Voltage Testing
The voltmeter (DVOM) must be connected in parallel with the load or circuit.
The DVOM has a high resistance and taps off a small amount of current.
A voltmeter must be used with the current on and with the correct polarity.
The red lead should be connected to the B+ side of the circuit and the black lead to the B-side of the circuit.
If the leads are reversed the reading will be a negative number.
- Select proper function and range of DVOM.
- Connect (-) lead of meter to battery B- or known good ground.
- Connect (+) lead of meter to test circuit.
Scheme 270
Amperage Testing
To measure amperage the meter must be installed in series in the circuit. The current flow of the circuit must flow through the meter itself.
Current must be flowing in the circuit.
Installing the meter in parallel with the circuit may cause damage to the meter, because of the increased current flow in the circuit, due to the low resistance in the meter.
| CAUTION | Most ampere meters or DVOMs are rated for no more than 10 amps. Current flow above 10 amps will damage the internal fuse of the DVOM and render it unable to measure amperage. |
- Select proper function of DVOM and move leads to proper position.
- Connect meter in series with (+) lead on the B+ side of the circuit.
- Connect (-) lead of meter to complete circuit.
Scheme 271
Resistance Testing
When set for resistance testing (Ohms) the DVOM must never be connected in a live circuit.
The component or portion of a circuit being measured, must be isolated from the power source.
Most modern day DVOM's are self ranging when set to measure resistance, so the meter can not be damaged by out of range measurements.
The test leads may be used without regard for polarity, unless the circuit contains a diode.
The DVOM functions by placing a very small amount of current on the circuit being tested, the red lead must be placed on the anode side of the diode.
- Select correct function and range (Most meters are self ranging in this function).
- Disconnect power to circuit.
- Disconnect any circuit wired in parallel with circuit being tested.
- Connect test leads.
Scheme 272
Continuity Testing
The DVOM uses its own internal power supply to test the continuity of the circuit. The DVOM must never be connected in a live circuit. Any circuits wired in parallel with the circuit being tested must also be disconnected.
Continuity testing verifies that circuit connections are intact. The continuity mode is extremely fast and is used to detect either shorts or opens that last as little as 1 ms.
When a change is detected the beeper tone is stretched to last at least 1/4 second so both shorts and opens can be audibly detected.
This is a valuable troubleshooting aid when diagnosing intermittent faults associated with wiring, connections, switches and other components of the circuit.
- Select correct function and range of DVOM.
- Disconnect power to the circuit.
- Disconnect any circuits wired in parallel.
- Connect DVOM leads to the circuit to be tested.
Scheme 273
Voltage Drop Testing
Voltage Drop Tests determine the resistance of an active circuit, a circuit with current flowing.
Voltage drop tests are preferred over simple resistance measurements because the power source is not removed from the circuit.
By measuring the voltage on both sides of a load, the amount of voltage consumed by the load is measured.
The voltage drops of each part of a series circuit added together must equal the power supply for that circuit while it is active.
- Select proper function and range of DVOM.
- Connect (+) lead to the B+ side of the circuit or component being tested.
- Connect (-) lead to the B- side of the circuit or component.
