“In the shop with Mr. A” – Voltage Drop Testing

Words and photos by Ron Alexander

As with most car enthusiast we are all drawn to the marvels of the mechanical machine. The moving part that performs the work that makes the magic happen is what we love to rebuild, free up, adjust, or just marvel at in its mechanical simplicity. There is nothing more rewarding as pulling a mechanical part that has not worked in 20 plus years, freeing up the rust, cleaning the gunk, replacing the wear items, and making it work again. The satisfaction of understating the operation and pride in the repair is hard to describe. A good can of penetrating oil, some hand sanding, maybe a few targeted whacks with your favorite BFH, and wam-oo it works! Pride in a job well done.

But in the area of electrical diagnostics, this is still an area of frustration for most. Based mostly on the lack of understanding of the operation of the wizardry and evil magic that occurs in even the basic electronic circuit. You can’t see how it works and if you ask your kid’s science teacher how it really works you find yourself dozing off in the middle of the explanation anyway. 

So, my suggestion is to skip the scientific explanation all together and look at your sink. Yup, your sink. If you consider the three major measurements taken in any electrical system you will understand.

The measurement of voltage is just another way of saying pressure. Pressure of the electricity, how much push do you have? PSI in your water system.

Amperage is the measurement of the volume of electricity flowing. How much water per minute flows from the faucet? If your pressure is down, the volume will be down.

The last measurement is restriction to flow, resistance and measure in ohms.

Considering the understanding of the above three, you can see the relationship and have a better idea of how things work. If you have a given pressure, let’s say 12 psi at the valve, we allow the flow of water with no restriction, you will have maximum volume. If we increase the resistance, the volume will decrease. This will also result in a drop in pressure in the flowing system downstream of the resistance. This is what we see when the green monster invades our copper wires. The green monster corrosion is a real issue with any vehicle that is exposed to water. As the resistance increases in the circuit, the volume of flow is decreased resulting in pressure drop and a lower output, dim bulb, slow spinning motor. The term voltage drop is used by automotive techs every day to help them diagnose and identify the location of the restriction. Simply measuring voltage in flowing system allows them to identify the location of the restriction. If you had a pipe in your home and the water was on fully flowing and you have full pressure on both sides of a union, you can say you have no restriction. If you have a 5 psi less on one side of the union, you would be seeing a pressure drop that represents a restriction. This is the same with an electrical system; if you fully run the starter motor and see a 5V drop in pressure across the red starter cable, you have a restriction in the cable. The key to keep in mind is you always need to have the circuit running and flowing when performing this test. If you don’t, you will have full pressure even after the restriction. Turn it on to test!

Congratulations if you made it this far without falling asleep, not the most exciting topic, but let’s face it, having some basic understating of electrical system operations will keep you from reaching for the BFH next time the starter turns slower than normal.

Tech article brought to you by:  Morrisville State College Automotive Program.


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