A slightly different 12v conversion

[radioguy41]

I recently converted Cyrus the Cub to 12 volts. I used the popular Denso 12180 35A alternator. I used the same model last year in a Lo-Boy I converted. These are sold as 1 wire alternators but I've never had great luck in getting them to self-excite on these low RPM Cubs. On the Lo-Boy it was easy to just run a wire with a diode from the coil IGN post to the alternator but this Cub has a magneto. The most common suggested solution is to add a switch to the dash but here's an alternative. I used the same oil pressure switch the Hobbs hour meter uses (part # 132244r91) Tee'd into the oil pressure gauge (see diagram). Works perfect.

For anyone with an original 20A ammeter I would suggest swapping for a 60A meter. Otherwise when the alternator kicks in it's going to bury the needle at first until it scales back.

Click on image for a larger view.

Click on image for a larger view.

[Slim140]

FYI this looks like the same alternator with same number but it is a true one wire alternator, doesn't need a hot wire to excite it.
https://www.dbelectrical.com/products/c ... 12180.html
I had one that looked like this as well from a different company and had to run a wire to it for excitement.

[radioguy41]

Yep, that's where I bought it. I found the self-excite to be hit or miss, not totally reliable, hence the mod.

[Slim140]

I guess we get what we pay for, lol.

[radioguy41]

I think the later model Cubs with the larger drive pulley on the fan hub probably has a lot to do with it also. Those will spin the alternator faster per engine RPM then the smaller drive pulleys do and with 60-70 years to mix and match, the later hub could show up on almost any year Cub.

From TM's website:

Early (165152 snd below)


Late (165153 and above)