Tech Stuff

Real-world voltage drop testing: Analyze problems and discover solutions

Alex Portillo is the head technician of Car Clinic, a state-of-the-art automotive repair facility in Mahopac, New York. He is a protégé of Jerry Truglia and has been trained by Automotive Technician Training Service and is TST certified. Alex’s real-world, in-depth diagnostic articles will appear in Auto Service Professional on a regular basis.


Figure 1: A theoretical headlamp circuit where the bulb is dim, even after it’s replaced.
<p>Figure 1: A theoretical headlamp circuit where the bulb is dim, even after it’s replaced.</p>

Not every electrical problem you run into is the result of an open in a wire or in a component. It might be rare, but electrical resistance at a connection or internal to a wire can cause very real driveability problems.

So, before we get into a case study that illustrates advanced voltage drop diagnostic techniques, we are going to cover what voltage drop testing even is for those who are not acquainted with using their meter or Power Probe for this task.

How to perform a voltage drop test

Essentially, a voltage-drop test measures how much resistance electricity runs into as it passes through a load. A reading of up to 200 mV is permissible, but in the real world it is rare you will see voltage drops more than a few dozen mV.

But, as a nice rule of thumb, 200 mV is the highest permissible voltage drop on all vehicle components aside from a computer component, where 100 mV or less is permissible.

Let’s work with an overly simplified headlamp circuit, pretending that after replacing the bulb the light is still dim (see Figure 1 on page 28). If it helps, follow along in our example with a black pen and a red pen, pretending they are your meter’s black and red test leads.

Always check system voltage first, using V DC on your meter.

• Check that the battery has 12.6 V.

• If voltage is low, the battery needs to be charged.

• Isolate where the problem is in the circuit by doing a quick voltage drop test.

• In order to find out if we have a good voltage drop on the feed side, all we do is put our red lead on the positive terminal of the battery and our black lead on the feed (positive) side of the lamp. Positive to positive.

• In order to find out if we have a voltage drop on the ground (negative) side, we put our black lead on the negative battery terminal and our red lead on the ground side of the lamp. Negative to negative.

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