March 2013 Articles & Features


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Foreign Service

By Dan Marinucci | March 2013

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Multimeters, amp clamps and oscilloscopes are used every day in shops across the country. Some common sense precautions will keep them working, and possibly save your shop some real money.

Supposedly, March comes in like a lion and goes out like a lamb. This reminds me of the technician who’s careless with a meter or oscilloscope. He roars like a lion when he burns up the instrument, but he’s meek as a lamb when pricing a new one.

This month, I’ll recommend sensible precautions readers should take with digital volt-ohmmeters (DVOMs, or multimeters). Two things prompted this theme: The first was witnessing a series of avoidable and very expensive mishaps. The other was the assumption colleagues made that the people frying these meters were novice techs. But, in fact, each incident involved an experienced guy who didn’t pay enough attention and/or rushed the job.

The most overlooked step to preserving any electronic instrument may be reading the operator’s manual carefully before turning on the device. Plus, vital reminders are printed on the front of the tool—useful stuff for techs who read and heed them.

Let’s visit the common multimeter first. Popular models measure volts, amps and ohms. Usually, the meter’s maximum voltage is printed on it. Note and respect that value; matter of fact, understand the circuit you’re testing and stay well below the meter’s maximum voltage.

The photo below shows a scorched, distorted circuit board from a meter, a prime example of what happens when a tech overlooks its maximum voltage capability. Someone connected this meter to the high-voltage side of an aftermarket HID (high-intensity discharge) lighting setup on a Honda. He either forgot or didn’t know that this side of an HID system is approximately 25,000 volts. Regardless of the exact voltage, it was far above the meter’s limit of 1000 volts.

Motor colleague and contributor Jorge Menchu urged readers to stop making assumptions. “There is such a vast variety of technology on today’s vehicles that it’s wise not to assume every system or circuit is the same as the last one you tested,” he cautioned. Instead, do some homework first so you know the system’s approximate operating pressure and volume (volts/amps).

If Menchu’s name isn’t familiar, he’s an ex-technician who founded Automotive Electronics Services (AES). AES ( manufactures a wide range of OEM and aftermarket test leads for scopes and meters. It also makes specialized electronic test components and database software.

Many multimeters have a single channel for measuring current; others have two. Channel A on my meter handles up to 10 amps and Channel mA µA handles up to 400 milliamps (.400 amp). Equally important, both channels are fused. Many techs I’ve met wrongly assumed the meter they bought was fused. When they made a mistake and released the proverbial smoke from the meter, they realized it wasn’t fused. Then they pitched this unrepairable meter in the trash.

Some meters contain relatively common glass fuses. However, higher quality meters usually have industrial-style, “fast-blow” fuses. Menchu repeatedly stressed that you must always replace a fast-blow fuse with the same type and rating of fuse the meter manufacturer specifies. A shortcut here could cost you a meter.

Some techs create another level of meter security by fabricating or buying a fused test lead. If they make a mistake, the inexpensive external fuse blows; the meter and its internal fuse survive. I began making my own fused leads years ago by carefully splicing a common ATO-type fuse holder into a test lead. If you don’t care to fabricate, contact your favorite equipment supplier about fused test lead availability. For example, ESI ( offers fused test leads Part No. 134. AES custom-builds fused leads for meters and scopes.

Also, Silvertronic, Inc. ( has a neat ATO-type fuse holder (Part No. 127112) equipped with common banana terminals. It connects in series between your meter and its regular test lead. Choosing kit Part No. 127129 gets you a fuse holder with a resettable 10-amp fuse. Finally, Fluke ( offers fused test probes under Part No. FTP-1.

Meanwhile, consider investing in a low-current amp probe or clamp. Many techs are familiar with inductive clamps on equipment such as battery load testers and engine analyzers; these are intended to measure current up in the range of 100 to 400 amps. But there’s a modern crop of clamps that excel at measuring relatively low current—perhaps 10 to 20 amps or less. Typically, the leads of a low-current clamp slip into the traditional terminals in the front of your meter. Like their higher-current cousins, these clamps just convert the current measurement into a low-voltage signal for your meter—or an oscilloscope, for that matter.

To me, an effective low-current clamp is an essential tool. First, it saves time because it eliminates the need to disconnect wiring in order to connect a meter in series in a circuit. This is extra helpful during an ignition-off battery drain test. Second, the inductive technique removes the risk of pushing excessive current through a meter. Third, there’s no harm if the current flow unexpectedly exceeds the range of the amp clamp; it simply won’t yield an accurate reading. The selection of these low-current clamps has steadily increased while the prices have decreased in recent years.

This column isn’t about key-off battery drain testing per se, but I believe it’s worth touching on that topic again. Suppose a battery keeps going dead because a component is shorted somewhere in the vehicle—staying on when it shouldn’t. Traditionally, some guys disconnect the battery while preparing to test ignition-off current drain. First, a tech may connect a meter in series between the negative battery terminal and negative cable. Second, he might connect a battery drain test switch in series in the same location. Regardless, either approach shuts off the entire electrical system. Experience shows that the shorted component may not act up after you reconnect the battery!

Many years ago, some Nissan specialists focused me on this issue and it’s served me well since then. I always keep the battery connected and then check ignition-off current drain with an inductive amp clamp. Of course, the tool has to be sensitive enough to read low-current values but large enough to fit around a battery cable. If you can’t find anything suitable, consider UEI’s Part No. CA30MA ( or an equivalent amp clamp.

Another option for measuring key-off battery drain is a stand-alone inductive tool called a clamp meter. There’s a wide variety of these on the market. Some electrical specialists favor ESI’s clamp Part No. 687 for two reasons: First, its lowest range is 0 to 4 amps (the lowest range on many clamp meters is 0 to 40 amps); second, its jaws fit around common battery cables.

Finally, always disconnect the test leads from the vehicle before changing ranges or functions, or turning off the meter. If the leads are connected and the meter knob is turned, you’ll commit the unforgivable sin of choosing resistance while connected to a live circuit—then pop goes the fuse or poof goes the meter!