Hello all,

Voltage Drop Testing is a very effective method of finding unwanted Resistance. I am no expert in using it. Here is my question - Have other folks on this site had Success using Voltage Drop?? If you have, can you tell us how you used it? Where did you place the probes of your voltmeter? Thanks.

Hombre427

 

You are pretty much required to use this method when troubleshooting the large starter circuit. Most people do not have the capability or the equipment to measure such large currents.

You will find also that you need to use this method when troubleshooting lighting circuits. If you don't, many times you will find you have 12v on the socket with the bulb out, but as soon as you put the bulb back in it doesn't work. This can be very frustrating to people who do not understand how current flow works in a circuit.

An easy way to think about it is like using a garden hose. The garden hose is the wire, the flow of water is the current, and the pressure of the water is the voltage. When everything is working as it should, you get good water flow and good pressure at the same time. A problem would be a small kink in the hose. If you open the nozzle just a little bit, you still have good pressure. Go ahead and open the nozzle up wide open, and the pressure drops to almost nothing and you get just a small dribble out of the hose.

The hose scenario is what can happen with electrical. You have a circuit, which has voltage and current flow. You need both to do work a lightbulb or turn a motor. Your meter reads the voltage (pressure) of the circuit. It needs very very little current (flow) to get a reading.

So say you have a wiring circuit with a corroded connection. This bad connection would be just like the kink in the garden hose. On the end of this circuit is a bulb and a socket. Take the bulb out and put your meter on the socket, you get 12v because the meter uses very very little current. Put the bulb back in, the bulb uses a lot more current, so the voltage drops very low (like the dribble from the hose) because enough current to run the bulb cannot make it through the bad connection (the kink).

So the best way to test circuits is to test them with your meter with the circuit powered and the loads present in the circuit. In the case of the bulb, keep all bulbs plugged in their sockets. The take your meter and poke the wire while the bulb is still plugged in.

Same with the starter. If you are having a problem with the starter not working correctly, the best way to test it and the battery is to get someone to get inside the vehicle and while you are probing with your meter, they are trying to crank the engine. Typically you would pick a spot to test, and then hollar for them to try and crank it while you watch the meter voltage reading. If that spot is good you move to another spot and then hollar again to try and start it. When you get past the problem area (kink), you will lose voltage when they try to start the vehicle. Before the problem area (kink), you will have fairly good voltage.

Another method that works well on lighting circuits is to use a testlight. A testlight is more crude than a meter, but that is why it works so well. It takes a lot of current to light the testlight, so it will present a larger load and you can find the bad spot without having the regular circuit bulbs in place.

 

Sorry for the previous grammar mistakes above. I fixed what I could find but I was interrupted earlier when writing it and had to submit it without proof-reading.

 

Those were great examples of using voltage drop testing.

 

There are quite a few videos about Voltage Drop on YouTube. Found by just searching. Its free !!