How to use a multimeter to test automotive switches

Would we like more videos like this showing how to diagnose things? 

In reply to David S. Wallens :

This is good content. I'm pretty familiar with this type of electrical diagnostics but I watched the video and looked for some tips. It was well done and you can always pick something up.  Thanks for making it. 

In reply to NY Nick :

Thanks for watching it–and thanks to Chris and Jesse for the heavy lifting.

Yeah, just some good, basic troubleshooting. 

Yes, we do.

^^^^^^^   Ditto

Testing static resistance can tell you if there's a path but not necessarily how good the path is. I've seen many switches that have continuity/ low resistance when tested but fail under load.

The way I explain it to my students is that a continuity test (and a resistance test to some degree) just tell you that there's a path but not how good that path is. A meter sending a tiny trickle of current through a switch may say the path is good and has 'x' Ohms of resistance. However, when the switch is in the circuit and has a current from a functioning circuit traveling through it, you often see unwanted resistance. Think of a 3 lane freeway in a zombie movie with abandoned cars everywhere. You ask someone to walk through a section and tell you if they can get through. They say they can and therefore there is a path. However, trying to send 3 lanes of traffic through that same section of freeway isn't going to happen because of all the unwanted resistance (abandoned cars) in the way. 

The best way to test a switch (or connection, or section of wire, or anything) for unwanted resistance is with a voltage drop test. This tests the component when it is loaded and the circuit is functioning. All components (other than the load(s)) in a circuit should basically have zero resistance and not drop any voltage when the circuit is on. Reading more than a few tenths of a volt of drop across a component or connection when the circuit is functioning indicates unwanted resistance in that component and it should be serviced or replaced. Note that the load in the circuit should drop voltage as it's where 99.9% of circuit resistance should reside. Anywhere else should effectively drop no voltage.

Performing a voltage drop test is easier than a resistance/ continuity test as well. there's no need to disconnect the component from the circuit. Just set the meter to DC volts, turn the circuit on, and put one lead on each side of the component you want to test. The meter will read the difference in voltage (or drop) between the two points you have it connected to.

TL;DR : voltage drop is a more accurate way to test swtiches than resistance or continuity tests.

I bought a hot rod shop ignition key assembly for Kimini, my mid-engine "Mini." I built a carbon fiber dash, just because I could, and during initial electrical checkout, had the dash assembly pulled out and sitting loose. Part way through checkout, I turned the ignition key and something briefly caught my eye - a whiff of smoke. Thing is, it was coming from a really strange location, right where the edge of the loose carbon dash just happened to be touching the chassis. It was ultimately traced to the brand new ignition switch, where when the key was turned just so - midway between Ignition On and Start - it briefly shorted to the housing of the key assembly, which was bolted to the electrically-conductive carbon dash, which would normally be bolted to the chassis. Who knows how that would have turned out had I not caught it then. So, yeah, pays to check things out before buttoning everything up!

BTW, GRM should probably get a new volt meter.  That one looks beat.