Electrical testing prevents expensive guess work. With knowledge and simple tools we can electronically eliminate possibilities instead of replacing parts. This is far superior to parts swapping as a new part is NOT a known good part. We could replace a component, with a defective part, misleading us.

Poor electrical connections cause many check engine lights and much unnecessary part replacement. A simple test can help separate problems from false symptoms. Most enthusiasts are familiar with voltage testing. We check voltage to confirm a current is flowing in a circuit. Not as familiar is voltage-drop testing, which provides a new scope of information to the diagnostic technician.

Understanding voltage and current flow

Voltage in a circuit is much like pressure in a hose. Pressure pushes the liquid through the hose, and voltage helps current flow through wires. A restriction in the hose, such as a kink, reduces the flow. Poor, loose or corroded connections act much the same, dropping voltage and reducing current flow. The clearest difference is, we need only one hose whereas an electrical circuit requires two wires for current to flow.

A voltmeter draws almost no current. Testing a poor connection may still show full voltage, until we load the circuit. For instance, we can have full voltage at a component, but lack current flow to operate it. Without testing, the non operative part may appear as the problem, though it is not.

A simple example might be a transmission solenoid, which does not allow the lock-up of the torque converter. This causes a check engine light to come on and a DTC saying a solenoid fault. This code may tempt a beginner to replace the solenoid, only to find the problem still exists. Worse, this may lead them to believe the torque converter is the problem, causing expensive trial and error tactics.

More experienced enthusiasts might check for voltage and ground, at the solenoid before deciding to change it. They find 12.5 volts between the solenoid-feed and the ground, and assume the part is bad. Decisions based on incomplete testing cause wrong guesses.

Voltage-drop testing

More informative is the voltage-drop test. This is where we test the circuit, under the normal operating load. Voltage-drop testing offers the advantage of revealing current available under a load. By testing while we apply a load, we gain far more useful information.

First, system voltage is checked for reference. Checking for voltage between the positive and negative terminals of the battery, shows 12.5 volts. This is near full voltage so we know the proper power is available. Next, we use the voltmeter to check for voltage between the positive battery terminal and the input of the solenoid, with the solenoid applied. Back-probing this connector allows testing under the normal system load.

An example of how voltage-drop testing works

With a good connection, very little voltage flows around the circuit. With a perfect connection, the voltage drop would be zero. In reality some resistance will exist and one-tenth of a volt drop is considered acceptable. A measurement of 0.1 volts confirms the power-side of the circuit is good. To flow, a current must also have a path to ground. Our testing is incomplete until we also check the ground circuit.

Testing between the ground side of the solenoid and the battery ground, shows four-volts. Voltage is flowing around the circuit, because the ground is incapable of supplying a proper flow.

Close examination of the ground connection reveals frayed wires and the source of the problem. A single strand of wire connects the circuit to ground. Unloaded, the circuit flows full voltage as the current requirement is negligible. With the solenoid activated, insufficient current is available and the lock-up does not engage. Repairing the ground connection corrects the problem.

Without such testing we can imagine what might happen. We replace the solenoid and the problem continues. We might assume the solenoid is defective and again replace it. Next we might guess the torque converter or even the transmission is bad. Replacing either will not correct the problem. The PCM issues the command for lock-up, so we might even replace this. The lack of diagnostic testing-knowledge costs a huge amount and does not correct the problem.

Voltage-drop testing is useful in testing almost any circuit that we can switch on. In part two we examine more complex examples of this procedure.

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