Here are the most common measurements made by mechanics who are troubleshooting, ranked in order of how frequently they're used:
Why this order?
Voltage will be the #1 measurement because it requires no alteration to the circuit before the measurement is made, and does not affect the circuit operation during the measurement. It's safe to say that approximately 75% of all electrical faults can be isolated through voltage measurements.
A resistance measurement is NEVER made in a live circuit; an ohmmeter is a source of voltage and acts much like an additional power supply.
A current measurement is seldom made because the ammeter must be connected in series with the device being tested. This requires wires to be disconnected or cut.
For your own safety, and for the safety and accuracy of the device, here are a few rules to remember before you use a DMM:
A voltage measurement is made by placing the test leads in parallel with the device or source under test.
Measuring voltage across any electrical or electronic component or device is also referred to as measuring the voltage drop.
Make sure that you dial the mode selector to the proper position to measure volts AC or volts DC.
Be sure to check that your voltmeter is operational before testing a live circuit. This can be done by measuring a known voltage.
A continuity test is the checking of an electrical circuit to see if it is in fact a complete circuit.
Push with the mode selector at the ohmmeter position to enter the continuity mode.
The meter operates exactly like an ohmmeter when testing continuity.
The continuity test features a beeper that sounds as long as there is a very low resistance path (less than a few hundred ohms). This allows you to perform quick continuity tests without having to watch the display.
The resistance reading represents the resistance of all paths between the probes.
An ohmmeter applies a small voltage across the resistance under test. The resulting current is dependant upon the circuit resistance between the probes.
It is important to isolate the load or resistance under test to ensure a correct measurement.
Do not touch the leads during the test as the resistance of your body may impact the measurement accuracy, because you have created an additional path for meter current flow.
To measure current, you must break the circuit being tested and place the meter in series with the circuit.
When measuring current less than 400 mA, insert the red lead into the mA/µA terminal.
To measure current above 400 mA, insert the red lead into the A terminal.
If you are about to measure DC current, don't forget to push yellow button to switch the meter to DC.
An example of when you might measure current is to verify correct solenoid operation on a hydraulic pump stroke controller.
Using your multimeter to read the current shouldn't take more then a few moments, and you can determine whether the solenoid is actually receiving the correct current. This will let you know if the problem is the solenoid, or the power supply, without ripping the pump apart and replacing parts unnecessarily.
First, put the multimeter in series with the stroker solenoid to be tested. A test adapter plug is a handy way to do this, or you might need to solder your own adapter.
Set the multimeter to measure DC amps, and take a moment to consider the expected current. Do you need to plug the red lead into the amps, or the milliamps jack on the meter?
If you are unsure, it's always best to start on the highest current range and work your way down.
Checking the manual on this hydraulic pump stroker control tells us that the stroker operates with a signal between 180 and 325 mA. If the signal sent is within those parameters, it would make sense to plug into the mA jack on your multimeter.
It's possible that the solenoid may be receiving a higher signal than expected. If this happens, you might blow a fuse on your meter, or, even worse, damage the meter.
The absolute safest scenario for your test meter is to start measuring through the amps jack. Then, once you have verified that the current is within the milliamp range, switch jacks for the finer milliamp resolution.
Since this particular stroker is proportional, you can expect to see a range of values during solenoid operation.