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:
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 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
System Description: When the switch is ON, the Engine control module (ECM) lights up the lamp.






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

Relationship Between Open/Short (high Resistance) Circuit And The Control Unit Pin Control: Case 2:

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