Using the DMM

Watt is being measured? Volts Amps Ohms Watts Weight

Resistance Tests Tests a circuit or component statically Does not confirm if something is “good” Can confirm if something is “bad” Useless for finding certain problems Good for finding other problems

Ohm reading? 1 ohm 2 ohms .5 ohms No ohms OL OMG?

Bulb resistances: Typically low Typically high Typically OL Typically OMG

Or the component is disconnected from the vehicle

Why does an open switch read OL? Low resistance High resistance Infinite resistance Confused DMM b/c its LOL without the L

A closed switch should read? 0.0 ohms Less than 1 ohm More than 1 ohm OL High resistance

Why a resistance test isn’t perfect How many ohms does this wire have? Is it OL? Low Resistance? How much current will it carry?

This may cause? A blown fuse No light operation Dim light operation Nothing 1 and 3 2 and 4 2 – 4 2 – 3

Unwanted Resistance Generally in the form of corrosion Can be green but may not appear at all Broken wire strands = increased resistance Fretting and thermal cycling May not show up during resistance measurement

Voltage Drop Testing Tests the circuit dynamically – this means the circuit must be in operation!!!! Shows how much voltage is getting to a spot Shows how much voltage is being used Shows how much voltage is left Much better than a resistance test

Voltage drop means? Voltage available to a load Voltage used by a load Voltage not used by a load Ooops, I dropped the DMM when I was measuring voltage

Why are the drops 12 volts? Because they’re parallel Because each gets battery voltage Because each has ground Because each is the only load

Did the last slide show voltage drop testing? Yes No Not sure

Not voltage drops The DMMs are placed to measure voltage at a point in the circuit

Testing voltage To test for voltage (available voltage or source voltage) – negative lead on ground somewhere Positive lead placed at test location – usually the power supply to a load Indicates the voltage difference between the two leads only

Checking fuses = checking for voltage

Circuits 101 Complete path for electron flow Positive side and negative side Contains Power (positive) Ground (negative) Circuit protection Control Load Conductors