Ammeter and voltmeter: purpose, difference and correct use in auto electrics

The electrical equipment of a car is a complex system where even small failures can lead to serious damage. To diagnose battery, generator or wiring faults, a car enthusiast cannot do without two key devices: ammeter and voltmeter. These devices are similar in appearance, but measure fundamentally different quantities - and it is dangerous to confuse them.

Many drivers mistakenly believe that a multimeter is enough to check the electrical system in the car. However, specialized instruments - an ammeter and a voltmeter - give more accurate results, especially when working with high currents (for example, when checking a starter) or when searching for leaks in the on-board network. In this article, we will look at how these devices differ, how to connect them correctly, and what errors can lead to a short circuit or failure of the device.

If you have ever encountered a situation where the battery is quickly discharged and the generator does not produce the required voltage, then without an ammeter and voltmeter it will be extremely difficult to identify the cause. These devices are like the “eyes” of an electrician, allowing you to see what is hidden from the ordinary eye: the current strength in the circuit and the voltage in its sections.

What is an ammeter and what is it for?

Ammeter is a measuring device designed to determine electric current strength in the chain. Current is measured in amperes (A), and it is this value that shows how much charge passes through a conductor per unit time. In auto electricians, ammeters are used for:

• 🔋 Checking the battery charging current from the generator.
• ⚡ Diagnosis of current leaks in the on-board network (for example, when the ignition is turned off).
• 🔧 Monitoring the operation of the starter - here the currents reach 200–400 A, and a regular multimeter may not cope.
• 💡 Checking the current consumption of individual devices (radio, headlights, heated seats).

The peculiarity of the ammeter is that it turns on sequentially into the chain. This means that the current must flow through the device, not past it. If you connect an ammeter in parallel (like a voltmeter), this will lead to short circuit - the device will burn out, and in the worst case, the car electrics will also be damaged.

Modern ammeters are analog (with arrow) and digital. The former are cheaper and easier to use, but less accurate. Digital models, e.g. Fluke 323 or Mastech MS8218, allow you to measure current with an accuracy of hundredths of an ampere and are often equipped with additional functions (memorizing peak values, auto-shutdown).

What is a voltmeter and its role in auto electrics

Voltmeter measures electrical voltage between two points in a chain. Voltage indicates the potential difference and is measured in volts (V). In a car, a voltmeter is used for:

• 🔋 Battery voltage control (norm: 12.6–12.8 V with the engine off, 13.8–14.4 V when the generator is running).
• 🔌 Checking the voltage drop on contacts, wires or fuses (over 0.5 V indicates poor contact).
• 🚗 Generator diagnostics - if the voltage at the battery terminals with the engine running is lower 13.5 V, the generator is not charging the battery.
• 💡 Checking the serviceability of sensors (for example, throttle position sensor).

Unlike an ammeter, a voltmeter is connected in parallel section of the circuit at which the voltage needs to be measured. This is safe, since the device has high internal resistance and does not create a load on the circuit. For example, to check the battery voltage, just connect the voltmeter probes to the “+” and “–” terminals - no open circuits are required.

They are often installed in cars built-in voltmeters on the dashboard, but they only show approximate values. For accurate diagnosis, it is better to use external devices, such as ANENG AN8008 or Victor VC9805A+, which measure voltage with an error of no more than 0.5%.

Key Differences Between Ammeter and Voltmeter

Although both devices are measuring devices, their operating principles and connection methods are radically different. The main differences are shown in the table:

Parameter	Ammeter	Voltmeter
Measured quantity	Current (A)	Voltage (V)
Connection method	In series (in open circuit)	Parallel (to two points in the circuit)
Internal resistance	Very low (so as not to influence the current)	Very high (so as not to consume current)
Danger of incorrect connection	Short circuit, device failure	Measurement error (the device will not burn out)
Examples of use in cars	Checking leakage current, battery charging, starter operation	Monitoring voltage of battery, generator, sensors

It is important to understand that The ammeter is never connected directly to the voltage source (for example, to the battery terminals). This will lead to a short circuit, since the device has minimal resistance. A voltmeter, on the contrary, can be safely connected to any two points of the circuit - it does not create a load.

