How to Perform Electrical Incident Simulation Tests

Circuit Inspection




INTRODUCTION
In general testing electrical circuits is an easy task if it is approached in a logical and organized method. Before beginning it is important to have all available information on the system to be tested. Also get a thorough understanding of system operation. Then you will be able to use the appropriate equipment and follow the correct test procedure.

You may have to simulate vehicle vibrations while testing electrical components. Gently shake the wiring harness or electrical component to do this.

NOTE: Refer to "HOW TO CHECK TERMINAL" to probe or check terminal.




TESTING FOR "OPENS" IN THE CIRCUIT
Before you begin to diagnose and test the system you should rough sketch a schematic of the system. This will help you to logically walk through the diagnosis process. Drawing the sketch will also reinforce your working knowledge of the system.

Continuity Check Method
The continuity check is used to find an open in the circuit. The Digital Multimeter (DMM) set on the resistance function will indicate an open circuit as over limit (no beep tone or no ohms symbol). Make sure to always start with the DMM at the highest resistance level.

To help in understanding the diagnosis of open circuits please refer to the schematic.
1. Disconnect the battery negative cable.
2. Start at one end of the circuit and work your way to the other end. (At the fuse block in this example)
3. Connect one probe of the DMM to the fuse block terminal on the load side.
4. Connect the other probe to the fuse block (power) side of SW1. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit the DMM would indicate an over limit or infinite resistance condition. (point A)
5. Connect the probes between SW1 and the relay. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit the DMM would indicate an over limit or infinite resistance condition. (point B)
6. Connect the probes between the relay and the solenoid. Little or no resistance will indicate that portion of the circuit has good continuity. If there were an open in the circuit the DMM would indicate an over limit or infinite resistance condition. (point C)

Any circuit can be diagnosed using the approach in the example.

Voltage Check Method
To help in understanding the diagnosis of open circuits please refer to the previous schematic.

In any powered circuit an open can be found by methodically checking the system for the presence of voltage. This is done by switching the DMM to the voltage function.
1. Connect one probe of the DMM to a known-good ground.
2. Begin probing at one end of the circuit and work your way to the other end.
3. With SW1 open probe at SW1 to check for voltage.
4. With SW1 closed relay and solenoid disconnected and the DMM leads across both fuse terminals check for voltage.
voltage; short is between SW1 and the relay (point B).
no voltage; short is further down the circuit than the relay.
5. With SW1 closed relay contacts jumped with fused jumper wire check for voltage.
voltage; short is down the circuit of the relay or between the relay and the disconnected solenoid (point C).
no voltage; retrace steps and check power to fuse block.

Any powered circuit can be diagnosed using the approach in the example.




TESTING FOR "SHORTS" IN THE CIRCUIT
To simplify the discussion of shorts in the system please refer to the schematic.

Resistance Check Method
1. Disconnect the battery negative cable and remove the blown fuse.
2. Disconnect all loads (SW1 open relay disconnected and solenoid disconnected) powered through the fuse.
3. Connect one probe of the ohmmeter to the load side of the fuse terminal. Connect the other probe to a known-good ground.
4. With SW1 open check for continuity.
continuity; short is between fuse terminal and SW1 (point A).
no continuity; short is further down the circuit than SW1.
5. Close SW1 and disconnect the relay. Put probes at the load side of fuse terminal and a known-good ground. Then check for continuity.
continuity; short is between SW1 and the relay (point B).
no continuity; short is further down the circuit than the relay.
6. Close SW1 and jump the relay contacts with jumper wire. Put probes at the load side of fuse terminal and a known-good ground. Then check for continuity.
continuity; short is between relay and solenoid (point C).
no continuity; check solenoid retrace steps.

Voltage Check Method
1. Remove the blown fuse and disconnect all loads (i.e. SW1 open relay disconnected and solenoid disconnected) powered through the fuse.
2. Turn the ignition key to the ON or START position. Verify battery voltage at the B + side of the fuse terminal (one lead on the B + terminal side of the fuse block and one lead on a known-good ground).
3. With SW1 open and the DMM leads across both fuse terminals check for voltage.
voltage; short is between fuse block and SW1 (point A).
no voltage; short is further down the circuit than SW1.




