Building a Digital Tach/Dwell meter from a DMM
John De Armond
>I have an el cheapo analog Tach/Dwell meter, and will be getting a Fluke
>DMM soon (I know about the Fluke 88 $$$), and I was curious about something.
>Would it somehow be possible to bypass the analog meter on my el cheapo,
>and run the leads that went to the meter into the DMM and obtain the
>readings? I'm not sure what the Dwell meter is measuring, current or
>voltage, and I don't what the relationship is of 30 degree dwell and
>amperes or volts.
No. The analog meters work by pulsing the meter and using mechanical
inerta to stabilize the pointer. A sampling instrument such as a
DMM would only display garbage. A large enough cap might smooth it
but the response time would be terrible.
A workable technique is to get one of the National semi tach chips
and build a tach to DC converter. It only requires this chip and
a couple of capacitors and resistors. There is a suffix that takes
inductive pickup signals directly. It still won't have the speed
of response of the Fluke 88 which actually measures the time between
ignition pulses and computes RPM.
My advice? Get a Fluke 88. I have one and absolutely love it.
John
--
John De Armond, WD4OQC | What's the difference between Jane Fonda
Rapid Deployment System, Inc. | and Bill Clinton? Hanoi Jane went to
Marietta, Ga | Viet Nam.
j...@dixie.com | Need public access in Atl? Write me.
Ian Woollard
Nope actually, dwell angle is easy to measure with most DMM's. DMM's work
by integrating a signal over a *LOOOONG* period, and come up with a digital
number based on zero crossings.
For dwell all you do is measure the battery volts, and the
DMM-averaged voltage on the *low* voltage side of the coil... Divide
one by the other. You now know what fraction of the time the points
are open for. The definition of dwell, is something like the angle the
points are open for in the distributor, so you multiply by 360 to get
an angle.
None of this gives you rpm of course, but I don't need to. On my car I
can see the dash rev counter while I adjust the engine... neat huh?
DISCLAIMER(S):
It may not work with all brands of DMM's.
The battery volts can vary while you do this, invalidating your results...
Working under your hood with the engine running can be dangerous, if
you hurt you or your dmm trying this, *you've* done something silly,
the above is only a guide!
-Ian.
"Any sufficiently advanced bug is indistinguishable from a feature."
-- Rich Kulawiec
"Home is where the .x11start is ..."
--
-Ian.
"Any sufficiently advanced bug is indistinguishable from a feature."
-- Rich Kulawiec
John Whitmore
>}dar...@felix.filenet.com (Dave Arnold) writes:
>}>I have an el cheapo analog Tach/Dwell meter, and will be getting a Fluke
>}>DMM soon (I know about the Fluke 88 $$$), and I was curious about something.
>}
>}>Would it somehow be possible to bypass the analog meter on my el cheapo,
>}>and run the leads that went to the meter into the DMM and obtain the
>}>readings? I'm not sure what the Dwell meter is measuring
>If you just want dwell, an ordinary analog ohmmeter will work. Stick it
>across the contacts; the dwell is proportional to the deflection of the
>needle from its rest position. Open ckt is zero dwell, short ckt is
>100% dwell. Read it off the volts scales.
THIS IS DANGEROUS. The scheme will work as proposed, IF you
detach the ignition coil primary wire (so you are just metering
a switch). If you leave the auto running and try this, you will
likely put a few hundred volts onto your ohmmeter (and fry it).
Dwell meters consist largely of protection diodes and current
limiting resistors, and can be used with the engine running normally.
John Whitmore