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 Posted: 04-21-2020 05:10 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 512
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Most ignition systems include a ballast resistor prior to the coil. If 12 volts is 'nominal', the resistor will drop the voltage delivered to the coil down to about 6 or 7 volts for normal running.

Then the starter-mounted solenoid has one extra small terminal. When the solenoid is energized, it turns power 'On' to the starter motor, and also 'On' to that extra spade terminal. A wire goes from that terminal directly to the coil's (+) input terminal, bypassing the ballast resistor and delivering a full 12 volts to the coil. That way the coil puts out a hotter spark only while the engine is being started. As soon as the key is released from the "Start" position, that bypass circuit turns off, and the coil goes back to running on reduced voltage via the ballast resistor. The whole purpose is to provide a hotter spark during start-up, and the bypass circuit is 'Off' all the rest of the time.

In Lotus cars, the wire from the ignition switch to the starter solenoid that 'tells' the starter to run is is White with a Red stripe (WR). The wire on the solenoid's second small terminal is output for the resistor bypass wire from the starter solenoid direct to the coil, and it's White with a Yellow stripe (WY).

From the ignitiion switch to the coil, the full 12V wire is White (no stripe) as far as the ballast resistor. From the ballast resistor to the coil, the reduced voltate wire is White with a Yellow stripe (WY). So at the coil's (+) input terminal, there will be two WY wires attached. One from the ballast resistor with reduced voltage for normal running, and one from the starter solenoid with full system voltage for starting only.

The wire and stripe colors follow a standard code. If you figure out the White, White-Yellow and White-Red wires in your J-H, you should find that the similar wires in a Lotus/ MG/ Triumph look pretty familiar.

Regards,
Tim Engel

Last edited on 04-21-2020 05:40 pm by Esprit2