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Author: Subject: Using multimeter to measure rpm
GreigM

posted on 18/8/09 at 03:52 PM Reply With Quote
Using multimeter to measure rpm

I think (hope) my tacho is reading wrong, so would like to get a true reading from the engine (fireblade engine) as to what the rpm are - so if anyone has done this and can give me tips, I'd very much appreciate it....i.e. where to stick the probes, what kind of reading I'll get and how to convert that to rpm.....
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MikeRJ

posted on 18/8/09 at 03:56 PM Reply With Quote
Unless you have an automotive multimeter with RPM facility then this isn't really possible.
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MakeEverything

posted on 18/8/09 at 04:02 PM Reply With Quote
Depends how its wired.

If its connected to the positive on the coil, then the voltage should be relative and directly proportionate to the RPM.

If its inductive, then the pulses will need to be calculated using the pre-programmed algorithm inside the rev counter, whatever it might be.

Hope this makes sense.





Kindest Regards,
Richard.

...You can make it foolProof, but youll never make it Idiot Proof!...

BLOG: http://21toc.com/blog.php?1742-MakeEverything

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BenB

posted on 18/8/09 at 04:34 PM Reply With Quote
You can get cheap optical tachos off Ebay for 20 quid or borrow someone else's. Little reflective sticker on the front pulley (or some other rotating thing) and jobs a good'un.

With a plain vanilla multimeter there's no way of checking without fabricating a frequency to voltage converter or other bit of electrickity.

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Dangle_kt

posted on 18/8/09 at 04:36 PM Reply With Quote
http://cgi.ebay.co.uk/MICRONTA-HAND-HELD-DWELL-TACHOMETER_W0QQitemZ160355504559QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item2555eed1a f&_trksid=p3286.c0.m14
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MikeRJ

posted on 18/8/09 at 05:15 PM Reply With Quote
quote:
Originally posted by MakeEverything
If its connected to the positive on the coil, then the voltage should be relative and directly proportionate to the RPM.



Connected across the coil the average voltage will vary with RPM if it's a constant energy electronic system (won't work with points or transistor assisted) but the meter may not like the 200-300 volt spikes!

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GreigM

posted on 11/9/09 at 10:14 PM Reply With Quote
So I got hold of an automotive multimeter and can get a reading, but at idle it seems to indicate around the 500-600rpm range, which for a bike engine would seem to be low...this is with the multimeter set on 4cyl - should I simply double this number for a bike engine? Is there a reason the number would read low (I'm assuming my car doesn't actually idle at 500-600 rpm)?

I suppose at worst case I could warm up the engine and see what reading it hits the limiter at - just a bit of a noisy exercise....

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matt_gsxr

posted on 11/9/09 at 11:55 PM Reply With Quote
hey there,

I have a UT105 multimeter. This is a basic multimeter that you can get from Maplin. It is well made, and also has some automotive functions that include RPM. It is good value (25) in my opinion.

On my bike engine (gsxr1100 running wasted spark) if I connect between the earth and one of the coils and set the multimeter at 4cyl then I get an RPM reading that is half of the true RPM.

I think it expecting a single coil that would pulse for every spark event (i.e. 2 sparks per rpm of the crank for a four stroke engine).

To get it to read properly you can connect it between the two coils, switched side (one side is always at 12V on mine and the earth is switched, this might vary on different models). This gives the correct number of triggers per crank rotation.

This seems to do the job for me.

Alternatively just double the value on the display in your head.

Matt

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GreigM

posted on 12/9/09 at 06:10 AM Reply With Quote
matt - many thanks for this, its the same model I've got from maplin and this sounds exactly what my problem is...nice to have my suspicions confirmed.
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