Diagnostics

Klein MM400 Review: The Fuse Is the Whole Point

An auto-ranging meter that publishes what most competitors leave out — the rating of the fuse protecting its current jack. That silence is the buying decision.

By Antoni DaskalskiPublished 13 August 20264 min read

Before you buy anythingThis page also considers a meter with a microamp range, a test light and a clamp meter. Why, and when each is the better answer.

Contents

This is a competent, unremarkable meter that does one important thing its cheaper competitors do not: it publishes the rating of the fuse protecting its current jack. For a tool whose worst failure mode is a short across a car battery, that single line is the reason to choose it.

A note on this assessment: researched from Klein's published specification, not used on a car here. See how we test.

The problem it addresses

A car owner reaches for a meter for six things — battery state, charging voltage, a fuse, a bulb, a suspect earth, and a parasitic drain. That list is set out in the multimeter guide for car owners, and none of it requires understanding the theory underneath.

What it does require is a tool that will not punish a mistake disproportionately. Every one of those six checks is a two-minute job; the only one that can go badly wrong is measuring current, and it goes wrong the same way every time.

How it is specified

Type: Auto-ranging
Model: MM400
Safety: CAT III 600 V
Current: 10 A AC/DC
Voltage: 600 V AC/DC
Resistance: 40 MΩ
Also measures: Temperature, capacitance, frequency, duty cycle, diode, continuity
Drop protection: 6.6 ft (2 m)
Fuse protection: 10 A jack fused at 10 A / 600 V, fast-blow

Read the fuse line first. It is the only line on that table that is about consequences rather than capability.

Why the fuse line is the review

Measuring current means putting the meter into the circuit rather than across it. In that mode the meter is deliberately a very low resistance — near enough a piece of wire — and it carries whatever the circuit sends through it.

The mistake almost everyone makes at least once is leaving the leads in the current jacks after a current measurement, then going back to check a voltage. Probing a battery terminal with the meter still in current mode puts a near-short across a lead-acid battery capable of several hundred amps.

In a properly fused meter, an internal fuse opens and the outcome is an evening's inconvenience. In an unfused one, the energy has to go somewhere and the meter is what is in the way.

Klein specifies a 10 A / 600 V fast-blow fuse on the 10 A jack and puts the fuse behind the same access panel as the battery. A remarkable number of budget meters — including some that look identical — publish neither the fuse rating nor confirmation there is one. Notably, Fluke's own web specification for its comparable model does not state a fuse rating either, which makes the Klein the easier meter to cite even where it is not the better meter to own.

What it does well

Publishes the fuse rating on the current jack — 10 A / 600 V fast-blow
Auto-ranging, which is what makes a meter usable without electrical theory
Covers all six checks a car owner actually performs
CAT III 600 V, correct for a vehicle and for domestic sockets
6.6 ft drop rating, which is the height of a bonnet
Fuse and battery behind one accessible panel

What it does badly

No microamp range, so a fine drain hunt runs out of resolution
10 A ceiling, and not continuous at that level
CAT III rather than CAT IV
Bead thermocouple is comparative rather than accurate
Awkward to read under a car

Ugly score 4, and it barely earns it

It is a yellow and black plastic brick with a rotary dial and two leads. It looks like every other meter, which is the point — this is a category where nobody has found a reason to change the shape in thirty years, because the shape works.

Four rather than lower because of where it ends up in use: face-down on a wing, wedged against a strut tower, or balanced on the battery while both your hands are elsewhere. It is a tool that never looks tidy in the moment of being used, and the rubber holster it comes in is the thing keeping it alive.

Where it stops being the right tool

The small end of a drain hunt

The parasitic drain test is the most valuable thing on the site's diagnostic list and this meter does the first half of it well. Put it in series with the disconnected negative lead and a large drain is obvious immediately.

The second half — deciding whether 30 mA is normal for this car or 30 mA too high — needs resolution this meter does not have. The method, and what the numbers should be, is in the multimeter parasitic drain test; the meter for the tail of it is a different purchase.

Anything drawing real current

Ten amps is the ceiling and it is not a continuous rating. Starter circuits, cooling fans and charging current are all beyond it, and the correct tool for those is a clamp meter, which reads current from outside the wire and never becomes part of the circuit.

Mains distribution work

CAT III 600 V is right for a car and right for a socket. Anything upstream of a consumer unit is outside both the meter's category and this site's scope.

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Klein Tools MM400 auto-ranging multimeter

Klein Tools

Check the listing states a fused 10 A jack. If a meter's specification is silent about its fuse, treat that as the answer.

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Who should buy it, and who should not

Buy it if you own a car and do not own a meter. It covers the whole realistic list, it removes the one step that stops people using a meter at all, and it is protected against the one mistake that turns a cheap tool into a dangerous one.

Skip it if you already have a fused auto-ranging meter — this is not an upgrade over one, it is the same class of tool. And skip it if the actual job is only is this wire live, because a test light answers that faster with nothing to set. The wider set of instruments that replace guesses with measurements is in diagnostic tools that stop you guessing.

Would we buy it again?

Yes, and the reasoning generalises past this product. In a category where the safety-relevant specification is routinely omitted, a manufacturer that publishes it has told you something about the rest of the tool. Buy the meter that states its fuse rating; if two do, buy the cheaper one.

The verdict

Fix score9/10

How well it solves the stated problem.

Ugly score4/10

How much it offends the eye. Higher is uglier.

Value8/10

Worth the money, clutter and shelf space.

Would we buy it again?: Yes
Best for: A car owner who wants one meter for battery checks, fuses, bulbs, earths and a parasitic drain test, and does not want to learn range selection first.

If this is not right for you

A meter with a microamp range: The last stage of a parasitic drain hunt is measured in milliamps and tenths of them, and a meter that starts at 10 A cannot resolve the difference between a healthy sleeping car and a slow leak.
A test light: Costs a fraction as much and answers the only question many jobs actually ask — is this wire live — without any settings to get wrong or any range to select.
A clamp meter: Measures current without breaking into the circuit, which is faster and safer for anything drawing real current, though it is poor at the small readings a drain test needs.

Questions people actually ask

Why does the fuse matter so much?

Measuring current puts the meter in series with the circuit, where it behaves like a piece of wire and carries whatever the circuit draws. The classic mistake — leaving the leads in the current jacks and then probing a battery terminal — puts a near short across the battery. In a fused meter that destroys a cheap internal fuse. In an unfused one the energy goes somewhere else, and Klein publishes a 10 A / 600 V fast-blow rating where a great many competitors publish nothing at all.

Is auto-ranging worth it, or is manual better?

For someone using a meter occasionally, auto-ranging is the difference between a tool that gets used and one that stays in the drawer — it removes the step where you have to know the answer's rough magnitude before you can measure it. Experienced users sometimes prefer manual because it settles faster on a fluctuating reading, which is a real but narrow advantage.

Will it do a parasitic drain test?

It will find a large one. The method is to put the meter in series with the disconnected negative lead and read the current, and 10 A is plenty of headroom for that. What it will not do is resolve the small end — distinguishing a normal sleeping current from a slightly high one needs milliamp and microamp resolution this meter does not offer.

Is CAT III 600 V enough?

For a car, comfortably — a 12 V system is nowhere near the category limits. For domestic sockets and appliance work it is also correct. CAT IV is for the supply side of a building's distribution, which is not work this site covers or recommends.

Does the temperature function have a real use on a car?

Comparative ones. Reading one brake disc against the other after a drive, or the inlet and outlet of a radiator, tells you something useful even if neither absolute number is accurate. Treat it as a difference gauge rather than a thermometer.

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