Re: [ihc] RE: Relaying George's power

["Allan Ericson" <lowvolt@xxxxxxxxxxxx>]

----- Original Message ----- 
From: "John Hofstetter" <hofs@domain.elided>
> > -Allan
>
> Well, Allan, for what's it worth, it did to me. <g>
>
> Here's the scoop. George has been looking for a trigger for a relay, a
> trigger that would activate the relay on both the start and run cycle.
>
> George says that there is not enough voltage at the + terminal of the
> coil during the start cycle to trigger the relay. What doesn't quite
> make sense is that George also feels that he has a problem with the
> resistance wire during the run cycle.
>
Okay. The short version: There's a wire that goes to the solenoid during the
run cycle. It has 12 volts on it when you start the engine. This terminal is
labeled "S" and is always present on a stock starter. On a points ignition,
there is also a terminal labeled "R" that also has 12 volts on it at that
time. That wire goes to the coil. Either wire will work to trigger a relay
during STARTING.

During run, any wire that's hot with the key on can be used.

The way to fire a relay with two separate feeds is to use DIODES. They can
be obtained at RADIO SHACK for a buck.

Each feed gets a diode. The end without the stripe goes to the wire. The end
with the stripe goes to the (+) terminal of the relay coil. The negative
side of the relay coil goes to chassis ground or battery (-).

Common diodes are part number 1N4001, 1N4002, 1N4003, and 1N4004. Type 914
will also work.

> With the MSD, and the ballast resistor as I described using it, he can
> use switched voltage going out to the coil, pick up his relay trigger
> there before it gets to the + terminal, and then run a lead from the
> junction he is making in the wire going toward the coil to the ballast
> resistor and then to the + coil terminal.
>
> John
>

The reason I don't understand is that the MSD uses the output of an
amplifier to drive the pulses to the coil. It's a really bad place to
trigger a relay. When you hook up the MSD, the wire that previously was used
to power the coil becomes a sense (system on) wire and feeds the small red
wire of the MSD. The wire that was previously used to feed the (-) side of
the coil becomes the "trigger" wire that tells the MSD when to fire. No
ballast resistor in the circuit unless you have a coil that has insufficient
resistance to keep from damaging the MSD.

Having said that, the ballast resistor will not make any difference if you
are simply using the old wiring to trigger the MSD and drive a relay. A
relay coil is normally more than 20 times the resistance of the ballast, and
unless the ignition coil is in the system (as in a stock setup) the relay
will work. If the stock system is used, however, the resistance of the coil
makes a voltage divider that keeps the relay from firing.

There are many ways to make the system work. Without knowing the reason for
this project, it's hard to say which one is the best approach.

-Allan