BASIC KNOWLEDGE REQUIRED

TM 9-2320-364-20-1

2-14. DDEC III/IV TROUBLESHOOTING (CONT).

Section II. BASIC KNOWLEDGE REQUIRED

Before using this manual, there are some areas that you should be familiar. With this basic knowledge, you will have success

using the diagnostic charts.

A. ELECTRICAL CIRCUITS

You should understand the theory of electricity and know the meaning of voltage and ohms. You should understand what

happens in a circuit with an open or shorted wire. You should be able to read and understand a wiring diagram.

You should be able to use jumper wires to make circuit checks.

B. USE OF DIGITAL VOLT-OHM METER

You should be familiar with the digital volt-ohm meter. You should be able to measure voltage and resistance. You should be

familiar with the controls of the meter and how to use it correctly.

Instructions for use of a typical digital volt-ohm meter are as follows:

Resistance Measurements

1. Connect the red test lead to the V-Ω (Volt-Ohm) input connector and the black lead to the com input connector on the

meter.

2. Set the function/range switch to the desired Ω position. If the magnitude of the resistance is not known, set the switch

to the highest range, then reduce until a satisfactory reading is obtained.

3. If the resistance being measured is connected to a circuit, turn off the power to the circuit being tested (turn off

ignition).

4. Connect the test leads to the circuit being measured. When measuring high resistance, be careful not to contact

adjacent points, even if they are insulated. Some insulators have a relatively low insulation resistance which can affect

the resulting measurement.

5. Read the resistance value on the digital display.

Continuity Checks

In addition to measuring the specific resistance value of a circuit, some meters will also tell if a continuous electrical path

exists. If a path exists, the circuit is said to have "continuity." (This continuity check can be used in any section of the DDEC

III/lV Troubleshooting Guide where the test is looking for greater than, less than, or equal to 5 ohms.) An open circuit

(broken electrical path) would have ∞ resistance and would not have continuity. To utilize this continuity feature of certain

meters:

1. Place the function/range switch in any Ω range.

2. Connect the red lead to the V-Ω connector and the black lead to the com connector on the meter. With the test leads

separated or measuring an out-of-range resistance, the digital display will indicate "OL" (overlimit; some meters show

"1 +", "↑", or simply "1").