Hioki Megohm meter

That's no megohm meter, that's a milliohm meter. Other extreme end of the scale.

A regular multimeter uses sort of everyday voltages to measure sort of everyday resistances.

A megohm meter uses crazy high voltages to detect crazy low microamps of current flowing through stuff you'd normally say has resistance too high to measure (like insulation).

A milliohm meter uses crazy high currents (well, for an ohmmeter anyway) to detect crazy low microvolts of voltage drop across stuff you'd normally say has resistance too low to measure (like thick actual wires).

Both of them have their uses for testing something like an MG. You can use the megger to see if the insulation on the windings has gone bad, leaking to the case, say. You can use the milliohm meter to see if the windings still look like the right length of wire (say, one of the coils isn't shorted to itself partway along). If you know a wire gauge and copper's resistivity, you can use a milliohm meter to roughly measure the length of wire on a spool.

But you can't use either instrument where the other is called for.

They both make you brush up against some extra, weird physics. The very high voltages a megger has to use make you think about capacitance, even of stuff like a wire or a motor that you don't think of as a capacitor. It's enough of one to remember 500 or 1000 volts for a while, and remind you later. And dielectric absorption, even weirder.

The high currents and tiny voltages used in a milliohm meter make you think about things like thermocouple effects when your test clips aren't the same metal as what you're clipping to. (You don't think about test lead resistance anymore, the way you sometimes do with your multimeter, because you won't be using a milliohm meter without 4-wire leads and Kelvin clips anyway.) And inductance. If you're measuring something with a big wire coil in it, like a motor, you should be thinking where the energy in that magnetic field is going to go when you unhook your leads.

The low-ohm meter I rescued from a salvage place is maybe from the 1970s or 80s. It's not as fancy (or compact) as the Hioki, but it has a certain charm.

 

In one sense, with measurement ranges from 3 mΩ to 3 MΩ, the RM3548 is both—but your point stands, since with a maximum open-circuit voltage of 5.5 V, it’s not useful for the high-voltage testing ordinarily done with megohmmeters. As the video mentions, the RM3548 is recommended by Toyota; they sell it as a special service tool (11413-00005) and published a training guide, “Milliohmmeter Usage,” QT016A, on techinfo.toyota.com.

A lower-priced alternative is the Kaise SK-3800 Handy mΩ Tester, which lacks temperature compensation and has a range that’s narrower (40 mΩ to 40 kΩ) but still enough for any test described in the Repair Manual.

For megohmmeters, there are many choices. In the U.S., Toyota’s training guide, “Megohmmeter Usage,” QT016B, describes the Fluke 1507, but in Japan, the Repair Manual series mentions the Kaise SK-3002 and Hioki IR4051-10 (page in Japanese). For the kind of pass/fail testing needed when diagnosing Toyota HV systems, however, almost any megohmmeter with a 500 V test voltage could be used.

 

That's a "nice bit of kit", but overkill for most folk - even rebuilders. You can look at GW Instek GOM-804 - $600-ish (or the 805 for a bit more $$) if you want "lab grade" equipment.

For the DIY crowd there's the Uni-T UT620C, found a seller at $200. Or the VICI VC480C - either side of $100. Both (probably) good enough for motor diagnostics.

Posted via the PriusChat mobile app.