Checking For Voltage Drop

Voltage Drop Test
Checking Voltage drop across connections or components as shown will indicate whether there is abnormal resistance creating an additional load in the circuit - consuming power and dropping the Voltage available to other parts of the circuit.

NOTE: Voltage drop measurements can only be made when the circuit is powered and there is normal current flow.

Testing For Voltage Drop:

The example illustrates troubleshooting the back-up light switch. The switch is in the circuit to switch power to the back-up lights ON and OFF. When Reverse gear is selected and the switch is closed, it should have very little resistance and not be a consumer.

If dirt or corrosion on the switch contacts creates resistance, some of the available battery Voltage goes into overcoming that resistance. Less Voltage is available to light the back-up lights, and they will not be as bright.

NOTE: An ohmmeter can measure resistance (or check continuity) only when the circuit is not powered, i.e. when there is almost no load.

Low Voltage:
A low Voltage reading across the two switch contacts indicates almost no difference in potential. Resistance across the switch is low - most of the battery Voltage is passing through the switch and is still available to power the lights.

High Voltage:
Any significant Voltage reading indicates a difference in potential across the switch contacts. Excessive resistance is loading the circuit, causing a Voltage Drop. Voltage is consumed overcoming the resistance of the switch, and less is left to power the lights.

Maximum allowable Voltage drops recommended by the Society of Automotive Engineers are:
0.0 Volt for small wire connections
0.1 Volt for high-current connections (Example: fuel pump, headlights)
0.1 Volt for ground (GND) connections
0.2 Volt for high-current cables (Example: battery/starter cable)
0.3 Volt for switch or relay contacts