Today I adjusted the valves and took it for a test drive. My 5 year old granddaughter wanted to drive it so I let her drive it around the yard as I walked alongside. After about 20 minutes of this we pulled into the carport where I was working on it and I noticed the alternator light was on.
I shut it down and came out about three hours later with my battery tester. I started it and throttled up to about 1/2 throttle. It measured about 13 volts and climbed to 14 slowly and steadily in about a minute and stayed there. This is what it did after I "repaired" it.
Given that it was 90 degrees out, I wonder if something is breaking down due to the heat. Maybe that one diode that tested low? Or the voltage regulator?
I'd say that in teaching a granddaughter to drive a tractor that YM1110D has just paid for itself. Anything else is gravy.
I doubt that the charging light is telling us much about the alternator. That guess on how heat is affecting that one diode is a possiblilty, but given how diodes work that's probably not it. In fact, the one diode that tested "low" is actually probably the best of the group rather than the worst. Theoretically a diode should have no resistance at all to current flow in one direction and infinite resistance in the other. You reversed the current flow from the battery in the multimeter when you swapped the multimeter leads.
Now it is true that diodes are heat sensitive, and that they will change state when heated .... but they tend to change state quickly - like flipping a switch - and their common failure mode is to short circuit internally - in which case alternator output goes directly to zero instead of being somewhat flaky as you are seeing.
WARNING! You can kill those rectifier diodes - and the ones in the voltage regulator - in a flash by hooking up a jumper battery or big battery charger backwards to your Yanmar and accidently getting the polarity wrong just for an instant. Those old Yanmars were among the first to use rectifiers in their alternator and they forgot to build back-bias protection. You can still jump or charge the Yanmar safely if you need to do so, but must be extra careful that the polarity is correct.
Given what you are seeing between pale red or strong red I'm guessing that the deficit is the Voltage Regulator although it could be the battery itself. Those early Zener-based VR were not very good, and using low quality parts made the worse. How the VR works is that when it sees more than 13.5 volts DC coming from the 4 rectifiers you tested on that plate, the VR reaches all the way back to the alternator coils and decreases one side of the AC waveform right as it is being produced from the coils before it even gets to those 4 rectifiers. At that point the rectifiers see one half of the incoming AC wave as being normal, but the other half of the AC wave is attenuated. Because the DC that the rectifiers are making for you is simply additive, reducing half the incoming wave causes the total output to the VR to drop below 13.5 volts. If you really crank up the RPM, the voltage regulator has the option to also put a resistor into the circuit so that the attenuated side of the alternator output is reduced farther. This all happens real fast, in fact it can happen many times a second. Once the voltage is reduced to 13.5, the VR turns itself off, whereupon the voltage immediately rises to over 13.5....making the whole cycle repeat itself.
If your charging system is working efficiently enough so that it charges at real low RPM (most don't) then you can even see the flicker as the VR regulates itself.
The charging lamp is a whole 'nother story. The charging light is a little sub-circuit within the voltage regulator and is highly dependent on the resistance of the bulb, the socket, and any corrosion there. By rights that charging lamp ought to have it's own little diode in a dedicated branch circuit that would only let it light up when more current is draining from the battery than is going into it. And it would light up proportionately to how much current it was missing. But that would have cost an extra dollar, so instead it shares a diode within the voltage regulator and what you get for that is irratic performance of the charging indicator.
Basically, if the charging light goes out when you rev the engine then don't worry about it. If the light is slightly on when you are operating, then figure that means you are slightly discharging the battery at that rpm, but that it is slight.
If you get curious about how it all works, try keeping the headlights on and see what difference that makes.
Are you even more curious? Then hook your multimeter across the battery terminals, strap the multimeter to the dash close to the charging lamp, and see what if you can combine the two pieces of information using your own inbuilt charging circuit.
Only if the light is on brightly before you start the tractor and doesn't change at all once you start up does it mean that something is wrong with the charging system and needs to be fixed - and I'd still believe the multimeter over the charging lamp.
Of course if the charging system does need attention, you can always elect to replace Yanmar's oddball sized garden tractor battery with a small automotive battery, and that has the advantage of letting you ignore that fact that it may not charge for months on end. By charging with a household battery "smart" charger every few months and not using the headlights much, you will find that a standard automotive battery will start and run that little tractor for months. Maybe all season. That's all the battery does in those old Yanmars, it just starts the motor. BTW, try to get the type of battery that is sealed or AGM type. I recommend you Do NOT use an open old style refillable type cheapie battery with the caps on top sitting that close to your nice (expensive) Yanmar radiator. If you need to expand the size of the battery box a little so as to fit a small automotive battery it will be one of the best things you will ever do for your Yanmar. Keep the expanded box as far away from the radiator as you can.
There are other ways to look at charging. For example, if you were to wire a voltmeter permanently into the circuit, or strap that multimeter to the dash, I think you will see a different charging picture than the charge lamp is showing you.
Bottom line. Like Winston. and California and Coy are saying, a flickering charge lamp at low to medium rpm is most likely your VR or your battery (or both) and probably not worth worrying about unless you want to change to a better battery - which you ought to do anyway. Then you basically don't care if it doesn't charge.
With a small car battery, you and your granddaughter can drive it all week anyway.
Enjoy,
rScotty
