How to Test the Voltage Drop of a Relay Contact

By Joe Glassford. To test the condition of the contacts (excessive resistance from pitting that causes an excessive voltage drop) this voltage drop test should be done:

Here we have a fuse located on the “ground” side of the relay switch. This is a common circuit arrangement you will see.

Check the wiring diagram to see where the fuse is located, if it is located on the voltage feed side ahead of the relay switch, your meter negative probe will then have to be placed on the input pin to the load instead of on the top of the fuse as illustrated.

The circuit must be on.

Voltmeter should be set to mV (millivolts), or on the lowest volt range.

Maximum allowable voltage drop for any relay or switch contact is 300mV (.300V).

If you want to test the contact resistance with an ohmmeter instead of doing this test, pull the relay, hot wire the coil to bring the switch contacts together, then check the resistance between (in this style relay) 30 and 87. If you read even 1Ω of resistance, the relay should be replaced.

Note: You can also just pull the relay up out of its female receptacle a little making sure all of the tangs are still making contact, then put you negative (-) voltmeter probe on the tang to terminal #87. This will also give you the voltage drop of the contact switch.