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Switching from a Gen to an ALT in a '47

Started by Justin Norwood, April 10, 2018, 08:47:56 AM

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Justin Norwood

Hi again!

as part of my 12v conversion, I am switching from the original generator to a GM CS121 alternator. I have a question about wiring the alternator...

There is an "ignition" wire on the alternator pigtail (see attached diagram) -- I am wondering what I should connect that into? Should that get connected to the IGNITION KEY circuit or the PUSH BUTTON START circuit?

To say it another way...should the "ignition" wire on the alternator get power when I turn the key to "ON" or when I push the "START" button?

Thanks in advance,
-Justin
1947 Cadillac Series 62 Sedan

Steve Passmore

If it's the type that needs 'Exciting' then it should have constant power, so that's ing.
Steve

Present
1937 60 convertible coupe
1941 62 convertible coupe
1941 62 coupe

Previous
1936 70 Sport coupe
1937 85 series V12 sedan
1938 60 coupe
1938 50 coupe
1939 60S
1940 62 coupe
1941 62 convertible coupe x2
1941 61 coupe
1941 61 sedan x2
1941 62 sedan x2
1947 62 sedan
1959 62 coupe

Justin Norwood

1947 Cadillac Series 62 Sedan

bcroe

I am pretty sure that NEW 35 OHM RESISTOR is replacing what normally
is the GEN light, certainly is on my 79 alt.  If you choose to have that
lamp for your car, you might notice taking a little more rpm to get the
alt started.  My generation solved that (on big diesel alts) by using 2
bulbs in parallel for more start up current.  One was hidden behind the
dash.  Bruce Roe

Justin Norwood

Thanks, Bruce. In lieu of the 35 ohm resistor, I am planning on running the following Dayton light in series in between the IGN switch and the "L" wire on the ALT pigtail:
https://www.ebay.com/itm/252717037321

This light fits nicely into the factory "L Brackets" on the bottom left of the dash -- the ones used for fog lights, the washer fluid button, etc.

Please let me know if you think there would be any issues with this approach.
Thanks in advance,
Justin
1947 Cadillac Series 62 Sedan

bcroe

Quote from: revvies
  In lieu of the 35 ohm resistor, I am planning on running the following Dayton light in series in between the IGN switch and the "L" wire on the ALT pigtail:
https://www.ebay.com/itm/252717037321

This light fits nicely into the factory "L Brackets" on the bottom left of the dash -- the ones used for fog lights, the washer fluid button, etc.

Please let me know if you think there would be any issues with this approach.
Justin   

That light can be used.  HOWEVER, since it is an LED light, the plus
lead needs to be wired to the IGN side to work.  In addition, it only
draws .017A, not enough to get the alt started.  The alt likely would
start anyway from residual magnetism when reved up.  The 35 ohm
10 watt resistor (recommended for this alt) could still be used to supply
.34A and your indicator just connected in parallel with the resistor.  If
you don't like resistors, a couple 194 pilot lights in parallel should work. 

Once the alt is generating, it supplies its own excitation current thru
the diode trio, and with 12V on both ends, the light goes out. 
Bruce Roe