Multimeter basics and buyer's guide

By Dave Mowitz
Ageless Iron Almanac Editor

A multifunction electronic tester, better known as a multimeter, can be a powerful weapon in the battle with electrical problems. But often multimeters are tucked away and forgotten because their owners never learn how to use them.

Multimeters can be intimidating tools at first glance, with so many different functions available and only brief, and even then, cryptic labels. But, for most mechanical chores, often all you'll end up measuring is DC voltage from 10 to 15 volts and resistance from 0 to 20,000 ohms.

Now, it helps to get the right multimeter for the job. Select a simple rugged meter built for the automotive trade. Desirable features include auto-ranging ability, slip-on alligator clips for test probes, audible continuity checker, and automatic power shutdown when not in use.

There are myriad chores a multimeter can tackle. And the more tasks you take on, the better you'll understand how and when to use a meter. Believe me, that understanding will open more diagnostic doors for you in the future. But I recommend that you start with the simple chores when first working with a multimeter. Here are two simple tasks to start with:

Testing a dead battery

Select DC Volts on the tester. Next, choose a maximum scale reading of 15 to 30 volts, depending on what your meter has available. If you are using an auto-ranging digital meter (which I prefer for its accuracy), you don't need to worry about choosing a measurement range. Just set the tester for DC Volts. And you'll need to calibrate an analog meter by touching the leads together and adjusting the calibration thumbwheel until the meter reads zero. Digital meters do not need calibration.

Now check battery voltage with the engine and accessories (such as lights) switched off. Touch the red test lead firmly to the positive post on the battery and the black lead to the negative post. Get a good connection because accuracy depends on it.

You should get a reading of 12 to 13 volts. If the reading is less than 11.5 volts, then you may have to charge the battery or jump-start the vehicle to get the engine running for the following test.

Next, with the engine running at fast idle and all accessories switched off, check the battery voltage again. It will be in the 13.5- to 14.5-volt range if the alternator is charging the battery.

If the reading is the same or lower than your engine-off reading, the battery is not being charged. If the reading goes higher than 15 volts and stays there, your voltage regulator (if your tractor employs one) is defective and should be replaced.

Measuring resistance in the spark plug wires

This simple test will tell you if the existing wires need replacing. Resistance is just what it sounds like. It is the amount of restriction in the flow of electricity through a circuit. It is measured in ohms. Analog-type meters, where the reading is displayed by a needle swinging across a scale, are the easiest to understand when measuring and comparing resistance.

All modern ignition systems use resistor-type spark plug wires to reduce radio frequency interference (RFI). These wires may develop too much resistance as they age, leading to hard starting, rough running, and missing under load. This problem is easy to pinpoint with a multimeter. To begin, set your meter to a maximum reading of around 20,000 (20K) ohms. With an auto-ranging digital meter, just set it to measure ohms — you don't have to manually select a range. On most analog-type meters, you'll want Rx100, or 100 times the indicated reading on the meter face. Your results may vary, so consult your meter's guide.

Next, remove one plug wire at a time, so there is less chance of reconnecting them in the wrong order. Get a good connection between your test leads (this is where those alligator clips come in handy) and the ends of the wire being evaluated. Don't forget to check the coil wire as well during your analysis.

Now, normal spark plug wire resistance runs from 3,000 to 7,000 ohms per foot of length. So it's normal for longer wires to have more resistance than their shorter neighbors. After testing replace any wire that has more than 15,000 ohms resistance unless a service manual specifies otherwise. And, if during the course of testing the wires you discovered that several of the wires are bad, it's best to just replace the whole set.

Buying a multimeter Multimeters can range in price from as low as $25 for a basic analog (dial-meter) volt-ohm-meter up to $200 and beyond for advanced diagnostic meters. If you haven't used or owned a multimeter, I highly recommend you purchase a basic machine, which will set you back $25 to $35. Some people prefer the older-style analogue multimeters (left) that provide readings with a dial (needle) scale. However, I have found that digital devices (right), which provide readings via a liquid-crystal display, provide readings that are easier to read. Regardless of the type of multimeter you choose, the $25 to $35 investment in a basic multimeter will give you a machine that is accurate enough for use on antique tractors or the family car (with accuracy ranges of plus or minus 3% to 4%). An absolute must for any multimeter is test leads. Almost all meters come with a pair of positive and negative probes. I highly recommend investing in a set of probes with alligator clips on the end of each wire lead, which makes the process of testing that much easier. Some multimeters have additional features such as transistor testing and ranges for measuring capacitance and frequency.

Learn more

A dandy Web site that provides an in-depth explanation of multimeter use can be found at: www.kpsec.freeuk.com/multimtr.htm. You can shop for a wide range of multimeters at the following Web sites:

• www.hobbytron.net
• www.fluke.com/products
• www.electronickits.com