Lotus Elan

Rpm - tach readings vs multimeter vs Ear

PostPost by: richardcox_lotus » Sun May 20, 2018 2:35 pm

Fettling today, wanted to check the timing, but I can?t see the tach from outside the car so hooked up my multimeter. The differences were surprising:

Tach in car showed 6-700 revs
Multimeter showed 1500-1700 revs
My ear thought about 1100 revs.

I?ve always suspected the tach in the car to be a little low but not that much out. Has anyone else had strange readings from a multimeter ?

Nb car is stock, with points.

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PostPost by: billwill » Sun May 20, 2018 10:10 pm

Sorry, but what kind of multimeter can count pulses ???

You would need a frequency meter with an adjustable input circuit as the pulses are not simple square shaped.
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PostPost by: Chrispy » Mon May 21, 2018 1:07 am

I've seen several multi's over the years with a RPM readout.
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PostPost by: richardcox_lotus » Mon May 21, 2018 5:59 am

Bill it?s a Gunson -G4187; now discontinued.
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PostPost by: pauljones » Mon May 21, 2018 12:38 pm

Hz x 60 = rpm

Ie 10hz reading is 600 rpm.

Different on wasted spark systems because you pick up the 2nd spark on the exhaust cycle. So for me i half to half the answer to correct it.
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PostPost by: bob_rich » Mon May 21, 2018 2:04 pm

Hi Richard

that looks an interesting multimeter. I could not down load the book anywhere but as is only has probes I assume ( for +ve earth) that it is placed on the coil CB terminal +ve probe and earth -ve probe so with a standard coil ignition system that would be 2 pulses per rev for a 4 cylinder engine.

Just guessing here but could you check it on say an "old fashioned" battery charger ? this would give a rectified AC pulses at 100pps and 20V or so peak which would be 3000rpm as a guide that the meter is working ok.

but without the book can't be certain if it would work !

hope this helps

best of luck

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PostPost by: ericbushby » Mon May 21, 2018 2:37 pm

My Fluke 87 has a frequency range on it, but I have not tried it.
I use a Tektronix THS720A digital oscilloscope with the low pass filter selected. It seems to work fine.
100 cycles/sec on the coil circuit is 3000 RPM.
Not usual garage kit I know, but I have it left over from retirement so I use that.
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