Got electrical problems? Do a voltage drop test.
rushpowersystems
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Just back from a vacation, just in time to fire the snow blower up, if you don’t know what a snow blower is I hate you and envy you!
So I see a few electrical questions that can be answered by a voltage drop test. To do one you need a digital voltage meter, the cheap ones at Harbor Freight or Radio Shack work just fine, however it is easier to use an auto ranging. If you are not using an auto ranging meter than just set it to the lowest scale.
So here is how you do it;
all tests are done with the engine off, make sure you are on volts and not ohms.
Set the meter to volts, now we need to base line the meter so hold both red (positive) and black (negative) leads on the negative battery terminal. Your gauge should read 0.00 anything other than this is the amount your gauge is off.
Now for testing hold the black lead on the battery negative and the red lead on the engine at a clean spot, the number you see is the voltage drop. 0.02 - 0.05 is what we normally see. However anything higher than 0.00 is resistance in the ground. Anything above 0.05 should be replaced. Test this on the frame and body as well.
Now do the same thing on positive and the alternator battery post. Same results should apply, however most factory wiring is a very bad design. Every alternator should go from battery terminal on the alternator, through a fuse, to the battery positive. Unfortunately many go through amp meters (which I hate) and into the key switch or fuse box. So it is not uncommon to see numbers in the 0.1 range, however in my personal vehicles I will get the number as close to 0.00 as possible.
Now lets say you want to test a older non computer controlled blower motor for example;
first we need to ohm test the motor, set the meter to ohm and unplug the connector at the blower motor. Touch the red lead to the positive terminal and the black lead to the body of the motor, normally we will see around 2 ohm but you should check it with a manual for your vehicle. Too high resistance and everything will dim and your voltage will drop when you turn the motor on. Too low resistance and you will blow fuses or burn up wiring, this is a direct short.
Next
turn the key on, turn the blower motor on high, do not start the engine. Test the voltage drop by unplugging the blower motor wire and touching the red lead to the wire terminal and the black lead to the battery negative, this will tell you how much you are losing in the wiring and switch. This should be under 1 volt although I like to see around 0.05, however I have seen working systems that are at 2.0 volts. If you find excessive move the red with to the resistor, then to the switch, and so on until you find the problem. Now turn the blower speed switch, each time you do so you will see the voltage drop number increase. This is because of the blower motor resistor. When the blower motor switch is on high it is a direct power to the motor, the lower speed settings on the blower motor switch routes the voltage through the blower motor resister, excessive drop in the resistor tells you that you have high resistance in the resistor and you need to do a ohm test on the resistor. Finally test the motor ground by doing a voltage drop from the motor body to the battery negative with the key off. If you do this test with key on and you have a bad ground your meter will become the ground path and you will fry your meter.
So I see a few electrical questions that can be answered by a voltage drop test. To do one you need a digital voltage meter, the cheap ones at Harbor Freight or Radio Shack work just fine, however it is easier to use an auto ranging. If you are not using an auto ranging meter than just set it to the lowest scale.
So here is how you do it;
all tests are done with the engine off, make sure you are on volts and not ohms.
Set the meter to volts, now we need to base line the meter so hold both red (positive) and black (negative) leads on the negative battery terminal. Your gauge should read 0.00 anything other than this is the amount your gauge is off.
Now for testing hold the black lead on the battery negative and the red lead on the engine at a clean spot, the number you see is the voltage drop. 0.02 - 0.05 is what we normally see. However anything higher than 0.00 is resistance in the ground. Anything above 0.05 should be replaced. Test this on the frame and body as well.
Now do the same thing on positive and the alternator battery post. Same results should apply, however most factory wiring is a very bad design. Every alternator should go from battery terminal on the alternator, through a fuse, to the battery positive. Unfortunately many go through amp meters (which I hate) and into the key switch or fuse box. So it is not uncommon to see numbers in the 0.1 range, however in my personal vehicles I will get the number as close to 0.00 as possible.
Now lets say you want to test a older non computer controlled blower motor for example;
first we need to ohm test the motor, set the meter to ohm and unplug the connector at the blower motor. Touch the red lead to the positive terminal and the black lead to the body of the motor, normally we will see around 2 ohm but you should check it with a manual for your vehicle. Too high resistance and everything will dim and your voltage will drop when you turn the motor on. Too low resistance and you will blow fuses or burn up wiring, this is a direct short.
Next
turn the key on, turn the blower motor on high, do not start the engine. Test the voltage drop by unplugging the blower motor wire and touching the red lead to the wire terminal and the black lead to the battery negative, this will tell you how much you are losing in the wiring and switch. This should be under 1 volt although I like to see around 0.05, however I have seen working systems that are at 2.0 volts. If you find excessive move the red with to the resistor, then to the switch, and so on until you find the problem. Now turn the blower speed switch, each time you do so you will see the voltage drop number increase. This is because of the blower motor resistor. When the blower motor switch is on high it is a direct power to the motor, the lower speed settings on the blower motor switch routes the voltage through the blower motor resister, excessive drop in the resistor tells you that you have high resistance in the resistor and you need to do a ohm test on the resistor. Finally test the motor ground by doing a voltage drop from the motor body to the battery negative with the key off. If you do this test with key on and you have a bad ground your meter will become the ground path and you will fry your meter.
