Testing Circuits

In general testing electrical circuits is an easy task if it is approached in a logical and organized method. Before beginning it is important that you have all available information related to the system you are going to test. You should also have a thorough understanding of how the system operates so that you use the appropriate test procedure and equipment.

While testing electrical components, if the incident is reported as intermittent, it may be necessary to gently shake the wiring harness or electrical component to simulate vehicle vibrations.

DIAGNOSTIC TOOLS AND EQUIPMENT





A Digital Multimeter DMM (10 megaohm input impedance) can safely be used to diagnose and test most vehicle systems.

Test lights are not recommended due to the possibility of inadvertently tapping into a computer or air bag circuit and causing damage.

Before attempting to diagnose a circuit for an open or a short, it is important that you:

1. Have the proper reference material available. Power Supply Routings Applicable Service Manual sections Fix Tips
2. Understand how the system works.
3. Have a good understanding of what condition you are diagnosing Situation when the problem occurs (weather, loads, etc.) What systems interact with the one you are diagnosing Is it intermittent or a consistent problem
4. Have proper equipment to perform the diagnosis such as a digital voltmeter, alligator clips, and probe leads.

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. Refer to the attached drawing for sample schematics.

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 (OL, 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 attached diagram.

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 1 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 swl. 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 swl 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 sensor. 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 above example.

Voltage Check Method

To help in understanding the diagnosis of open circuits please refer to the previous diagram. 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 swl open, probe at swl to check for voltage.
voltage; open is further down the circuit than sw1.
no voltage; open is between fuse block and swl (point A).

4. Close sw1 and probe at relay.
voltage; open is further down the circuit than the relay.
no voltage; open is between sw1 and relay (point B).

5. Close the relay and probe at the sensor.
voltage; open is further down the circuit than the sensor.
no voltage; open is between relay and sensor (point C).

Any powered circuit can be diagnosed using the approach in the above 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 sensor disconnected) powered through the fuse.

3. Connect one probe of the ohmmeter to the load side of the fuse terminal and 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. With swl closed, relay disconnected and probes at the load side of fuse terminal and ground 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. With sw1 closed, relay contacts jumped with jumper wire and probes at the load side of fuse terminal and ground check for continuity.
continuity; short is between relay and sensor (point C).
no continuity; check sensor, retrace steps.

Voltage Check Method

1. Remove the blown fuse and disconnect all loads (i.e. swl open, relay disconnected and sensor disconnected) powered through the fuse.

2. Turn the ignition key to the ON or START position and 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 swl open and the DMM leads across both fuse terminals, check for voltage.
voltage; short is between fuse block and swl (point A).
no voltage; short is further down the circuit than sw1.

4. With sw1 closed, relay and sensor 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 swl 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 sensor (point C).
no voltage; retrace steps and check power to fuse block.

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 resistance can change the way a circuit operates.

Electronically controlled circuits are very sensitive to proper grounding. A loose or corroded ground can drastically alter an electronically controlled circuit. These circuits generally operate in the 5 volt range. The components in these circuits can be seriously affected by a voltage change as low as one tenth (0.1V) of a volt. A poor or corroded ground can easily affect the circuit by that amount. 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 screw or clip.
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 one or more of the ground wires does not 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. Part of the available voltage is used by the resistance. When there is excessive resistance less voltage is available for other loads (lights, motors, etc.) in the circuit. Since each resistance in a circuit uses voltage, a voltmeter can be used to isolate problems.

A voltage drop across closed contacts can indicate excessive resistance. This can cause the circuit to operate incorrectly. Remember a switch is not a load. During diagnosis, use a voltmeter to measure the voltage drop across each switch contact while the circuit is in operation.





Check the wire in the illustration. If an ohmmeter is used to measure resistance (circuit off), the single strand of wire still making contact would give a 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. Using the voltmeter this will be picked up as a slight voltage drop.

Unwanted high resistance can be caused by many factors as illustrated below:
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 in isolating excessive drops in low voltage systems, such as those in "Computer Controlled Systems". Circuits in the "Computer Controlled Systems" operate on very low amperage. Any variation in resistance in the system due to poor connections, improper installation, improper wire gauge or corrosion can adversely affect the systems operation. A step by step voltage drop test can be used to identify a component or wire which is operating under too much resistance.

Circuit Inspection:

1. Connect the voltmeter as described in illustration, starting at the battery and working your way around the circuit.

2. An unusually large voltage drop will indicate a component or wire that needs to be repaired. As you can see in the illustration above, the poor connection causes a 4 volt drop.

The chart that follows illustrates some maximum allowable voltage drops. These values are given as a guideline, the exact value for each component may vary.

COMPONENT VOLTAGE DROP
Wire negligible <.001 volts
Ground Connections Approx. 0.1 volts
Switch Contacts Approx. 0.3 volts
Starter Solenoids Approx. 0.5 volts