There are four basic ways an electrical circuit can fail and at least three out of the four share one thing in common - current flow!

An open circuit will result in no current flow. Excessive resistance in the circuit will result in lower than normal current flow. And a short to ground before the load will most definitely result in higher than normal current flow!

What else can affect current flow? What about the work the circuit is doing?

Starters, fuel pumps, blowers, window motors - all will demand more or less current depending on the load they are trying to overcome. For example, a fuel pump that is trying to move fuel from an empty tank (sucking air) will draw very little current while one trying to push fuel past a restricted filter will demand much more - so measuring current in this circuit can provide even more diagnostic information not necessarily related to an electrical failure.

What about relay controlled circuits? If an ECU doesn't see all the needed requirements to activate a relay, the controlled component won't function. Think of an A/C compressor as an example. The ECU will not allow compressor operation of the refrigerant charge is too low. That can be seen by looking for the current flow in the control side of the relay. A shorted compressor clutch coil will draw more current but not build sufficient magnetic strength to engage the compressor. That can be seen in the current flow on the component side of the relay. And if the current flow is normal? Perhaps the clutch is mechanically defective or the air gap is too great.

Again, current is being used as a diagnostic aid in determining the root cause of the problem!

There are several other ways current can be used for diagnostics. What applications can you think of? Check out the February 2016 The Trainer video!

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