If I remember correctly, the classical ignition circuit produces an
instantaneous voltage of about 20,000 V between the centre electrode of the
spark plug and the battery. If the ignition coil is wired correctly, this
voltage is negative at the plug with respect to the battery. If the car is
positive ground wired, the continuous 12 V voltage at the battery is added
to the around 20,000 V, and you would have some like 20,012 V voltage
difference between electrodes at the spark plug. In the case of a negative
ground, the 12 V would be subtracted, to yield about 19,988 V. Hence, the
difference would be 24 V over about 20,000 V.
Jesus
-----Mensaje original-----
De: owner-mg-t@autox.team.net [mailto:owner-mg-t@autox.team.net] En nombre
de Dave & Liz DuBois
Enviado el: jueves, 07 de julio de 2005 18:49
Para: Bob Howard
CC: wtsnyder@bellsouth.net; mg-t@autox.team.net
Asunto: Re: TD Negative Ground Conversion
" Some say that pos earth gives better sparkplug efficiency". The
polarity of the spark plug voltage remains the same by reversing the
primary leads of the coil when the conversion is made, so that is not
realy an issue. Other than matching the polarity to a modern am/fm
radio/cd or the addition of an alternator, there is no advantage to
having a negative ground over a positive ground vehicle. Conversely,
there is no real advantage to keeping a vehicle as positive ground.
Basically the conversion is a wash, and only needs to be done for
permenantly installed equipment that can't be matched to the positive
ground. Most CB units will work on either ground polarity, but
radios/cd players can't.
Bill, your decision not to try to isolate the installed unit to positive
ground was a wise one, as most attempts to isolate the equipment from
ground end in failure and the results can be disastorous to the
vehicle's wiring. This is particularly true of the antenna installation
and even if it is successful, the antenna efficiency suffers greatly
from being isolated from ground. One thing that should be done on any
vehicle whose polarity has been changed, is to install a plate stating
that the ground polarity has been changed from normal. This plate that
states WARNING This Vehicle Wired NEGATIVE EARTH, can be obtained from
Moss Motors under part number 215-605. On the T series and MGAs , this
plate should be positioned next to the regulator and on the MGB it
should be on the horizontal panel next to the bonnet latch. I will send
the instructions on a seperate e-mail.
Cheers,
Dave
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