The dwell for the 25D4 distributor is 60 degrees, the 45D4 is 51 degrees,
and the 35D8 27 degrees, each with a tolerance either side. Dwell can be
expressed either as a percentage, as an angle or as a period and represents
the proportion, or ratio, of the closed period to the open period of the
points. When expressed as an angle it relates to the percentage of 90
degrees for a 4 cylinder engine or 45 degrees for an 8 cylinder engine. In
percentage terms the 25D4 is 67%, the 45D4 is 57%, and the 35D8 is 60%.
A dwell meter can be as simple as an analogue voltmeter connected across
the
points. As the points open and close the needle tries to move between
system voltage and zero volts, but because of its damping it actually
fluctuates very little, indicating an average voltage equivalent to the
proportion of the time the points are closed and open. If you know the
system voltage, say 14.5v, you can read off the average voltage and
calculate its percentage. For a 25D4 the expected 67% of 14.5v is 9.715v.
But dwell measures the length of time the points are closed, whereas what
the meter is displaying is the length of time they are open, so we have to
subtract 9.715v from 14.5v to 'invert' it to get the correct expected
reading i.e. 4.785v. However if you connect the voltmeter from the
coil -ve/CB to a brown wire the meter will be registering when the points
are closed and you won't have to do the subtraction.
The problem with this method is that you have to know what the system
voltage is at the time you take the 'dwell' reading and use that in your
calculations, and that voltage will vary over time and engine speed. The
high-voltage 'spike' at the instant the points open, around 250v, will also
have a slight effect on the 'dwell' reading. A more accurate method is to
use an ohmmeter, which if it has a scale calibrated from 0 at zero
deflection (points open) to 100 at full scale deflection (points closed)
allows you to read off the percentage directly, and in my telecom days we
used this method to measure the ratio of pulsing contacts all the time. An
analogue ohmmeter uses an internal battery to drive a current through the
meter and the external circuit, but you can't use an analogue ohmmeter when
the engine is running because the voltages from the cars electrics
interferes with the voltage from the internal battery. Neither can you use
most digital voltmeters because they use sampling periods, unlike the
continuous monitoring of an analogue instrument, and this sampling means
that the reading jumps about all over the place according to whether the
points are open or closed at the precise moment the sample is taken. So
unless you have a second voltmeter measuring the system voltage for the
volt
meter method, or can spin the dizzie off the car for the ohmmeter method
(which is exactly what I do when changing points and setting them up for
the
first time), it remains a beautiful theory.
As to what goes on inside a digital auto multimeter with dwell scales in
degrees, percent and period I haven't a clue, it all happens inside that
little black chip, with various filtering that ignores the effects of the
spike etc. But the analogue voltmeter method should be good enough to
verify the calibration of a specialised instrument.
PaulH.
> ----- Original Message -----
> From: "James Nazarian" <jhn3@uakron.edu>
> To: "mgs" <mgs@autox.team.net>
> Sent: Thursday, August 14, 2003 4:33 AM
> Subject: Dwell
>
>
> > I can't seem to find what I am looking for in the archives so I'll ask.
> >
> > What are the normal dwells for 4 and 8 cyl engines, MGB and Olds V8 if
it
> > matters?
> >
> > Does anyone have a schematic for a dwell meter? I'm curious what makes
> them
> > tick. I'm also trying to find a way to verify the calibration of one.
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