----- Original Message -----
From: Rick Huber <rickhuber1@home.com>
Cc: MG V8 List <mgb-v8@autox.team.net>
Sent: Saturday, August 04, 2001 6:50 PM
Subject: Update - Intermittent Coil Problem
> The new (in 99 but now spare) coil, that I thought had
> failed 1 1/2 years ago is a Wells C 841, 12 volt unit that says on the
side
> "use with external resistor", which to me means lower voltage with a 12
volt
> coil.
More like a 6v coil for use in a 12v system, I would have said.
> When I test the + terminal on the coil with the ignition switched on, I
show 14
> to 15 volts. When cranking, it drops down to about 12 volts, so that
resister
> in the wiring isn't dropping the voltage to the coil.
If the points are open you will see full system voltage on the coil +ve
regardless of the ballast. The ballast only drops voltage when a current is
passing through it, and with the points open there isn't any. However if
you have 14 to 15v with the engine stationary, and 12v while cranking, and
you mention 20v and 24v later on, I seriously suspect the accuracy of
whatever you are measuring with. On the coil +ve you should only have
12-point-something max with ignition on and engine stationary with points
open, about 6v with 6v coil and external ballast and points closed. About
10v while cranking, and about 10v (6v coil) or 14v (12v coil) when running.
> Looking at the 75 MGB
> wiring diagram it shows a WLG wire from the fuse box and the distributor
through
> the squiggly line (in line resistor I presume)
The coil ballast.
> to the + terminal on the coil.
> There is also another WLG wire directly from the starter solenoid to the +
> terminal on the coil, which must provide 12 volts while cranking. After
the
> car is started, power must drop off of this wire, and then the car runs
with
> power through the in line resistor.
Indeed.
> However, when the engine is running on fast
> idle, I'm reading 24 volts on the + terminal and about 12 on the -
terminal. At
> normal idle, it's 20 volts on + and 12 on -, so no indication of a voltage
drop
> through the in line resistor.
What are you measuring with? You will get spikes of 250v or more on the
coil +ve, and these cause a digital meter problems, which is why an analogue
meter is best in some circumstances. 20 and 24 volts on an analogue meter
is just not possible in a 12v system - unless your electronic ignition is
generating very large feedback voltages, in which case any attempt to
measure the supply voltage at the coil is doomed.
> One thing not shown on the wiring diagram is a green wire from the - side
of
> coil through a small cylindrically shaped piece of blue plastic (like a
> condenser in a distributor) that goes to ground.
Sounds like a radio interference suppressor, although it should be connected
to the coil +ve. On the negative it would be paralleling the normal points
condenser, if you had one, and adversely affecting the spark.
> When I had my wife crank the
> engine for me to test the coil voltage, I saw that this spade connector
was
> shorting out to the body of the coil making a spark. I'm not sure what
this
> function is, but if was shorting out, that can't be good, and maybe that
has
> been the cause of my problem.
If the ground terminal was flapping about and intermittently touching the
coil body I would not be that surprised to see some sparks, but if the
suppressor were fully or partially short that would be A Bad Thing.
> Another oddity compared to the wiring diagram is that I have 3 wires from
the
> Mallory - red, brown, and green. I've got the red hooked up to the +
terminal
> on the coil, the green to the - , and the brown to ground. In the wiring
> diagram it shows 3 wires, a WU which goes through a "distributor
resistor", a W
> which connects back to the WU after the "distributor resistor" before
going
> through the in line resistor and then to the + side of the coil. The
third
> wire, WB goes to the - side of the coil. With my mallory set up, I'm not
using
> a "distributor resistor" and the brown wire is running to ground instead
of the
> + side of the coil, but that's how Woody said to set it up.
Well now, you can't expect an after-market system to hook-up exactly the
same as the factory electronic-trigger system. You need to look at the
Mallory documentation, not the factory diagrams. The most important thing
to find out is what voltage the Mallory should run at. If it says 12v you
will have to fit a 12v coil and a full12v feed from the white, and forget
the loom ballast (white/light-green) and distributor ballast (white/blue)
feeds, taping them up out of harm's way.
PaulH.
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