GROUND INSPECTION
Ground connections are very important to the proper operation of electrical and electronic circuits. Ground connections are often exposed to moisture dirt and other corrosive elements. The corrosion (rust) can become an unwanted resistance. This unwanted resistance can change the way a circuit works.

Electronically controlled circuits are very sensitive to proper grounding. A loose or corroded ground can drastically affect an electronically controlled circuit. A poor or corroded ground can easily affect the circuit. Even when the ground connection looks clean there can be a thin film of rust on the surface.

When inspecting a ground connection follow these rules:
1. Remove the ground bolt or screw.
2. Inspect all mating surfaces for tarnish dirt rust etc.
3. Clean as required to assure good contact.
4. Reinstall bolt or screw securely.
5. Inspect for "add-on" accessories which may be interfering with the ground circuit.
6. If several wires are crimped into one ground eyelet terminal, check for proper crimps. Make sure all of the wires are clean, securely fastened and providing a good ground path. If multiple wires are cased in one eyelet make sure no ground wires have excess wire insulation.

VOLTAGE DROP TESTS
Voltage drop tests are often used to find components or circuits which have excessive resistance. A voltage drop in a circuit is caused by a resistance when the circuit is in operation.

Check the wire in the illustration. When measuring resistance with ohmmeter contact by a single strand of wire will give reading of 0 ohms. This would indicate a good circuit. When the circuit operates this single strand of wire is not able to carry the current. The single strand will have a high resistance to the current. This will be picked up as a slight voltage drop.

Unwanted resistance can be caused by many situations as follows:
- Undersized wiring (single strand example)
- Corrosion on switch contacts
- Loose wire connections or splices.

If repairs are needed always use wire that is of the same or larger gauge.




Measuring Voltage Drop - Accumulated Method
1. Connect the voltmeter across the connector or part of the circuit you want to check. The positive lead of the voltmeter should be closer to power and the negative lead closer to ground.
2. Operate the circuit.
3. The voltmeter will indicate how many volts are being used to "push" current through that part of the circuit.

Note in the illustration that there is an excessive 4.1 volt drop between the battery and the bulb.




Measuring Voltage Drop - Step by Step
The step by step method is most useful for isolating excessive drops in low voltage systems (such as those in "Computer Controlled Systems").

Circuits in the "Computer Controlled System" operate on very low amperage.
The (Computer Controlled) system operations can be adversely affected by any variation in resistance in the system. Such resistance variation may be caused by poor connection, improper installation, improper wire gauge or corrosion.

The step by step voltage drop test can identify a component or wire with too much resistance.







CONTROL UNIT CIRCUIT TEST

NOTE: Trouble diagnosis indicates work procedures required to diagnose problems effectively. Observe the following instructions before diagnosing.

1. Before performing trouble diagnosis, read the "Preliminary Check", the "Symptom Chart" or the "Work Flow".
2. After repairs, re-check that the problem has been completely eliminated.
3. Refer to Component Parts and Harness Connector Location for the Systems described in each section for identification/location of components and harness connectors.
4. Refer to the Circuit Diagram for quick pinpoint check. If you need to check circuit continuity between harness connectors in more detail, such as when a sub-harness is used, refer to Wiring Diagram in each individual section and Harness Layout. Refer to Harness Connector.
5. When checking circuit continuity, ignition switch should be OFF.
6. Before checking voltage at connectors, check battery voltage.
7. After accomplishing the Diagnostic Procedures and Electrical Components Inspection, make sure that all harness connectors are reconnected as they were.

FUSE

Fuse:

a. If fuse is blown, be sure to eliminate cause to problem before installing new fuse.
b. Use fuse of specified rating. Never use fuse of more than specified rating.
c. Do not install fuse in oblique direction; always insert It Into fuse holder properly.
d. Remove fuse for clock if vehicle is not used for a long period of time.


FUSIBLE LINK

Fusible Link:

A melted fusible link can be detected either by visual inspection or by feeling with finger tip. If its condition is questionable, use circuit tester or test lamp.

CAUTION:

a. If fusible link should melt, It Is possible that a critical circuit (power supply or large current carrying circuit) is shorted. In such a case, carefully check these circuits and eliminate cause of problem.
b. Never wrap periphery of fusible link with vinyl tape. Extreme care should be taken with this link to ensure that it does not come Into contact with any other wiring harness, or vinyl or rubber parts.