Multimeter Question

Pete7 · May 13, 2012

Had to replace the alternator in my 340 the other day. Wanted to check the new alternator today and I tried to use my cheap digital multimeter. With the tractor running I got volt readings at the battery that changed consistently and were from 12 to 19 volts. Numbers just jumped all around. Thought I had a problem for awhile and I started checking wires and connections. Couldn't find a problem. Got out my old analog multimeter and got a steady 14.5 volts while running. Was the digital multimeter the issue (it reads OK on my car and on my Super A ( has a alternator too). I'm guessing the 340 must be creating some type of "noise" that makes the meter erratic?? The 340 has a coil while the Super A has a magneto, if that makes any difference.
Pete

Bob M · May 13, 2012

You hit it right on the head Pete!

Most of those cheap digital multimeters go crazy with the electrical noise in old tractor electrics - especially if the tractor has battery ignition or a mechanical voltage regulator. The rapid opening/closing of electrical contacts creates all sorts of transient voltages that confound cheaper meter's measuring circuits.

FWIW I trust only my old Simpson analog VOM and Fluke 87 (industrial quality DMM) when fooling with tractor electrics. My $6 Harbor Freight DMM is pretty much worthless for this kind of work.

buzzinduzzin · May 13, 2012

Can a bad diode also make a meter jump around?

old · May 13, 2012

Yep a lot of the cheap digital meters freak out around the engine static and that is why you get that type of reading. I have heard but do not know if it is true or not but by placing the meter in aluminum foil that helps shield the static

Multimeter Question

Pete7

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Bob M

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buzzinduzzin

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old

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