lever.

Step 2: Install the memory saver as per the manufacturer’s instructions.

Step 3: Open the hood. Release the latch inside of the car. Stand in front of the car and just lift up the hood.

Step 4: Disconnect the negative battery cable. It needs to be positioned away from the battery.

Step 5: Decide where you want to install the gauge. First, you should see how the gauge mounts: it may mount with attachment tape, or it might be mounted with screws.

If it has a screw mount, you need to ensure that it is mounted in a spot where screws will not hit anything inside the dash.

Step 6: Run the wiring just between the gauge and the battery. With the use of the appropriate size wire, run wiring from where the gauge will be mounted to the positive battery terminal.

  • Tip: While running the wire from inside of the vehicle and into the engine compartment it is easiest to run it through the same seal as the vehicle factory wiring.

Step 7: Connect the connectors to the wire you just ran and the fusible link. Strip ¼-inch of insulation from each of the end of the fusible link. Install an eyelet connector and crimp it into place on one end, and crimp a butt connector on the other end.

After that, connect it to the wire that you ran to the battery.

Step 8: Remove the nut from the positive battery cable end clamp bolt. Install the eyelet and tighten the nut back into its place.

Step 9: Install an eyelet to the other end of the wire. Eyelet is installed where the wire will mount to the gauge.

Step 10: Find the wire that goes to the lighting circuit. Use your wiring schematic to find the positive wire that supplies voltage from the light switch to the lights.

Step 11: Run a wire from the place you are mounting the gauge to the lighting circuit wire.

Step 12: Remove ¼-inch of insulation from the circuit end of the gauge wire. With the help of a three wire connector, crimp that wire to the lighting wire.

Step 13: Install an eyelet to the end of the wire you just ran from the lighting circuit wire. Remove ¼-inch of insulation from the gauge end of the wire and then, install an eyelet connector.

Step 14: Run a wire from the gauge towards a grounding point under the dash.

Step 15: Add an eyelet to the wire running to the grounding point. Remove ¼-inch of the insulation from the wire, install an eyelet, and crimp it into the place.

Step 16: Install the eyelet as well as the wire to the ground connection.

Step 17: Add an eyelet to the end of the wire that will connect to the gauge. Remove ¼-inch of insulation from the wire at the gauge and then install an eyelet.

Step 18: Just connect the three wires to the gauge. The wire going to the battery goes to the signal or towards the positive terminal on the gauge; the wire connected to the ground goes to the ground or to the negative terminal. And the final wire goes to the lighting terminal.

Step 19: Install the gauge in your vehicle. Ensure to mount the gauge according to the gauge manufacturer’s instructions.

Step 20: Wrap the wire loom around any of the exposed wiring.

Step 21: Install the negative battery cable and tighten till it’s snug.

Step 22: Just remove the memory saver.

Step 23: Start the vehicle and verify that the gauge is working or not. Turn on the lights and ensure the gauge is illuminated.

Video of how to install a Volt Meter in your vehicle

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