Marinucci: Fuses are cheap insurance for quality job

Spare fuses for test meters are cheap insurance against bungled repair jobs and angry customers.

Ultimately, a blown fuse may upset a customer by unexpectedly delaying a critical repair job.

Worse yet, a popped fuse may prompt a frustrated technician to abandon a diagnosis and then guess at the correct solution for the vehicle's problems.

In my last column, I described the meters as well as the reasons these fuses may fail. That column lays the groundwork for this one.

The key is that many professional voltmeters also measure current (amperes). Typically, a specialized fuse protects the meter's current-measuring function. A tech's mistake may blow the fuse.

The ceramic-style fuse, which is used in both automotive and non-automotive meter applications, is designed to react quickly to over-current conditions. These are often called fast-acting or fast-blow fuses.

Note that a typical fast-blow fuse is an integral part of the meter's safety rating. Don't shortchange that rating by substituting a cheaper fuse for the proper type.

An OEM-style fast-blow fuse may cost within the range of $7 to $10.

Some bosses assume that professional meters always come with a spare fuse or fuses. They also may think that competent techs purchase their own spares.

Experience suggests otherwise. For one thing, a professional meter may not be packaged with spare fuses. For another, most techs I encounter have not purchased spares, either.

One preventive measure is to inventory all the meter fuses within your service department. Then invest in fast-blow fuses for all these meters.

Tell your techs that the first spare fuse is on the house, but the next one comes out of the employee's pocket.

One potential source for the proper fuses is the vendor(s) that sold the meters. Others are the internet or the electrical department at a local home center.

After all, a fast-blow fuse in an automotive meter also may appear in an industrial, commercial or non-automotive application.

More than once, I have found a fast-blow fuse in a home center that's identical to the OEM fuse in a digital, multifunction automotive meter.

A second preventive step is encouraging techs to use a fused test lead. As its name suggests, this test lead contains a fuse holder with a replaceable fuse.

Suppose a careless tech subjects the meter to excessive current. If so, the condition blows the fuse on the test lead, leaving the OEM fuse inside the meter intact.

The meter manufacturer — or an electrical specialty supplier — may offer a fused test lead. For example, Snap-on Inc. offers test lead kit MTTL504.

Remember that measuring current (amps) with a traditional meter requires disconnecting wires in order to place the meter "in series" within the circuit of interest. Or, it means connecting the meter to a fuse panel with a special adapter.

Third, consider upgrading to an inductive current-measurement device, such as a clamp meter. Two examples of popular automotive-specific clamp meters appear in the accompanying photographs.

One of these tools comes from Electronic Specialties Inc.; the other comes from Snap-on.

Inductive devices such as clamp meters feature spring-loaded jaws. The user squeezes open the jaws and slips them around the wire being tested. A sensor inside those jaws monitors the volume of current flowing through the wire.

This inductive measurement method has been a staple of automotive diagnosis for years. (I first used it on cars back in the late 1970s.)

One giant advantage of the inductive technique is that it saves the time and trouble involved in disconnecting wires or tapping into a fuse panel.

Another major feature of inductive amps measurement is that it eliminates the risk of blown fuses in meters.

What's more, the latest automotive clamp meters feature exceptional sensitivity. For example, the models shown here measure current values effectively down in the milliamp range. (A milliamp, abbreviated as mA, is one-thousandth of an ampere.)

Meanwhile, the clamp meter's combination of slip-around jaws and milliamp sensitivity is especially timely today for troubleshooting battery drain problems. These conditions occur when a component remains on after the driver leaves the vehicle.

Then the "live" component discharges the battery.

My field experience has been that battery drains have become fairly common conditions on modern vehicles.

Sometimes, troubleshooting the cause of a battery drain is the toughest troubleshooting challenge a technician faces.

To me, an appropriate automotive clamp meter is a universal battery drain tester, whose jaw openings are large enough to slip around a common battery cable.

A tech can park a problem vehicle and work on other repair jobs. After a period of time, on-board electronic components are supposed to shut off.

Simply slipping a clamp meter's jaws around one of the battery cables enables a tech to measure battery drain quickly and accurately.

An excessive milliamp reading here confirms that something inside that car has not shut itself off. This job requires additional diagnostic time.

A reasonable rule of thumb is that this reading should be approximately 50 milliamps or less. Typically, the battery drain measurement on a healthy vehicle is closer to zero.

Meanwhile, the reading on a problem vehicle tends to be substantially greater than the 50-milliamp guideline.

Bottom line: The inductive current-measurement method is a practical complement to traditional digital meters in any service department.