Date: August 25, 1998
It is my understanding of dwell meters that they attempt to measure the
duration of the charging time relative to the cycle time of a distributor.
In the case of a positive ground system a dwell meter set up for negative
ground would see the inverse of what you expect to see. Therefore, when
you read a dwell of 20 degrees it probably is a dwell of 90-20 = 70 degrees.
The tach cicuit just counts transistions so it does not matter which
polarity the system uses.
Bob Reisse
76 MGB
NAMGBR 8-3559
At 03:32 PM 8/25/98 -0400, Dave Ahrendt wrote:
>Hi again,
>
>Well the coil discussion has died down a bit so I thought it may be safe
>to ask another EE type question.
>
>Is there any issue using a generic "Sears Craftsman" type tach/dwell/etc.
>meter
>on a positive ground car? When I try to check the dwell on my MGA I
>connect the +cable from the meter to frame; -cable to battery power and
>the sense lead to the distributor. I expect that the dwell should be about
>60 degrees on this car with the points set to .015". The meter shows
>a dwell of about 20 degrees. If I use the meter to set the dwell to 60 then
>the gap is huge.
>
>I wonder if there is some sore of RC circuit in the meter that is getting
>confused. There is nothing in the meters manual that says anything
>about positive ground. So how does a dwell meter work anyhow?
>
>BTW, the tach seems to track pretty close to the mechanical tach.
>
>Thanks, Dave
>
>
>
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