Real-world voltage drop testing Leave no stone unturned to analyze problems and discover solutions By Alex Portillo 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 certifed. Alex’s real-world, indepth diagnostic articles will appear in Auto Service Professional on a regular basis.
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ot 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.
Voltage drop tests Bad reading: More than 200mV Bad reading for computer electronics: More than 100mV example with a black pen and a red pen, pretending they are your meter’s black and red test leads. Always check system voltage frst, 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. How to perform a voltage drop test • In order to fnd out if we have a good Essentially, a voltage-drop test measures voltage drop on the feed side, all we do is how much resistance electricity runs into as put our red lead on the positive terminal it passes through a load. A reading of up to of the battery and our black lead on the 200 mV is permissible, but in the real world feed (positive) side of the lamp. Positive to it is rare you will see voltage drops more positive. than a few dozen mV. • In order to fnd out if we have a voltage But, as a nice rule of thumb, 200 mV is drop on the ground (negative) side, we put the highest permissible voltage drop on all our black lead on the negative battery tervehicle components aside from a comminal and our red lead on the ground side puter component, where 100 mV or less is of the lamp. Negative to negative. permissible. Let’s work with an overly simplifed head- Positive to positive, negative to lamp circuit, pretending that after replacing negative: that’s a voltage drop test! Let’s try to visualize this. If the feed side the bulb the light is still dim (see Figure 1 on page 28). If it helps, follow along in our of the lamp is good and the ground side
26 | May/June 2013
