Alternator wiring

[Steve Forrest]

Some time ago there was a picture posted (I think) in this forum on how to wire an alternator in when converting to a 12V system.

My question is...what is the reason for putting a diode into the circuit which goes to the ignition switch? I recently changed my alternator (neither were MH) and there was no diode in the circuit. I put a small one in (1 amp rating). When the tractor worked a bit (plowing snow) it started running poorly. Took the diode out, runs fine.

My guess is the diode was too small, which is fine...take it out and it runs. But, what is the purpose? Am I going to do any damage to battery/alternator/regulator leaving the diode out?

Thanks, Steve.

 

[Anonymous-0]

The purpose of the diode is to keep the battery from being drained back through the alternator.

 

[old]

Well first off the other guy is worng because it wired correctly that wire will have no voltage on it with the switch turned off. The reason for a diode is so the alternator excite windings will not back feed the ignition and cause you not to be able to shut the engine off. As far as the diode it needs to be bigger then a 1 amp one or it will burn out and cause problems I use a radio shack part #276-1661 which is a 6 amp diode. On some installations there is no need for the diode and on some it is needed. But a ballast resister ot part #194 light bulb will also work
Hobby farm

 

[Bob]

That's NOT correct, Jon,

"old" has it right.

 

[Bob]

The diode should be in the wire to the alternator's #1 terminal, and NOT elsewhere, where it will affect the power feed to the ignition coil's primary.

 

[Anonymous-0]

My mistake misunderstood. Are you saying a diode will not work for this purpose. Just curious as my shops massey field rep uses them religously. Maybe he is wrong.

 

[Bob]

If we are talking about the same diode, in the lead to the #1 terminal on the alternator, it's purpose is to prevent current from backfeeding OUT of that terminal and into the ignition system, preventing engine shutdown when the igntion is switched "OFF".

It can also prevent damage to the alternator if there's a failure in the charging circuit.

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIA1A.gif">

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIB1A.gif">

Once the engine shuts down and the alternator is no longer ouputting current, the ignition switch side of the diode (anode), and the alternator #1 terminal side of the diode (cathode) are both at ZERO Volts, so the presence or absence) of the diode is has NO effect on the battery being drained.