How to properly connect an ammeter: step-by-step instructions

Connecting the ammeter requires caution, as an error may result in a short circuit. Let's consider the process using the example of checking leakage current in a car:

1. Turn off the ignition and make sure that all consumers (headlights, radio, heating) are turned off.

2. Remove the “–” terminal from the battery.

3. Set the ammeter to direct current (DC) measurement mode to the limit 10 A (if the leakage current is unknown, start with the maximum limit).

4. Connect red dipstick ammeter to the battery terminal, and black dipstick - to the removed wire. Thus, the device ends up in an open circuit.

5. Take the readings. Normal leakage current for a modern car is up to 50–70 mA. If the value is higher 100 mA, look for a "voracious" consumer.

To measure large currents (for example, when testing a starter), use current clamps. They allow you to avoid breaking the chain:

• 🔧 Switch the clamp to DC measurement mode.
• 🔄 Grasp with pincers one wire (not a bundle!) - for example, a positive cable from the battery to the starter.
• 📊 Start the engine and take readings. Normal starter current - 150–400 A (depending on the car model).

⚠️ Attention: When measuring currents above 10 A Do not hold the ammeter probes with bare hands - a short circuit may cause burns. Use insulated clips (crocodile clips).

How to use a voltmeter: practical tips

A voltmeter is easier to use than an ammeter, but there are some nuances here too. Let's look at typical tasks:

1. Checking battery voltage:

• 🔋 Connect the red probe to the positive terminal, the black one to the negative terminal.
• 📉 With the engine turned off, the normal voltage is: 12.6–12.8 V. If below 12.2 V, the battery is low.
• 🚗 When the engine is running, the voltage should be 13.8–14.4 V. If higher 14.5 V — The generator voltage regulator is faulty.

2. Search for voltage drop:

If, for example, the headlights are dim, check the voltage drop on the wires:

1. Connect a voltmeter to the battery terminals and record the value (e.g. 12.7 V).
2. Connect the probes to the headlight lamp contacts (with the lights on).
3. If the lamp voltage is lower by 0.5 V or more, this means there are problems with the wiring or contacts.

3. Checking the generator:

Start the engine and turn on the load (headlights, heater). The voltage at the battery terminals must remain within the limits 13.8–14.4 V. If it sags below 13 V, the generator can't cope.

What to do if the voltmeter shows 0 V?

Check:

- correct connection of the probes (red to “+”, black to “–”);

- serviceability of the voltmeter fuse;

- battery charge (perhaps it is completely discharged);

- integrity of the probe wires (a common problem with cheap multimeters).

⚠️ Attention: When measuring voltage in AC circuits (for example, in the ignition system), use a voltmeter with the function AC (Alternating Current). Connecting a device designed for direct current only may give incorrect readings.

Typical mistakes when working with ammeter and voltmeter

Even experienced car enthusiasts sometimes make mistakes that lead to equipment failure or inaccurate measurements. Let's look at the most common ones:

1. Connecting the ammeter in parallel:

This is the most dangerous mistake. The ammeter has minimal resistance, and when connected in parallel to a voltage source (for example, a battery), it creates direct short circuit. Consequences:

• ⚡ The device fuse blows (at best).
• 🔥 Probes or wires melt.
• 💥 Possible fire (especially if the battery is faulty).

2. Wrong choice of measurement limit:

If the approximate current or voltage is not known in advance, start with the maximum limit on the meter. For example:

• 🔋 Choose a limit for the battery 20 V (constant voltage).
• ⚡ For leakage current - 10 A, and then clarify.

If you connect the ammeter to the limit 200 mA into the starter circuit (where the current 300 A), the device will burn out.

3. Ignoring polarity:

If the polarity is incorrect (red probe to “–”, black to “+”):

• 📉 Analog devices will show a value with a minus sign or break.
• 🔋 Digital multimeters usually give a negative reading (but not all models are reverse polarity protected!).

