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Voltage Drop Testing

6673 Views 5 Replies 5 Participants Last post by  Subvet

Some of the most difficult and challenging problems to solve are in the electrical system. Since electricity is invisible to us, we are reduced to using tools and devices to measure and verify it's presence. For this discussion we really need to understand a few things.

  1. In order for electricity to flow, it needs to travel in a closed loop. If the loop (circuit) is not complete, we have only a potential circuit. So if we have no loop, we have no circuit, and no flow.
  2. Every circuit contains electrons. Even open circuits. Even a scrap of old wire is full of electrons. If you push more electrons in at one end, then electrons would flow out the other. Sort of like a conveyor belt. The more you put in, the more you get out.
  3. Any resistance in a circuit will reduce the number of electrons that are able to flow through it. Resistance can be poor connections, small wire size, poor quality wire, corrosion,
In any circuit with excessive restrictions to electron movement flow, we will experience poor performance. We can have the right amount of electrons pushing, but not get the full flow at the other end. The headlight circuit is an excellent illustration of voltage drop. Did you know your headlights are not receiving full voltage? A popular electrical modification is to add an under hood relay to boost the available voltage to headlights due to voltage drop. The larger amperage circuits like the Starter Circuit and Charging Circuit are particularly vulnerable to these restrictions of current flow. The large gauge wire on these circuits can mask the problem and make diagnoses difficult. Checking these circuits with an ohm meter may yield false results. A better way of testing these circuits is when they are under a load. By using this testing method, we can test small portions of our circuit to find the potential problems. When having starting or charging problems, we will want to concentrate on the battery posts, terminals, cables, connections and grounds.

  • Using the Voltage Drop Testing method on portions of the circuit while it under load will reveal the "restrictions to electron flow", by showing us a voltage differential on our volt meter.
  • A reading of 0 volts when using this method is good. The higher the number, the bigger the problem.
  • Voltage Drop testing can find problems on both the positive and negative side of the circuit.
  • Voltage Drop Testing is ideal for testing higher amperage circuits, like starters, and alternators.
To test the starter circuit: Measure from battery positive post to cable end, then test each cable until you get to the starter. Then do the same on the negative side, starting at the battery post, and following it to body ground and engine ground. Then do it between body and engine. Remember, the circuit has to be under a load for this test to reveal the problem. Any voltage being displayed during the test reveals a potential resistance to electron flow.

A resistance test using an ohm meter is not an accurate way of determining if a battery cable is good. According to this ohms test, this battery cable is good!

A Voltage Drop Test can find all sorts of losses when used correctly on high amperage circuits. The circuit needs to under a load to perform the test.

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Good one Doc, not a lot of folks know how to do a voltage drop test on a component system. This is the most accurate way to diagnois a faulty circuit, relay, component or ground.
Here is a pictorial post of how to use drop voltage testing for a starter circuit. You might find it helpful, if not to troubleshoot your particular issue but to further understand the concept.

How To voltage drop test a starter circuit

Posted by BogHog
i have a 97 f150 that the battery dies over night i dont see anything left on any ideas
IF you disconnect the battery and it is dead in the morning after you had charged it, you have a bad battery and it needs replacement. IF it is good in the morning, then you may have a parasitic drain on the system killing your battery.

How to find a short/ parasitic drain
Good one Doc, not a lot of folks know how to do a voltage drop test on a component system. This is the most accurate way to diagnois a faulty circuit, relay, component or ground.
I had a 67 Taurus that developed a hesitation problem when accelerating rapidly in traffic. After some trouble shooting I found the alternator voltage would drop 2-3 volts when engine speed was accelerated. Replaced the alternator and the hesitation problem went away.
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