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Performing a voltage drop test

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If an engine is cranking slowly or doesn’t crank at all, the first things people usually think of are the battery and starter. But the problem could be as simple as a connection -- corroded, loose, frayed, or broken ground or positive battery cables, wires or terminal ends. There are many ground cables, wires and straps going from the engine to the frame, frame to body, engine to body, etc. And, while your vehicle may still start with one of these corroded, loose or broken, it creates higher resistance which could cause it to crank slowly. And, the higher resistance could also prevent the battery from being fully charged.

Check your connections first

Some problems are pretty obvious. If you twist a terminal end and it’s loose or you see crusty corrosion bubbling up from it, you know you need to tighten or clean that connection. But, corrosion can also build up under the insulation of a battery cable, between the terminal end and the post, or between the terminal end and the cable where you can’t see it.

Check for damage to the terminal end where it is attached to the post and the cable, if you don’t have a good connection due to damage you’re going to have higher than normal resistance. An out of round or over-sized terminal end could cause a bad connection as well; so, even though it’s tight on the post, it’s not making full contact with the post. I’ve also seen cases where someone has replaced a bad cable, but installed one smaller than the application calls for, causing excessive resistance. All of your cables, straps and wires must be clean, tight and making good contact for your starting and charging systems to work properly.

Testing for voltage drop

The best way to pinpoint a problem like this is a voltage drop test. You create a load in a circuit while using a multimeter to measure the voltage drop. You will have little or no voltage drop in a circuit where all the connections in the are good, and see 0.1 to 0.4 volts on the meter. Anything more than a 0.2 volt drop indicates a problem in that circuit, whether it be a bad connection, corrosion, etc.

Examples

  • Checking for a voltage drop in the connection between the positive battery post and terminal end - Set your multimeter to the 20 volt scale. Disconnect the ignition coil or unplug the fuel pump relay, so the engine will not start. Connect the positive lead of your meter to the battery post, and the negative lead to the terminal end that is clamped to the post. Have someone crank the engine for about 10 seconds while you watch the display on the meter. With a good connection you will see no voltage drop.
  • Checking for a voltage drop in a battery cable - Connect the meter positive lead to the terminal end, and the meter negative lead to the eyelet on the other end of the cable. Again, have someone crank the engine while you watch the display on the meter. If the voltage drop is higher than 0.2 volts, there is a problem with the cable.
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