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Old 12-22-2011, 07:07 PM
Schurkey Schurkey is offline
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Older Snap-On Counselor II (MT3000 or MT3000A) is both extremely affordable and wonderfully equipped for automotive work if you get all the accessories with it. Unlike the more modern automotive scopes, the Counselor series does not have "known good" patterns stored in memory for various electrical devices. It's also not very portable. It will run on 12V, so you "could" take it along on a test drive--but how do you get the cables from engine compartment to passenger seat?

Here's a screen shot of the 'scope connected to read amperage draw of the electric fuel pump of my '88 K1500 work truck:

Looking closely at the "humps", you'll see that they are similar but not identical, and the pattern repeats every eight humps. This shows that the fuel pump motor has eight bars on the commutator. A motor with twelve commutator bars will have a repeating pattern every twelve humps.

The vertical position shows that the fuel pump draws just under six amps average, with a peak of 6.444 amps.

By "freezing" the pattern, and using the cursors--the solid and dotted horizontal and vertical lines--I can tell that it takes 9.42 milleseconds for all eight bars to peak, which would be one revolution of the electric motor. Simple math tells me that one revolution in 10.16 milleseconds is equivalent to (60,000 / 10.16 = 5905) 5905 RPM; a reasonable speed for this fuel pump.

Since the average amperage, the shape of the amperage peaks or "humps" is reasonably symmetrical, and pump RPM are proven to be good, and the fuel pressure is acceptable, I know for sure that this pump is in fine condition.

Just a quickie example of how wonderfully handy an automotive oscilloscope is.

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