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Multi-meter usage primer

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[Note: Some of this gets very detailed, but don't worry about all the heavy detail stuff you don't understand if you don't want to (like accuracy, or what resistance you're measuring across compared to the meter's, or what the meter's loading is). Just understand the simple basics of how you connect the meter to measure voltage, resistance, and current.]

It helps to think of electrical current flowing through a wire analogous to water flowing through a pipe. Voltage is the water pressure, resistance is the pipe diameter (larger diameter has less resistance and allows more water to flow), and current is the water itself (amount flowing).

Voltage:
Voltage is ALWAYS measured across a wire or component or can be at a point. If at a point, the meter's negative lead is always connected to chassis/body/engine ground or battery negative terminal. If across a wire or component, the leads are connected on each side of it. Current may or may not be flowing through the wire or component. You usually cannot damage a voltmeter no matter how you connect it (even if you accidentally connect it in series or have a digital one on the wrong range) since it has very high internal impedance (total resistance to all frequencies). Of course if you connect it in series, any devices on that circuit will now not operate since you just inserted a very high impedance. If you know the approximate voltage (eg. 15v), set the scale range to handle it. If you don't know, then set the scale range to the highest one and work your way down. Digital ones will display an error if you use too low a range or it is connected backwards. Some of the more pricey ones even automatically adjust the range or if connected backwards. When measuring DC pulses (eg. CAS signal) the reading will be the average. When using an AC scale the reading value will be bogus since all multi-meters assume you are measuring a sine wave (which cars don't have). The fact that you get a reading however is meaningful.

Voltage measuring accuracy (can skip if just want basics):
For analog (needle) multi-meter voltage measurements the higher scales have higher resistance so they load down the circuit less. But the meter still always places resistance in parallel with the load on voltage scales. So again, you will get an incorrect and misleading reading on low scales where the meter's internal resistance is significant compared to the resistance of the load (usually when meter's resistance is less than 20 times that of the load). This decreases the resistance of the circuit you're measuring (since you just placed resistance in parallel with it) and usually results in a lower voltage reading than you actually have. With most car circuits however, the resistance of the device/circuit you are measuring across is so low compared to the voltmeters resistance that you don't have to worry about any inaccuracy due to voltmeter loading. Digital multi-meters (voltage settings) and oscilloscopes don't have the problems like analog multi-meters do since they usually have 10 meg-ohms input impedance on all voltage scales. Analog multimeters typically have 20,000 ohms/volt on dc scales (so a 15v scale has 300,000 ohms input impedance).

Resistance:
Resistance is ALWAYS measured across (NOT in series with) a wire or component that has NO voltage present or current flowing through it. Voltage or current will affect the reading giving you a bogus result and also may easily damage the meter. Also be aware that measuring something that has capacitors on it (like measuring from one battery cable to the other without the battery there - yet the ECU, radio, alarm, etc are still connected which have internal capacitors across their 12v power inputs) will give constantly changing and bogus results while the capacitor is charging (from the ohmmeter's voltage). It usually appears to be an exponentially increasing resistance and you will then have a voltage there which again makes the reading bogus. Normally you disconnect whatever you are trying to measure or what will give bogus results. Start off on the lowest scale range and work your way up until you get a believable reading. Also note that on analog multi-meters the probe polarity is reversed on resistance scales - on digital ones it is not.

Current:
Current is ALWAYS measured in series with a wire or component. Placing the meter in parallel will nearly always damage the meter. Using a scale range that is too low may also damage some meters. It's always safest to start out on a high scale and work your way down. Don't use the meter on something that exceeds the meter's capability or you will damage it or blow an internal fuse/wire. A good example of this is trying to measure starter current which is in the 50-150 amp range with your typical 10 amp meter. Normal engine current draw also exceeds 10 amps as may just turning on a lot of things without the engine even running.

Current measuring accuracy (can skip if just want basics):
On smaller current settings the resistance of the meter is higher and when that resistance gets significant compared to the resistance of the circuit you're measuring the current of (usually when the meter's resistance is more than 5% of the circuit's resistance), the meter "loads" down the circuit and you get an incorrect and misleading reading. The smaller the setting (or scale) the higher the meter's resistance. This resistance is placed in series with the load which will give you a lower current reading than you actually have (since you just placed more resistance in the circuit). This is why the lower scales which you would think would be more accurate actually are worse if the meter's resistance isn't way way less than the circuit's. [hint: So when you don't know the circuits resistance, which is often the case, use the higher setting - you get less digits but they are more believable and true.]

Other (can skip if just want basics):
With all digital meters any reading on any scale is always +/- one least significant digit. Examples: 120.5V is 120.4-120.6V; and 5mA is 4-6mA; and 5.0mA is 4.9-5.1mA; and 5.00mA is 4.99-5.01mA; and 1 ohm is 0-2ohms; and 1.0 ohms is 0.9-1.1 ohms; and 0.99 ohms is 0.98-1.00 ohms; and 0 ohms is 0-1 ohm, and 0.0 ohms is 0-0.1 ohm, etc.

Test leads connection (aka probes):
- On the cheap analog (needle) meters connect the black (negative) lead into the minus jack and the red (positive) lead into the plus jack.
[Note: On nearly all of these analog meters the polarity is reversed on the ohms scales (positive is on the minus jack). This only makes a difference if your measuring across a semiconductor (eg. diode or transistor) or something that is polarized (eg. capacitor). You can tell because you'll get a different reading with the test leads reversed. The actual reading is bogus in all these cases but you may be trying to figure out which end of a diode is the cathode so knowing the meter's polarity is then necessary.]
- On digital meters for volts and ohms, connect the black (negative) lead into the COM jack and the red (positive) lead into the V/omega symbol jack.
- On digital meters for current, connect the black (negative) lead into the COM jack and the red (positive) lead into the 10A jack for the 10 amp scale or the A jack for all other lower current scales.
 
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