I had a moment of electrical clarity while I was drinking my coffee this morning and realized that I was seriously overthinking this problem.
It's not a rocket!
Once I understood that the TDCM, actually the Battery Discharge Lamp Control (BDLC), was getting juice to turn the light on that eliminated one circuit. So, since there is only one circuit left Occam's razor kicked in. It only stood to reason that this circuit was the culprit. However, it had a few possible reasons that it wouldn't work.
The first thing I checked was for juice at the fuse which I didn't do yesterday when I looked at the fuse. There was juice. First possible denied. The next possible was the connection to the TDCM. This was raised by Chuckv and skwirl. Ah HA! Found it! I found and fixed a loose wire that had pulled loose from the plastic connection when this first started, I did not see that another wire had also pulled completely out. Of course it was the red wire from fuse 2. The wire that controls all the lamps.
Battery voltage comes to the BDLC through this wire and energizes (what I assume is a) electric switch in there that stays open as long as it's energized, when it looses this voltage (at least down to 12.3 volts) the switch closes and sends all the lamp control volts to ground, thereby completing the lamp control circuit and lighting the lamps. It's actually very simple.
Of course there is a little more to it I assume, like maybe a time delay or something for when the ignition switch is first turned on; IE: the lamp lights, the timer times out, and the lamp goes out until the engine starter engages (assuming there is at least 12.7 volts in the battery) at which time the draw on the battery by the starter reduces the battery voltage at fuse 2 to less than 12.3 volts, which in turn de-energizes the switch and closes the circuit once again. Then the lamp lights again until the alternator is supplying enough charge current to get the battery voltage above 12.7 volts, and the light goes out again. If the battery voltage never goes below 12.3 volts while cranking, the light should not light up.
So, it's all done! Everything on my 318 works again (at least the way it was before I messed with it). There are many here that helped me get to the point that it all became clear to me. Thank you! At least it's a little clearer than the mud I was previously looking at it through.
I have 4 318's and I found from experience that the charging module is a weak spot. I had the same experience as you and changing the module fixed it. You shouldn't have to run the engine at WOT for the light to go out. One of my 318's is a FEL and there is no way I'm going to run it at screaming WOT while using the bucket! The light will come on for maybe 15 seconds after the engine starts and then goes out, even at idle. The Onan engines have a monster fan that moves a huge amount of air even at idle. I wouldn't worry about overheating.
Congrats on finding and repairing the issue on your 318.
85 JD 212 - 46 in deck
77 JD 212 - 38 in deck - 43 blade - 37a thrower
88 JD 318 - 54 in quick hitch 4 way hyd blade - Ruegg cat 0 3pt hitch - rear blade.
Ruegg front weight bracket - Aux hyd positive lockout
72 JD 140 H3 - 48 in deck.
The charge system control inside the mystery box is still baffling me, IE: why the light comes on with the turn of the key and then goes out in a few seconds. Electricity doesn't move that slowly! Either the voltage is there or it isn't, but the box reacts otherwise. A timer? A thermal switch? I'm not well versed in electronics so I actually have no clue what they would have used to effect this action. Or why. But whatever it is it is very hardy, it's still working after decades of use and abuse.
Maybe if someone had one that didn't work they could send it to me so I can play mad scientist and dissect it over the winter months and learn some more about it? Or not if someone already has and can write a good paper on it it's internal parts and how they do what they do.
There are diodes in the wiring harness of the Yanmar powered tractors like the 322at the key switch that implement a "lamp test" feature for those lights that are not often lit, like the low fuel and the engine coolant. Here is the manual information on what he lamps on the dash should do when the key is turned to start and the engine not allowed to crank (note the use of the PTO switch or the neutral lever movement to prevent cranking..)
dash lamp tests.jpg
The TDCM contains a simple voltage comparitor circuit to sense the low battery voltage and light the charge lamp. It also has a one-half second delay timer IC that 'de-bounces' the seat switch to eliminate annoying engine kills on rough terrain, but still meets the "operator not present" safety function if the user falls completely off. There is a bypass of the seat switch detection function provided by a transistor driven by the X23 pin 7 connection from the starter interlocks as well. On the 318 class tractors this comes from the S2 terminal of the key switch, on the 322 it comes from the drive circuitry of the neutral start relay. The internal relays inside the TDCM and their attendant circuitry provide the power switching for the PTO engagement and for the ignition itself.
Here is a "black box" view of the TDCM from its eternal connections/functions/voltage measurements that should be helpful in understanding how it all works. This list is written from a 318 point of view, but applies equally to the TDCM as used in the 322. I generated this list some time ago because many other forum members in addition to yourself had similar questions on the module.
TDCM voltage measurements.jpg
Hope this begins to remove the mystique of the TDCM for you...
That does a great job explaining the why, but I was wondering about the how. I will admit I'm not well versed in electrical theory, for instance; why a diode instead of a transistor. That's a little too in depth for me. So, with that said, if the internals are full of diodes, IC's, transistors, and other mysterious little cylinders, do-dads, and circuits that speed up, slow down, and generally alter the flow of electrons, then I'll let it go and be satisfied that when it works I don't need to worry about it.
Randy, A diode is just like a check valve in a hydraulic system...it allows flow in one direction. A transistor is a little more complicated, but it's basically a switch. Bob
'80 317 w/18hp B&S and divert valve for rear hydraulics, 3 pt hitch, 5' york rake
'82 314 w/rear PTO for tiller
54 4 way blade
Mod 48 deck & Mod 462 TracVac
Thanks Bob! That helps clear up a few things.