guidance for digital multimeter

Hello All,

I have done a comprehensive search and have found much information about using a DMM for automotive electronic projects and the importance of doing so.  There is an overwhelming wealth of information on these forums that provides much guidance.  However, I am having difficulty using a DMM and, even though there is much information, my questions have not been answered in a way that makes me understand.  I have learned how to test for a good ground using continuity.  I would like to know how to physically use the DMM to test wires for alarm installation, power windows, and trunk solenoid. 

Many guides here say "test the wire with a DMM" but what am I testing for, and how do I set my DMM to test it?  I know there is way more information you will need, so maybe we can start with these questions below and continue from there.  I know they may sound quite elementary but thats the exact state that I'm in:

• How do you test for constant 12v and what should it read? (and how do you physically set the DMM)
• When testing power windows, what are you testing for? (and how do you physically set the DMM)
• How do you test if a wire is positive or negative? (and how do you physically set the DMM)

Hopefully we can start here and move on to some more questions.

Thanks,

Scott

There should be an easy guide... One main rule is never set to test Resistance on a circuit with power (voltage). Resistances should always be unpowered. The safest setting is the highest voltage, but for 12V systems, the 20V DC is fine. "Voltage meters" are "high impedance" which simply means they take negligible current. (As opposed to current meters which are essentially a short circuit - ie, never test across ANY battery with the DMM set to Current. Even a 1.5V battery will usually blow the internal DMM fuse, or if on the 10A setting, be destructive or hazardous). FYI - A voltmeter is current meter with series resistance. The resistance is usually in Mega-Ohms (hence negligible current unless using very high voltages). You'll see most voltmeters rated with 10MOhm or 20MOhm input impedance - ie, very high resistance. To measure vehicle voltages, the DMM's black (-ve) probe is usually connected to ground - ie, chassis/body, GND, battery -ve aka 0V (zero Volts). Then the red (+ve) probe shows +ve volts across the connection. (If a minus "-" sign appears in front of the reading, then it's -ve with respect to the -ve/black probe. The above assume the black probe is in the black or GND or 0V socket of the DMM, and red is in the appropriate socket - eg, voltage, current, Ohms.    Sometimes a test light is more suitable for testing - a mere 12V 3W bulb is all that's needed. Whilst that won't indicate polarity (unless you add a diode, or use 2 lamps and diodes, or LEDs), it will load the circuit as well as being easier & quicker to determine if +12V is present.    Optional rant: The last paragraph refers to cases like the recent relay (thread) where the DMM measured +12V on the GND side (85) of a relay coil. That's because the relay-coil's resistance was negligible compared to the DMM so the coil had no voltage drop and the DMM measured the +12V #86 side's voltage (since there was nothing else to pull #85's voltage down). A 3W test lamp (being about 48 Ohms) would have pulled down the voltage to 6V or less. It's dimness would have indicated "less than 12V". That's for automotive relay coils that are usually of similar resistance (typ ~50 to ~80 Ohms). If the coils were higher resistance (200-400 Ohms etc) then the bulb may not even glow - ie, too small a current thru the coil resistance). "Resistive Voltage Dividers" explain what happens above. Be careful, there are somethings NOT to test - eg, oxygen sensors for resistance, spark=plug or other high-voltage sources.

Frankly on your car Scotty, an incandescent bulb test light would be far more easy to use and convenient. e.g. Snap-On CT-45. The bulb draws the current to trigger locks etc, but it's bulb acts as a fuse to protect the circuit, ground the wrong wire you will at least blow a fuse if not expensive processor damage. You'll do no harm with a test light, also faster response. Yes as a pro, I have the above, plus LED testers, power probes and DMMs. The test light is used in 90% of cases unless I'm playing with battery diagnostics or finding tach (which can still be done with an LED).