Noob with a question….312A Diode Issue…

[Jake217 20]

I’ve been lurking around here for about 3 months and I find this site to be an outstanding resource. I’m hoping someone can point me in the right direction. I promise to post pictures of my herd once I can get this machine into the frame.

I just picked up a 1986 312A in pretty good shape, K301 and Eaton 700. It runs great, but after a few days I noticed the battery weakening and sure enough it is not charging. This machine has the unregulated 5amp charging system with the 70 watt light set up.

The lights were working fine. They get brighter as the engine RPM increases, so I assumed the stator is OK. I started looking for a wiring issue, by checking for continuity from the stator output connecter to the battery etc.

The trouble began when 4 of the connectors fell off the ignition switch. The connector is long gone and someone in the past did some rewiring and replaced the ignition switch connector with individual connectors. Anyways, when I put the connectors back on, I reversed the connectors for the B and R tabs on the ignition switch. As soon as I switched the key to the run position I smelt something burning and sure enough it was the diode in the charging harness coming out of the K301. I put the connectors on correctly, but now the diode gets really hot as soon as I put the ignition switch to Run.

I assume the diode is shot and I have a new one on the way, but I don’t want to take a chance on burning out the new diode as well. So I have some questions:

Will a bad diode continue to get hot (really, really hot) even if everything is now wired correctly? Or, does it sound like I have another wiring issue somewhere because a diode whether good of bad should not get hot? I know nothing about diodes other than the fact that I think they are magically converting K301 AC output to DC without a rectifier. Is it possible I have fried the stator as well? Isd it possible the diode is still good?

Also, the Kohler manual says to remove the charging wire from the battery when performing some of the tests on the charging system. There is no charging wire attached to the battery on this machine, just the main cable to the solenoid. Does Kohler mean that I should take the battery to solenoid cable off? That doesn’t make sense to me.

I studied the Demystification guide until my eyes watered and it was no help. Page 7-48 of the guide shows a diagram of the charging system with no regulator, but the diagram of the gauge circuit that immediately follows does show a regulator – that one is still confusing me. I’m pretty sure the voltmeter is shot. I put direct battery power to it with no response.

Any help or clarification would be greatly appreciated. Even though I’ve owned Horse’s for years, I have learned a tremendous amount from this site in the past few months. Thanks.

[Save Old Iron 1,564]

Jake,

with a dual output stator, your wiring should look something like this

(click on images to expand)

Please note there are two separate windings in the stator, each one is independent from the other and each is connected to the stator core which is grounded to the tractor's electrical system.

Yellow windings in diagram provide the AC output for your headlights - the faster the engine runs, the brighter the lights get

Green windings provide a similar AC unregulated output which has the negative part of the AC clipped off by the diode embedded in the stator wiring. This diode is the equivalent of the rectifier part of a rectifier / regulator assembly.

The winding for the battery charger side of the stator only has a sufficient number of wire wraps to supply 16 volts overall output at full engine speed. The limited stator windings "self regulate" by lacking the ability to produce excessive charge voltage at full engine speed.

To check the diode in the DC output charger circuit, follow the outlines in the images below. One of the conditions on the multimeter readouts will match up with your diode. I'm guessing it will be low ohms readings on both orientations of the meter leads - the diode is probably shorted allowing current flow FROM THE BATTERY TO THE STATOR LEAD AND THEN TO GROUND. This "reverse" flow is normally stopped by a properly functioning diode. A shorted diode allows reverse current to flow INTO the stator and possibly overheat and ruin the stator windings.

[Jake217 20]

Thanks a million, Chuck.

It makes sense now that you explained the diode operation. I'll pull the flywheel this weekend and check the stator as well. It sounds like I might have fried that too.

[Save Old Iron 1,564]

You can check the diode in the wiring without removing the flywheel - I'm guessing the diode is shorted but may not necessarily have damaged the stator. The diode does not have to be removed from the wiring. Peel back the heat shrink insulation around the diode. If the diode is cracked and obviously overheated, just cut it out and solder in a new one. A defective stator WILL NOT cause the diode to go bad again. A defective stator with a good diode will just not charge the battery - no harm will come to the new diode and may save yourself the hassle of removing the flywheel for no reason.

Radio shack has suitable replacement diodes - 50 volt 6 amp diodes for less than $1.50 at any local store.

http://www.radioshac...ductId=12673860

[Jake217 20]

Excellent! Thanks Again.

[Save Old Iron 1,564]

You're welcome.

And of course pay attention to the direction in which the diode is soldered into the harness. The silver / white band on the diode body will be pointing toward the connector - away from the stator.

[Duff 206]

OK, Chuck, are you messing with our minds? On your third post above (the one with the ohm meters) there's a little ant-like thingy running around the bottom of the post. I thought something had gotten inside my laptop screen and I was about to dismantle it until I realized it was electronic buggery, not the real deal!

Duff :laughing-lettersrofl: