In The Shop: Those Darned Test Lights

Published on: 18:58PM Dec 11, 2019

This is for all you die-hards who insist on testing DC electrical circuits on farm equipment with those darned, antique test lights. You know, the testers that look like a pointed screwdriver, with a little light bulb in the handle and a 6-foot-long wire coming off the handle for you to clamp to a good ground point?

"I don't need no fancy voltmeter," I've been told by more than one farmer. "I can tell all I need with my test light."

Really? Can you tell the difference between 4.5 volts and 4.0 volts? Because on modern farm equipment, a half volt on a low-voltage circuit is the difference between working like it's supposed to and an armrest display flashing "Warning" or "Diagnostic Error Code."

If you've resisted buying a voltmeter because you didn't want to spend the extra money, or were intimidated by all the dials and settings on digital voltmeters, here's what you need: a digital test light that displays voltage readings on a small display in the handle, instead of illuminating a little light bulb.

Test as you normally would with your trusty old test light. Clamp the end of the cable running from the handle to a known ground point, then stick the point of the test light against the wire, stud, or circuit you want to test. A small digital display in the handle will read out the voltage to within 0.1 volt. Instead of trying to judge the strength of voltage by the brightness of that little light bulb, you get an accurate voltage reading on the display in the handle. 

They cost from $20 to $40 at auto parts stores, and are well worth their price. Christmas is coming; ask Santa for one, or give one to a friend or family member who needs to retire his antique test light.