Want Better Garage Results? Ban Test Lights!

Electrical problems can be one of the most difficult upsetting issues a truck owner can have, and for good reason! When you do an oil change, you understand, X amount of minutes to replace the filters, Y amount of minutes to drop and refill the oil. It makes sense, you see it. Hours and hours to find where to fix a single wire somewhere in the cab or chassis… It can hardly get more frustrating. This is why it is terrifying to hear “we replaced the light and checked the fuse, but, we don’t know why it won’t work”, only sometimes it’s a wiper function or a fan, or worse, a starter.

How the test light leads us astray

I feel the need to begin by saying that like most things in life, the test light can be a good thing. It does have its place. I go into that a bit later on. The test light, though, on the other hand, has its limitations, and we ask it to do too much. To get the light to work, it takes a minimal amount of voltage and amperage. A wire that is too frayed to work a headlamp can still illuminate the much smaller, weaker bulb (often nowadays a single LED). On the same note, a weakened wire too weak to power an electric motor (window, fan) will still illuminate a test light.

To clarify: when I say a weakened wire I mean one that is frayed (almost completely worn through), rotting or rusted (verdigris) or has a bad or loose connection somewhere.

Now, when power is verified at an electric accessory with a test light, what happens is, the power is assumed to be good. With that assumption that the power is good, the component must not be and therefore, should be replaced. If the part is in stock, then all that is lost is an electrical component that often cannot be returned if not needed, and the time (and cost) of labor. If the part being replaced is not in stock or close by, costs are compounded. And the real issue is still not solved.

What you can do to improve results in your garage

In more and more shops across the country, all electrical problems are being reported with an attached file presenting voltage, amperage and/or resistance at different harness connections. Now, when it’s your fleet being repaired in your shop, this can be a fairly straightforward initiative to add to your existing processes. When a problem is being repaired elsewhere or on the road, you need to ask for these numbers. In the above situation where power was detected and found to be good, what was the voltage? A test light can easily light up at 3-4 volts, but for most components and accessories 12-14 is needed to work properly. Asking to have these numbers beforehand is a great way to get the mechanic on the other end to think about checking the wiring properly, therefore, not shooting in the dark with a test light. Furthermore, if this information is a problem for the garage to give you, maybe your choice of garage should be reconsidered. Any garage with staff equipped and competent with electrical equipment should be the only ones doing electrical work.

The right tools for the job

Now that I trash-talked the poor little test light, I should say, I do believe there can be an appropriate time and place for a test light. Where quick checks are all that is needed, or the ensuing repair or part replacement is simple and inexpensive, like a small light bulb. The moment there is a doubt, the moment there is something that doesn’t add up, the test lights time is up.

With all the available attachments for the multimeters, there are no more good excuses for the speed and ease of a test light. Not only are multimeters themselves much less expensive for solid, reliable models they are less and less expensive.

Electrical problems can be tough enough and at times difficult to understand. Anything that can make the job of solving these issues easier should be done. Using the right tool for the right job is always the best, easiest way to be more efficient and get better results, no matter the job.

Drive safe,

Kevin Gauthier