Multi-Meter Series: An Introduction

If you get a check engine light and read the codes, do you know what to do with it? Do you replace the sensor or is there may be a problem with the wiring?

The driver’s report claims a light has burned out. You replace the light and the new one doesn’t work. What now? The test light is a go to for most mechanics. Test lights are great tools for quick readings but test lights are not perfect and can lead to bad diagnostics. One example is a trailer stop light problem I once had. In order to save time (and use my 15$ tool rather than my 300$+ tool), I take out my test light to check my power and ground. The tractor is off and 53 feet away I’m reading 12.2 volts. Great. So, I plug in a new light and tell the driver to push on the brakes. Nothing happens. So, I check my new light with a known power/ground (two test leads on a battery) and it works. Something doesn’t add up. I grab my voltmeter and I have a good ground and voltage at the back of the tractor, the front of the trailer, and back of the trailer. I check the amperage, there isn’t enough to light a single incandescent light.

Long story short, a circuit breaker in the tractor fuse box is half burned but there is just enough life left in it to let a tiny bit of voltage through. Just enough for a LED test light, but not enough for a set of incandescent lights. The part of the story I left out is that another mechanic who only owned a test light had spent two hours not understanding the problem, replacing lights and pigtails, searching high and low but using the wrong tool for the job.

Now a 5-10$ light isn’t a big deal. Engine sensors and electric motors are big deals. Diagnosing problems by replacing parts can be very expensive. Unfortunately, not everyone has had good multi-meter training. Many who have been trained can forget, it’s definitely a perishable skill.

So, whether it’s for the first time or just a refresher, what follows in the coming days is a primer on everything you need to know all in the name of being a better mechanic. We`ll be covering all the basics of voltmeters, ammeters, and ohmmeters as well as practical things like the voltage drop test and how to use an amp clamp to make testing easier.

Stay tuned…!

Drive safe,

Kevin Gauthier