Need help using voltmeter...

[x_Tallon_x]

Hello
I'm new and I found it hard to figure out where this would fit in so I picked this forum. I'm in my automotive core program and I am having a bit of trouble in one of my classes.
When diagnosing a circuit how do you measure for high resistance using the voltmeter in a DMM?
Also I need to know exactly what to measure to find opens, shorts, Available Voltage and Voltage drops/total VD.
I can use a meter and I know how the circuits work I just have trouble figuring out where to measure and test is coming up.

Using simple series and parallel circuits like headlights/rear lights, by the way.
I can't find any sites that show how this measuring readings stuff works.
Thanks.

[shorod]

So one of the study questions is specifically to measure resistance using the voltmeter function of the DMM, or just by using a DMM? If it's just by using a DMM, then you want to use the resistance setting designated by the symbol for "ohms" which is the Greek letter Omega. To measure the resistance using this setting, you will want power removed from the circuit, then place one of the probes on one side of the item to be measured and the other probe on the other end of the item to be measured. The reading on the meter will be the resistance.

If the requirement is to measure the resistance in a powered circuit using just the voltmeter setting, I'm not sure that's possible. By Ohm's Law you would need to understand two out of three of the variables to determine the resistance. You can determine the voltage drop, but that's only part of the answer in determining the resistance. To determine the voltage drop you would place the two probes across the item being measured for drop like you did in the prior description. Instead of the resistance measurement though you would select DCV or Volts DC. The drop across the part (wire, light bulb, fuel pump, connector, etc.) would be indicated on the meter in Volts DC. To determine the resistance through the device though would also require knowing how much current is flowing through it, which is a the second measurement. Once you have those two pieces of information, the DC resistance would be calculated by the formula:
Resistance = (Voltage drop / Current through the device).

For example, if you measure a voltage drop of 1 volt through a connector and determine that there is 10 amps flowing through the connector, the DC resistance of the connector is R = 1 / 10, or 0.1 ohms. Light bulbs are a bit trickier to measure since the resistance will change slightly as the bulb filament changes temperature.

-Rod