Nelson Riedel wrote:
>
> Helped a fellow club member fix an interesting problem with the MSD ignition
> in his '73 TR6. If this is old news --- sorry.
>
> The car ran great but wouldn't stop when the ignition was switched off. If
> he tapped his brakes, the engine would shut off. (He said that was good
> British logic; if you want it to stop, hit the brakes.) He said the MSD
> module was installed as per instructions, wires directly to 12 volts and
> ground and also a wire to the white ignition wire at fuse block. ( I assume
> there was also a wire or two to the coil, didn't think to check.)
>
> There was obviously some source of 12 volt power to the ignition wire with
> series resistance such that it was loaded down by the brake lights (also
> powered from the white wire --- but after the fuse so it is the green wire).
> Checked out Dan Master's schematic for the '73 and noted the Alternator
> Warning Lamp (IGN) from the alternator voltage sense circuit connects to
> the white wire. We pulled the lamp and everything worked properly (except
> the lamp). We installed a diode in series with the warning lamp and
> everything now works as it should.
>
> The problem may have been compounded by engine modifications such as three
> big Weber Carbs and I believe a shaved head & high lift cam. The idle was
> set faster than norm to smooth out the jumping at the curb. That may
> cause the alternator output voltage to be above normal for idle.
>
> Nelson Riedel
> Granville, OH
Nelson,
I forwarded your email to an 'automotive electric aware' engineer friend, and
his response is as follows: Hope this helps you somewhat.
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This problem is well known as is not just unique to Lucas wiring. Same
symptoms can be caused by high speed heater fan(s) and this was a common
problem on the early BL Princess (wedge). This car had (for the period) an
ultra modern and efficient heating/demisting system with three (I think)
speed fans. If the fans were on fast speed and the ignition then turned off,
the engine would keep running for some considerable time afterwards. The fans
(like most simple electric motors) act as a generator, albeit not a very good
one if they are turning fast enough when the electrical supply is switched
off. BL cured this problem by fitting inline diodes to the electrical feeds
to the fans. A diode is basically a component that only allows electrical
flow in one direction (a one way valve).
Regards
Steve
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Forwarded by Leon F Guyot
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