petrock thats some good information, you should make that a sticky in the tech area.

 

and dosnt the multimeter have to be set in ohms?

 

I'm not sure how to do that.


No, you want the multi-meter set to volts. A voltage drop test, as it's name implies, is testing volts. An ohm (a.k.a. resistance) test is not as reliable or as accurate as a voltage drop test. An ohm meter (or a multi-meter set to measure ohms) applies a very small amount of voltage/amps to the circuit and measures the difference between the two leads to determine the resistance. In simpler terms, it is doing a voltage drop test with a very small amount of voltage flowing through it and converting the voltage reading into ohms (google: ohms law).

There are two problems with an ohm test in my opinion:

• The small amount of power the ohm meter sends through the circuit isn't enough for a good test. The proper way to test a circuit is with the normal amount of power it usually has flowing through it (e.g. in this case 12 volts by having the lights on).
• An ohm test is only useful if you have the ohm spec for the circuit. All circuits will have some amount of resistance in them. Even a short piece of wire has resistance. So without that ohm spec your getting a meaningless reading from the ohm meter.

A voltage drop test does not have either of these problems. You test the circuit with the proper power flowing through it (e.g. the lights turned on) and you're measuring the effect of the resistance on the circuit not the amount of resistance. Any resistance on the circuit will reduce the amount of voltage flowing through the circuit (again, google ohms law). The multimeter when set to read voltage is just comparing the voltage reading at the negative lead with the voltage reading at the positive lead. So, for example, if the multimeter reads 12.6 volts from the negative lead and 12.0 volts from the positive lead, it will display .6 volts which is your voltage drop between the two leads. The general rule of thumb is a voltage drop of .5 volts is acceptable on a 12 volt circuit and a voltage drop of .2 volts is acceptable on a 5 volt circuit. Any more then those values and you have too much resistance on the circuit. Hope that makes sense...

 

Yea that is some really good info, I have yet to try it due to lack of time and money (I don't own a multimeter yet) but I will soon give it a whirl. Thanks so much for taking the time to write that, I'll be sure to post back with an update...

 

Multi-meters run anywhere from $5 to $200. You can pick up a good one for less then $40 from radio shack or your local auto parts store.


No problem. Teach a man to fish... :-)