Multi-Meter Series: The Practical Differences Between A/C and D/C Voltage

Just about the entire world runs on alternating current (A/C) voltage. Cars and trucks or anything else that runs off a battery will run on direct current (D/C) voltage. Many don’t know what the difference really is, or what it means for technicians and the diagnostics they have to perform. There is only one difference between the two types of current but that can create many problems for techs. In this article, we’ll look at what that difference is, how it affects us, and how new technology is changing the way trucks run, and the way we diagnose them.

A/C voltage and why it runs our world

Long story short, A/C is cheaper and more efficient over long distances. This is important when you look at where power is generated and where that power is used (homes and businesses). Also, it takes a lot more amperage to create the same heat (and damage) that high voltage will create in a wire or cable carrying D/C voltage. What this means is that a voltage spike or a short circuit will do less damage in an A/C system.

If you’ve had the unfortunate experience of shorting a cable for a fraction of a second, then you know how fast a wire will burn up. Although you would want to avoid ever causing a short, it is a little less aggressive in an A/C circuit. If you want to learn more about the intense battle between Edison and Tesla (not the car guy Elon Musk, but the inventor guy Nikola Tesla) there are a ton of great articles, books, and documentaries on the topic. For that reason, I’ll let that lie, suffice to say that our power grids are A/C by our choice, batteries are D/C their nature.

A/C or Alternating Currents alternates. It literally goes back and forth the way a cylinder moves up and down as it cycles in an engine.

Our vehicles being run on D/C has a lot less to do with a choice having been made than by the reality that batteries supply a steady and direct current. Now, while D/C is simply the way it is by its nature that doesn’t mean that A/C cannot be used on a vehicle. And it certainly doesn’t mean that A/C can’t be used on a truck. A simple transformer can turn a D/C line to an A/C one.

There is even a truck model with a throttle position sensor that takes A/C and sends out D/C as a signal to the ECM (electronic control module). The point I’m trying to make is this: Before assuming that there is a fault ‘upstream’ (closer to the power source) always make sure that you are looking for the right sort of voltage. There are even models of transmission that use a ‘doubler’. A doubler does exactly what it sounds like, it turns 12V to 24V. So, you can spend days looking at 12.6V DC when you should have somewhere around 24-26V DC. And don’t forget, an ABS sensor output is A/C.

One obvious takeaway is that, more than ever, a test light will not help you in many diagnostic tests.

Closing point

The reason that not looking for the right current is a problem is that with A/C being a wave for a signal, and D/C being a straight line for a signal, the multi-meter will look for the wrong sort of information. Like anyone or anything looking for one signal but receiving another, it will misunderstand. This means taking longer to find the real problem and possibly replacing electronic parts (that can’t be returned).

A common complaint is that it is very hard to come across electrical plans if you don’t work for a manufacturer or a licensed dealer. Information is usually available, but it sure isn’t cheap. One solution is to, when what you’re reading doesn’t make sense, simply try different ranges and functions on your multi-meter to make sure you aren’t missing anything. In the same way that if you aren’t confident with the ground you are using you try different points.

In the next article, we’ll get more in-depth in multi-meter troubleshooting techniques.

Drive safe,

Kevin Gauthier