How to... check your classic car’s electrics

| 24 May 2019
Classic & Sports Car – How to... check your classic car’s electrics

Many of us have been left stranded and confused by electrical problems but, if you work logically, you can usually track them down surprisingly quickly.

Think of a circuit in terms of its elements: at its most basic, it will simply contain the battery, a switch, a fuse and a consumer, such as a bulb, plus the earth-return circuit.

A multimeter is an essential piece of kit to help you test each of these. More expensive units will include a larger range of functions, and should have an auto-ranging facility. This enables you to merely select, for example, ohms.

The meter will then automatically adjust to display small figures for a coil’s primary circuit, or large ones for its secondary circuit. Even basic multimeters without this function, however, will give you the readings you need once you’ve established the correct range.

Remember basic maintenance: ensure that connections and terminals are clean, and that earthing points are rust-free.

When testing the battery on a negative-earth car, connect the meter’s black lead to a good earth, and the red lead to the test point. On positive-earth cars, it’s the other way around. 

COMMON FUNCTIONS

Classic & Sports Car – How to... check your classic car’s electrics

The meter shown here is a basic model but it includes the most commonly used functions.

The ‘volts DC’ range is in the upper-left quarter. The continuity function is the white buzzer symbol near the bottom.

Ensure that the leads are positioned in the correct jacks for the measurement.

TESTING THE BATTERY

Classic & Sports Car – How to... check your classic car’s electrics

To ensure that your battery is in good order, select ‘volts DC’. If it’s not a self-ranging meter, turn the dial to the 20V setting.

Place the probes on the relevant battery terminals. You should see around 12V. Just above is fine; below 11.5V means that the battery needs charging or replacing.

CHARGING SYSTEM

Classic & Sports Car – How to... check your classic car’s electrics

Start the car’s engine and give it a few seconds to settle. With the meter in the same configuration as for the battery test, place the probes across the terminals again.

This time, you’re looking for a reading of 14-15V. A lower reading indicates that the battery isn’t receiving sufficient charge.

MEASURING RESISTANCE

Classic & Sports Car – How to... check your classic car’s electrics

Never measure resistance in a live circuit, so if you want to check a coil, first you have to disconnect it and/or remove it.

Select ‘ohms’, and connect the meter across the coil terminals. Check your car’s manual for the correct reading; it differs on cars with ballast and non-ballast ignition systems.

CHECKING CONTINUITY

Classic & Sports Car – How to... check your classic car’s electrics

This enables you to prove whether switches or fuses are working.

The continuity is illustrated via an audible buzzer. Select ‘continuity test’ and connect the probes to the test piece.

The buzzer will sound if a closed circuit is established, such as when a fuse is intact or a healthy switch is in its ‘on’ position.

POWER AT A COMPONENT

Classic & Sports Car – How to... check your classic car’s electrics

You can find out whether power is reaching a component by testing its socket. When the circuit is energised, you should see a reading of around 12V.

If you do, and the component doesn’t work, you’ve established that the problem is with the part itself rather any other bit of its circuit.

BATTERY DRAIN PART ONE

Classic & Sports Car – How to... check your classic car’s electrics

This solution needs your classic to be of an age where it has a decent number of fuses.

Connect the leads to the correct jacks, as shown, and select the 10A range.

Disconnect the battery’s negative cable and put the multimeter between the cable and its terminal, so it’s ‘in series’ with the battery.

BATTERY DRAIN PART TWO

Classic & Sports Car – How to... check your classic car’s electrics

Do not put any additional loads through the battery during this test, and do not start the engine.

Without a drain, the readout should be almost zero. Anything above about 25mA shows a drain.

Pull out fuses one at a time. When the reading drops, you’ve found the circuit that the drain is on.


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