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Dealers, particularly for General Motors want their technicians to use True RMS multi meters such as the FLUKE 87V.

There are other meters made by FLUKE such as the 88V, which is an "Automotive Meter," that does not have True RMS. This is a more expensive and feature rich meter than the 87V but is frowned upon.

Is a True RMS meter somehow better?

Technicians primarily measure DC and True RMS is related to AC. Why do dealers care, as long as it is a respectable meter?

  • i've never heard this before. maybe for taking ac voltage readings from sensors. e.g. passive wheel speed sensors, some crank and cam sensors. – Ben Dec 3 '16 at 21:51
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The difference between a "true RMS" meter and a plain old garden variety AC meter is the ability to accurately measure the voltage of non-sinusoidal waveforms, for example a square wave. "Normal" meters tend to under report the values of non-sinusoidal signals, so you might see readings that were lower than expected. This could lead to misdiagnosing problems, I suppose.

To really answer the question it would help to know where GM thinks a true RMS meter would be used. I'm guessing on engine or wheel sensors, but I don't have any experience with GM vehicles – all of my SAABs were pre-GM.

I suppose it could also be the GM sees a need for techs to be able to make accurate non-sinusoidal readings in the future – perhaps for electric cars, or because of new sensors – and so they are trying to prime the pipeline by encouraging techs to buy the tools GM expects the techs will need in the future.

Fluke has a page on their website that explains true RMS.

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It's about measuring the "odd" waveforms: wheel sensors, crank sensors cam position sensors that are square or sawtooth in form, rms means "root mean square" that is square all the individual values, then take the mean of those squares, then, finally, take the square root to get the final value.

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