12 volts to coil only at start up

[50 F1]

A friend told me it would be a good idea to put 12volts to my coil just while the motor is cranking over to give it more juice to start.  Is this a good idea and if it is should it come from the ignition switch from the "S" post or from the "S" post ont the started or does it matter being they are hooked up with the same wire?    1968 327

Thanks Mike

[enjenjo]

It comes from the R post on the starter. What do you have for the ignition?

[50 F1]

original points distributor

[enjenjo]

Okay. from the R terminal on the starter to the + terminal of the coil. From the ignition switch I terminal, to the ballast resistor, and from the resistor to the  + terminal on the coil.

[50 F1]

Thank You

Mike

[wayne petty]

a quick test...

a test light/circuit tester..

clip attached to the coil positive terminal ..  probe shoved into something grounded so you can see the test light from the drivers seat...

turn the key on..   with points medium bright..

turn the key to crank the engine.. while cranking.. bright light...


return the key to the run position.. with points .. medium bright light..


with HEI ignition.. take the coil cover off .. shove probe of tester into the top of the cap. next to the RED wire..   ground clip on intake..  perform same test...

this time.. should be bright light in RUN. CRANKING and returned to RUN.  no drop outs..  


hope this helps..

[txturbo]

A friend told me it would be a good idea to put 12volts to my coil just while the motor is cranking over to give it more juice to start.  Is this a good idea and if it is should it come from the ignition switch from the "S" post or from the "S" post ont the started or does it matter being they are hooked up with the same wire?    1968 327

Thanks Mike

very good idea. That's how the factory has always done it.

[PeterR]

A friend told me it would be a good idea to put 12volts to my coil just while the motor is cranking over to give it more juice to start.  Is this a good idea and if it is should it come from the ignition switch from the "S" post or from the "S" post ont the started or does it matter being they are hooked up with the same wire?    1968 327

Thanks Mike

This text only applies to early ignition systems fitted with a ballast resistor.

While the engine is running, power from the Ignition terminal on the ignition switch passes through the ballast resistor to the coil + terminal.   The ballast resistor drops the 12V from the ignition switch to about 8V at the coil.

This means under normal operation conditions the coil is operating on 8V.

During cranking the large current drawn by the starter motor causes the battery voltage to drop as low as 10V and if the ignition coil was only fed from the ballast resistor, the coil voltage would drop to about 6V making starting very difficult.  

The diagram below shows a starter solenoid with an auxiliary terminal which is labelled I or R.   During cranking the Start terminal on the ignition switch provides power to the S terminal on the solenoid.   The solenoid armature pushes down on the internal copper disk to join the B terminal to the M terminal and the R terminal.  See blue arrow in diagram.

This means during cranking, battery power is fed directly to the coil, by-passing the ballast and compensating for the lower battery voltage.

If the solenoid does not have an R terminal the boost supply to the coil must not be connected directly to the S terminal on the solenoid.  Connecting the S terminal to the coil will certainly provide the boost during starting; however during normal running, current will back feed from the ignition coil to the solenoid winding.   The additional current passing through the ballast will cause a larger voltage drop across the resistor resulting in reduced coil voltage.  Also the starter solenoid coil will be partly energised whenever the engine is running and it is only the voltage drop across the ballast that prevents the solenoid armature moving against the return spring and activating the starter.  

If the starter solenoid does not have an R terminal all is not lost.  The S terminal can be used with a blocking diode to prevent back feed, or preferably the S terminal can activate a small relay which then supplies battery power to the coil during cranking.