A look at circuit voltage

Part two: Frank Massey examines some fault finding basics

Published:  04 April, 2013

By Frank Massey

Of course, you can dim a light bulb right down to zero volts but for most electronic components there is a threshold voltage below which they won't operate at all. This includes ABS pump motors, ABS solenoids, variable rate steering solenoid valves, electronic shock absorber motors and even electronic modules. Low circuit voltage is usually caused by excessive resistance at some point in the circuit. Usually, this means a loose, poor fitting or corroded connector, a faulty switch, relay or a ground connection.

Every electrical circuit requires a complete circuit to operate. Correct voltage supply on load won't do any good unless there's a return back to the battery. The ground path in the case of metal bodied vehicles may be the vehicle body itself. A poor ground connection has the same effect as an additional component or resistor, it will share its proportion of the load if the circuit isn't complete and no current will flow.

Make sure there's no voltage in the circuit when disconnecting it from a power source by pulling the fuse or by testing downstream from the circuit switch or relay. Ohmmeters can't handle normal battery voltage and should you accidentally complete a circuit through the meter it may cause permanent damage.

Ohmmeters are great for measuring circuit resistance but you have to use care when checking electronic components. An ohmmeter works by applying a small voltage through its test leads and this voltage can be enough to damage some electronic components (such as the oxygen sensor). A high impedance (10,000 mega ohm) meter should be used for electronic testing.

Tips on testing voltage

Sometimes undersized wiring can cause low voltage. It's not something you'll find with original equipment wiring circuits but it is a common mistake that's made in many repairs or where additional equipment has been installed.

? 18 gauge wire - 6 amps

? 16 gauge wire - 8 amps

? 12 gauge wire - 15 amps

? 10 gauge wire - 30 amps

? 8 gauge wire - 40 amps

For checking voltage drops and circuit continuity a regular digital multimeter can be used but there are many jobs where the first tool you reach for will be a dedicated automotive tester. The most basic of these devices is just a probe with a bulb but more sophisticated machines are available from Draper, Gunson and Sealey, they offer a variety of features including auto-ranging and voltage selection. The newest device on the market, the one that Frank uses, is the Power Probe Hook. As well as being able to automatically select range and meter, the Hook also has a function allowing you to power on components at a moment's notice.

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