Digital multimeters: Tools that tell the truth

Publish Date: 
Sep 29, 2012
By Tracy Martin
Having a digital multimeter is as basic as a 3/8-in. drive ratchet when it comes to any type of electrical work on motorcycles, ATVs and watercraft. Photos by Tracy Martin

LET’S SAY A motorcycle, personal watercraft or an ATV is in your shop because the engine doesn’t start. You connect the factory diagnostic scanner and the trouble code for a crank sensor is listed. Simple; just replace the crank sensor.

But first you have to get to the crank sensor, which means you have to remove the engine cover. But before you remove the engine cover, you have to drain the engine oil.

If you’re lucky, the Parts department has the crank sensor you need, but perhaps they will have to order it. Do you work in an independent shop? Then you’ll have to order the part from a dealer — and remember, there are no returns on electrical components, so you’d better hope this will fix the no-start problem.

Once you replace the sensor you will need a gasket for the cover and new oil for the engine. You’ll charge the customer for the labor, crank sensor and four quarts of oil for the engine (so far, you’re making money). But then you turn on the ignition, press the start button, the engine cranks over… and nothing. It doesn’t start. You reconnect the scan tool and the same trouble code shows up (bad crank sensor).

So instead of moving on to the next job, you’re working on this one for free until you figure out the real problem.

If this scenario hasn’t yet happened to you, it will. Let’s rewind the previous diagnosis, and then add a few extra steps.

The onboard computer’s diagnosis of a faulty crank sensor is only as good as the information the computer receives. When the ignition is turned on and the starter motor turns, the computer expects to see a signal from the crank sensor; if it doesn’t, it sets a trouble code indicating the sensor is faulty.

You need to verify whether the crank sensor is working independently from the computer’s diagnosis. Unplug the sensor where it is connected to the wiring harness and connect a digital multimeter. Crank the engine over and watch the meter’s display; if you know what the numbers mean, it’s easy to determine whether the sensor is working.

When you know the sensor is in good shape regardless of what the computer is saying, you can then start determining the root problem. Unplug the large connector at the computer. Use a wiring diagram to determine which wires are coming from the crank sensor. Connect your multimeter to the wires, and check the crank sensor signal like you did before. This time, the numbers may indicate that the signal is not present. With a closer look, you may find corrosion (green crud) on the inside of the main computer connector which you can remove with a wire brush and electrical cleaner. Plug the connector back into the computer, press the start button, and the engine fires up. Perform a quick check with the scan tool; if there’s no crank sensor code, the problem is fixed.

The extra steps in using the multimeter to verify the sensor’s operation only take a few minutes, but they save hours of labor along with extra parts.

FEATURES TO CONSIDER

The analog bar graph (above arrow) on the Fluke 175 digital multimeter can update up to 40 times faster than the numbers, making these types of multimeters ideal for indicating an intermittent faulty sensor signal.

Knowing how to use a digital multimeter can save lots of headaches. But what’s not as easy is knowing which one to purchase, and which features are useful.

Let’s take a look at the Fluke 175 multimeter as an example of a high-quality professional test tool. Other multimeter manufacturers offer similar features at varying prices (there are hundreds of choices — just search for multimeter online and you’ll find out). The Fluke Corp. has been manufacturing digital multimeters for many years, and the Fluke 175 is a good example of what’s available for midrange (about $250) automotive-type units.

There are two basic kinds of multimeters: averaging, and True RMS (root mean square). Both measure voltage and resistance, but the RMS type can more accurately measure distorted sine waves. AC pick-coils, often used on powersports vehicles as engine speed sensors and ABS sensors, output a lopsided waveform (signal); an averaging type of meter may not be able to read these outputs. A True RMS meter can accurately read the frequency produced by these sensors.

A feature that’s important to have is an analog bar graph (see photo). This part of the display is located beneath the numbers and is made up of segments that move horizontally across the display. The bar graph serves as an electronic meter needle, indicating changes to the meter reading much faster than the numbers can update. (continued)