4. Measuring voltage under load without taking into account consumers:

For example, if you measure the battery voltage with the headlights on, the readings will be underestimated. For an accurate diagnosis:

• 🔋 First measure the voltage with the car turned off.
• 🚗 Then - with the engine running without load.
• 💡 And only then - with switched on consumers.

⚠️ Attention: Never touch the voltmeter probes with both hands when measuring high voltages (for example, in the ignition system). This may cause electric shock through the heart!

How to choose an ammeter and voltmeter for a car

For auto electricians, both universal multimeters and specialized devices are suitable. When choosing, pay attention to the following parameters:

1. Device type:

• 🔧 Multimeter (for example, Fluke 17B, UNI-T UT139C) - universal, but for high currents (over 10 A) current clamps are required.
• 📊 Specialized ammeter (for example, Mastech MS2108A) is more accurate for measuring current, but does not measure voltage.
• 🔋 Voltmeter with recording of peak values (for example, Victor VC890D) - useful for diagnosing voltage surges in the on-board network.

2. Specifications:

Parameter	Recommended value	Explanation
Maximum measured current	20 A (for multimeter), 400 A (for current clamps)	To check the starter you need a limit of at least 300 A.
Voltage measurement accuracy	±0.5%	Error over 1% may give false results when diagnosing a generator.
Overload protection	fuse 10 A/250 V or automatic shutdown	Cheap devices without protection will burn out if there is a connection error.
Additional features	Resistance measurement, diode test, thermocouple	Allows you to diagnose not only current and voltage, but also open circuits.

3. Ease of use:

• 🔋 Screen backlight — necessary for working in poorly lit places (for example, under the hood).
• 📏 Probe length - no less 30 cmto reach deleted contacts.
• 🔋 Auto shutdown — saves device battery power.

For most car enthusiasts, the optimal choice is a mid-class multimeter (for example, ANENG AN8002) with current clamps. Professionals should consider models with advanced features, such as Fluke 88V, which allow you to measure signal frequency and temperature.

FAQ: Frequently asked questions about ammeters and voltmeters

Can a multimeter be used instead of an ammeter and voltmeter?

Yes, the multimeter combines the functions of both devices. However, to measure large currents (for example, when testing a starter), you will need current clamps, since standard multimeter probes are not designed for currents exceeding 10–20 A. Also specialized devices (for example, ammeters with shunt) give more accurate results in narrow ranges.

What happens if you mix up the ammeter and voltmeter when connecting?

If you connect ammeter in parallel (like a voltmeter), a short circuit will occur - the device will burn out, and in the worst case, the battery or wiring will be damaged. If you connect voltmeter in series (like an ammeter), it simply will not show any values, since it is not designed to pass current.

How to check leakage current in a car without an ammeter?

You can use the multimeter in ammeter mode:

1. Turn off the ignition and all consumers.
2. Remove the “–” terminal from the battery.
3. Connect the multimeter into the gap between the terminal and the wire (mode 10 A DC).
4. If the leakage current exceeds 50–70 mA, pull out the fuses one by one to find the “gluttonous” circuit.

⚠️ Without an ammeter or multimeter, it is impossible to determine the leakage current - it is not visually noticeable.

What voltage should be on the battery when the engine is running?

Normal values:

• 13.8–14.4 V — the generator is working, the battery is charging.
• 12.6–13.0 V — the generator is not charging enough (there may be problems with the belt, diode bridge or voltage regulator).
• Above 14.5 V - overcharging, which leads to boiling off of the electrolyte and failure of the battery.

Measure the voltage with the headlights and other consumers on - this way the load on the generator will be real.

Is it possible to measure the starter current with a regular multimeter?

No, standard multimeters are not rated for current. 200–400 A, which the starter consumes. For this task use:

• 🔧 Current clamps (for example, UNI-T UT204), which measure current without breaking the circuit.
• 📊 Specialized ammeter with shunt (for example, Mastech MS2108A).

Trying to measure the starter current with a conventional multimeter will lead to a blown fuse or the device itself.