In a message dated 97-09-09 12:56:04 EDT, ingate@shiseis.com writes:
> My question is; where does the ballast
> wire go? It dissapears into the wiring harness and I cannot find the
other
> end!
> If I could find the other end, I could easily check the resistance and see
> if it
> matches the spec. Does anyone know the resistance of the ballast wire?
Shane:
You won't find the other end - it goes into the harness, where it connects to
the white wires, hidden in the harness. You can still check the resistance,
though.
Just disconnect the white/yellow wire from the coil, and place one ohmeter
lead on the end of this wire, and place the other ohmeter lead on the closest
white wire you can find. This will give you the resistance of the resistance
wire plus the resistance of the white and white/yellow wires. Unless you have
bad connections (and I am assuming you have cleaned ALL your electrical
connections as a routine matter), the resistance of the other wires will be
negligible, less than the tolerance on the resistance wire.
Disclaimer: I don't have a '74 to look at, so I am assuming the resistance
wire to white wire connection is the one that is in the harness, and the
connection to the white/yellow wire is the one you can get to. It may be that
it's the other way around, with the white connection free of the harness.
As to the value of the resistance, I don't have that data, but typically they
are in the range of 0.75 - 2.5 ohms. Hopefully, someone else on the list has
that value, or would be willing to measure theirs for you. (If any one would
like to do that for Shane, just follow the procedure above. Assuming the
connections are good, this should get you close enough so that he can test
his own).
There are two other possibilities I thought of after my last post. It's
possible that the coil is bad. If it is drawing more current than it should,
but not drastically more, it could heat up the resistance wire, but not the
regular wires. Heat (power) is equal to current squared times resistance, and
voltage drop through the resistance wire is current times resistance. As you
can see, with a larger current, the resistance of the ballast wire will
produce more heat, whereas, the low resistance of the plain wire produce
negligible heat, untill the current gets very high. ie, current squared times
zero resistance produces zero heat. (of course, the resistance of regular
wire is not zero, but dang near for this purpose. If the current gets high
enough, even the extremely low resistance will produce heat. For example,
with a wire resistance of 0.01 ohm, and a curent of 200 amps, the resistance
will produce 400 watts!!!).
The same analysis holds for the voltage drop. Pulling 6 amps through a wire
with 0.01 ohms resistance will drop 0.06 volts. Within the tolerance of your
meter, you will still measure 12 volts at the coil. With a 1.75 ohm
resistance wire, otoh, 6 amps will drop 10.5 volts, leaving the 1.5 volts you
measured, (and producing 37.5 watts).
If not the coil, it is possible that you have a high resistance "short" to
ground somewhere along the ignition wires, not enough to cause a burn down,
but enough to increase the current as described above. Check resistance to
ground, with the w/y lead disconnected from the coil. It should be infinity.
Hope this helps. I apologize for the length of the post, but I don't know how
to be succinct.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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