Current in an electrical system

Why Current Matters in Automotive Electrical Systems

When diagnosing an electrical fault, one of the first things a technician will often check is voltage. But voltage is only part of the story.

You can have 12 volts showing perfectly at a connector and still have a circuit that simply cannot do its job.

Why? Because the circuit also needs to be capable of supplying the required current under load.

In automotive electrical systems, we're generally working with relatively low voltages — traditionally around 12 volts — but potentially very significant currents. A control module, fuel pump, electric motor, lighting circuit or actuator may have the correct voltage present when measured with a high-impedance multimeter, yet the voltage can collapse as soon as the component attempts to draw current.

This is where understanding current becomes extremely important.

12 Volts Doesn't Always Mean a Good Circuit

Imagine measuring a power supply at a module connector and seeing 12.6 volts.

At first glance, everything looks fine.

But what if that circuit has a corroded terminal?

What if a wire has 30 strands of copper inside it, but only three are still intact?

What about an overheated connector, partially burnt fuse holder, pitted relay contacts or a switching device inside another control module that is no longer turning fully on?

A digital multimeter may still happily display battery voltage because the meter itself draws almost no current.

Connect the actual load, however, and suddenly the weakness in the circuit becomes apparent.

Voltage tells you electrical potential is present. Load testing tells you whether the circuit can actually deliver.

Where Does the Current Go?

Technically, we don't normally describe this as a "current drop" in the same way we describe voltage drop. Instead, unwanted resistance in the circuit restricts current flow and produces a voltage drop when the circuit is loaded.

That resistance can come from:

• Corroded connectors and terminals
• Overheated wiring
• Broken or missing conductor strands
• Poor crimps
• Damaged fuse holders
• Pitted relay contacts
• Poor earth connections
• Internal PCB or connector damage
• Failing semiconductor switching devices

Some of these faults can be almost invisible until the circuit is asked to work.

So How Do We Test It?

One method is to operate the component normally and use a current clamp to measure its actual current consumption. This can be extremely useful because we can compare the measurement against a known-good component or an expected operating value.

Another useful diagnostic technique is load testing.

A known load  traditionally something as simple as an appropriately rated incandescent bulb  can help determine whether a circuit can actually deliver useful power. If the supply looks perfect with a multimeter but collapses when an appropriate load is connected, we know there is a problem that needs further investigation.

Voltage-drop testing across connections, terminals, earths and wiring while the circuit is operating can then help locate exactly where that unwanted resistance exists.

But There Is a Catch

You cannot simply connect a large load to an unknown circuit and see what happens.

Before load testing, the technician needs to understand what the circuit is designed to supply.

A 5-amp load on a circuit designed for 20 amps may be perfectly reasonable. Put a 20-amp load onto a delicate electronic output designed for a few hundred milliamps and you may destroy the driver before you've diagnosed anything.

Fuses provide protection, but they should never be treated as your primary method of determining whether a test load is safe.

This is where knowledge, circuit diagrams, known-good measurements and experience become important.

Start building a reference library. Measure known-good fuel pumps, lamps, motors, displays, modules and actuators with a current clamp. Learn what normal looks like.

Because electrical diagnosis isn't simply about asking:

"Do I have 12 volts?"

The better question is:

"Can this circuit deliver the voltage and current the load requires — while maintaining that voltage under operating conditions?"

That distinction can be the difference between finding the actual fault and replacing a perfectly good control module.