International DT466 – DIAGNOSTIC TOOLS AND ACCESSORIES – Electronic Circuit Testing – Ohmmeter

CAUTION: To prevent damage to the test meter, only use the ohmmeter on circuits when the power is OFF.
Power from 12 V systems may damage the meter.
The ohmmeter measures resistance (ohms) in a circuit. Ohmmeters use a small battery to supply voltage and current flow through the circuit being tested.
Based on Ohm’s Law, the ohmmeter calculates resistance in the circuit by measuring the voltage of the meter battery and the amount of current flow in the circuit. Range selection and meter adjustment are not necessary with the DMM.

Resistance measurements are used to determine the resistance of a load or conductors, the value of resistors and the operation of variable resistors.

To measure the resistance of a component or a circuit, remove power from the circuit. Isolate the component or circuit from other components and circuits so that the meter current (from probe to probe) only flows through the selected component or circuit. When measuring the resistance of the load, most of the current flow from the meter will go through the indicator lamp because it has less resistance.

Remove one connector to the load. It is not always apparent when a component must be isolated, so it is a good practice to isolate a component or circuit by disconnecting one circuit. Place the ohmmeter leads across the component or circuit to display the resistance in ohms. When checking a sensor or variable resistor such as the fuel level gauge, heating the element or moving the arm should move the meter through a range of resistance that can be compared to a specification.

Open electrical circuits can be diagnosed using an ohmmeter. Disconnect the power supply to the circuit and isolate the circuit from all other circuits. The circuit between the light and the ground is disconnected to prevent reading a circuit that may be shorted to ground ahead of the load device as a continuous circuit. Connect the ohmmeter to the open ends of the circuit. A high reading (infinity) indicates an open circuit. A reading near zero indicates a continuous circuit. With the Fluke 88 Digital Multimeter (DMM), an open circuit will read OL (over limit).

Checks for short circuits are similar to checks for open circuits. Isolate the circuit from the power source and
the ground point. Connect the ohmmeter between an isolated circuit and a good ground point to check the circuit for a short to ground. A short to ground will be indicated by a reading near zero. A circuit that is not shorted to ground will cause a high meter reading.

Measuring Duty Cycle with FLUKE 88
When measuring duty cycle, ensure that the large dial on the meter is pointing to volts DC, the DUTY button is set to the Duty Cycle function, and the trigger has a positive slope.

Use the following procedure to check duty cycle:

1. Turn the large dial on the meter to volts DC, indicated by V RPM.

2. Press the % DUTY button to select duty cycle mode. The screen on the meter will show TRIG (with a _ under the        TRIG) in the lower left hand corner of the screen. A percent sign will appear on the upper right hand corner of the      screen.

3. In duty cycle mode, press the ALERT button to change from negative to positive trigger slope. The slope is                    indicated by a plus or minus sign below TRIG in the lower left hand corner of the screen. A percent sign will                  appear on the upper right hand corner of the screen.
4. After the meter has been set to the correct settings, connect meter as indicated in Pin-Point Diagnostics.


24. November 2021 by samuel
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