Circuit Inspection

DESCRIPTION

• 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.

OPEN	A circuit is open when there is no continuity through a section of the circuit.
SHORT	There are two types of shorts.
	SHORT CIRCUIT	When a circuit contacts another circuit and causes the normal resistance to change.
	SHORT TO GROUND	When a circuit contacts a ground source and grounds the circuit.

TESTING FOR "OPEN" 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.

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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). Check to always start with the DMM at the highest resistance level.

To help in understanding the diagnosis of open circuits, please refer to the previous schematic.

• Disconnect the battery negative cable.
• Start at one end of the circuit and work your way to the other end. (At the fuse block in this example)
• Connect one probe of the DMM to the fuse block terminal on the load side.
• 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)
• 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)
• 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 previous 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.

• Connect one probe of the DMM to a known good ground.
• Begin probing at one end of the circuit and work your way to the other end.
• With SW1 open, probe at SW1 to check for voltage.

  voltage: open is further down the circuit than SW1.

  no voltage: open is between fuse block and SW1 (point A).

• 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).

• Close the relay and probe at the solenoid.

  voltage: open is further down the circuit than the solenoid.

  no voltage: open is between relay and solenoid (point C).

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

TESTING FOR "SHORTS" IN THE CIRCUIT 

To simplify the discussion of shorts in the system, please refer to the following schematic.

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RESISTANCE CHECK METHOD 

• Disconnect the battery negative cable and remove the blown fuse.
• Disconnect all loads (SW1 open, relay disconnected and solenoid disconnected) powered through the fuse.
• Connect one probe of the DMM to the load side of the fuse terminal. Connect the other probe to a known good ground.
• 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.

• 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.

• 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 

• Remove the blown fuse and disconnect all loads (i.e. SW1 open, relay disconnected and solenoid disconnected) powered through the fuse.
• Turn the ignition switch to the ON or START position. Verify battery voltage at the battery + side of the fuse terminal (one lead on the battery + terminal side of the fuse block and one lead on a known good ground).
• 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.

• 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.

• 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.

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:
  • Remove the ground bolt or screw.
  • Inspect all mating surfaces for tarnish, dirt, rust, etc.
  • Clean as required to assure good contact.
  • Reinstall bolt or screw securely.
  • Inspect for "add-on" accessories which may be interfering with the ground circuit.
  • If several wires are crimped into one ground eyelet terminal, check for proper crimps. Check all of the wires are clean, securely fastened and providing a good ground path. If multiple wires are cased in one eyelet check no ground wires have excess wire insulation.
• For detailed ground distribution information, refer to GROUND DISTRIBUTION .

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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 DMM, 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

• Connect the DMM across the connector or part of the circuit you want to check. The positive lead of the DMM should be closer to power and the negative lead closer to ground.
• Operate the circuit.
• The DMM 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.

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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.

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CONTROL UNIT CIRCUIT TEST 

System Description

• When the switch is ON, the control unit lights up the lamp.

CASE 1

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Terminal No.		Description		Condition	Value (Approx.)	In case of high resistance such as single strand (V) *
+	-	Signal name	Input/Output
1	Body ground	Switch	Input	Switch ON	Battery voltage	Lower than battery voltage Approx. 8 (Example)
				Switch OFF	0 V	Approx. 0
2	Body ground	Lamp	Output	Switch ON	Battery voltage	Approx. 0 (Inoperative lamp)
				Switch OFF	0 V	Approx. 0

• The voltage value is based on the body ground.
• *: If high resistance exists in the switch side circuit (caused by a single strand), terminal 1 does not detect battery voltage. Control unit does not detect the switch is ON even if the switch does not turn ON. Therefore, the control unit does not supply power to light up the lamp.

CASE 2

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Terminal No.		Description		Condition	Value (Approx.)	In case of high resistance such as single strand (V) *
+	-	Signal name	Input/Output
1	Body ground	Lamp	Output	Switch ON	0 V	Battery voltage (Inoperative lamp)
				Switch OFF	Battery voltage	Battery voltage
2	Body ground	Switch	Input	Switch ON	0 V	Higher than 0 Approx. 4 (Example)
				Switch OFF	5 V	Approx. 5

• The voltage value is based on the body ground.
• *: If high resistance exists in the switch side circuit (caused by a single strand), terminal 2 does not detect approx. 0 V. Control unit does not detect the switch is ON even if the switch does not turn ON. Therefore, the control unit does not control ground to light up the lamp.