# SERIES 638 BASIC KNOWLEDGE REQUIRED

The following items must be read and thoroughly understood before using this manual. ‪

1. The engine and ignition should always be off before the harness connectors are disconnected or reconnected.
2. When disconnecting harness connectors, be sure that the pulling force is applied to the connectors themselves and not the wires extending from them.

#### Electrical Circuits

A basic understanding of the following will enable the user to use the troubleshooting flowcharts: ‪

• You should understand the theory of electricity and know the meaning of voltage and ohms. You should understand what occurs in a circuit with an open or shorted wire and be able to read and understand a wiring diagram.
• You should be able to use jumper wires to make circuit checks.

#### Digital Volt-Ohm Meter

A basic understanding of the following will enable the user to troubleshoot the Turbotronic 638: ‪

• You should be familiar with the digital volt-ohm meter and 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 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 volt-ohm 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 relatively low 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 may be used in any chapter of this manual 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 will not have continuity. To use this continuity feature on certain meters: ‪

1. Place the function/range switch in any volt-ohm range.
2. Connect the red lead to the volt-ohm 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" (over limit; com meters will show 1 +", ", or simply 1").
3. Put one test probe at one end of the wire or circuit to be tested. Use the other test lead to trace the circuit. When continuity is established, an ohm symbol will appear in the upper left corner of the digital display. If contact in the wire is maintained long enough (about 1/4 of a second), the OL will disappear and the resistance value of the wire or circuit will appear next to the symbol.
4. If your VOM will not work in the manner described above, refer to OEM guidelines.

Voltage measurements

1. Connect the red test lead to the volt-ohm input connector and the black lead to the com input on the meter. If a DC-AC switch is present, make sure it is switched to the DC position.
2. Set the function range switch to the desired volts position. If the magnitude of the voltage is not known, set the switch to a range which will be able to read most voltages seen on a vehicle (a 20V range will do). Then reduce the range until a satisfactory reading is obtained.
3. Connect the test leads to the circuit being measured. In the flowcharts, voltage measurements are always given as being taken at pins, sockets, battery +, or ground. Following the voltage measurement point, the color test lead to be used is given in parentheses (red is the volt-ohm connection, and black is the com connection). Example: If the procedure states, take voltage reading at socket C2 (red lead) to socket K1 (black lead)", the hook-up would be as follows; see Figure 24167 .

Typical Volt-Ohm Measurement