Briggs and Stratton Charging/Electrical System

c3pilotOctober 28, 2010

Hello All,I have a Craftsman II Garden Tractor/mower the numbers are:

Model: 917 255915

Serial: 3147SO 2717

Briggs 18 HP

Model: 422437

Serial: 0758 01

Code: 871029 12

The mower is in really rough shape but it runs. Im trying to figure out what regulates the battery charging? There is only one wire coming out from under the flywheel that runs to the diode then to the lights and the L pin on the ignition switch. Is there a regulator under the flywheel?

I thinking about wiring it up to have a master power switch with push button start. Any help would be great.


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Please state the color of the wire that comes from the flywheel area.

Please state the color of the plug on the wire that comes from the flywheel area.
This information will help identify the type of charging circuit installed on the engine.

    Bookmark   October 28, 2010 at 11:50PM
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The wire is Black and plugs into a white barrel connector with no locking tabs. The wire coming out of the connector is red and goes through what looks like 2 diodes into a white plug that splits and goes into the light switch and the L pin of the ignition switch.

I'm reconsidering the master switch/pushbutton start and will look at installing a new ignition switch. I found the owners manual online and they do have a great wiring schematic with the ignition switch logic. Some of the unkowns are, what regulates the battery charge? Does the wire from the diode going to the L pin on the ignition switch send power through the ignition switch (B pin) back to recharge the battery?

I did run the engine with no battery using jumper cables and the mag ground wire disconected. The Black wire from under the flywheel was disconected (origonally thought that it was the mag grounding wire) and arced to the block several times. Would this have shorted anything under the flywheel, is there a test procedure for checking the output of that wire and what should it be putting out. I will also want to wire in the PTO switch. Can I run that straight from the batt or should it be from the L pin of the ignition, if from the batt should it have a fuse? What exactly does the L stand for on the ignition switch?

I will be rewiring this mower and removing all of the safety switches (all are missing or corroded beyond repair). I know this may be frowned upon by some. If it matters there is no mower deck on the machine and the only real use for the tractor is to pull around a small trailor and have fun with the kids working on a project in the garage.

Thanks again for the help.


    Bookmark   October 29, 2010 at 8:54AM
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As you can see on the schematic, the 2 diodes result in a plus + and a minus - circuit.

The + does go to the ignition switch and is used to recharge the battery and power the PTO clutch. (note that when the key switch is ON, B & L are connected as per the "truth table")

The - is used for the headlights only.

Note also the schematic shows 22 VAC for test purposes BEFORE the diodes with the lights off. (AC, not DC.)

    Bookmark   October 29, 2010 at 3:40PM
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rosemallow(z7 Md)

Some of the unkowns are, what regulates the battery charge?
The charging system in reality puts out little current. The battery in the circuit is the regulator. You cannot overcharge the battery and if the battery is low you will need to charge it on a charger. If you took the battery out and measured voltage it would be about 33 volts, but with the battery connected it will drop to the battery voltage.

    Bookmark   October 29, 2010 at 5:54PM
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Good info guys, thank you. I have most of the wiring harness done and will install tomorrow. Got to admit that I do not know much about the electronics, but I do love the challange of it. I have even gained a new respect for the parts used on these mowers. I was able to disassemble the headlight and PTO switch today and refurb them to usable status, and will use them in the new wiring system.

Do I really need the ammeter? If the clip that holds the fused wire on the ammeter is totally rusted over will the ammeter give an accurate readout?

How can you test the diodes?

Thx again for the help.

    Bookmark   October 29, 2010 at 9:11PM
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There's no guarantee that any ammeter will give an accurate reading.
You don't "need" an ammeter, but it's nice to know that the charging system is working. It also can give an idea how well the battery is charged, since the rate of charge tapers off as the battery reaches full charge.

Diodes can be (usually) tested with the OHMS scale on your voltmeter.
It should conduct in one direction and show open in the other.
You may have to experiment with different Ohm "ranges" to find one that works on your meter.

Personally, I'd just use the voltage function and check to see if I'm getting the + or - volts where/when I should.

    Bookmark   November 1, 2010 at 6:37PM
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Your comment:
"The wire is Black and plugs into a white barrel connector with no locking tabs"
I checked the Briggs and Stratton alternator chart and the PN 391595 was offered on your engine. This has the black wire and a white plug. This alternator is only 1/2 the size of the full circle alternators. Output is rated at 14 volts AC and 5 amps. The alternator does not require a regulator. A diode is not included with the alternator so the diode is probably part of the tractor wiring harness. The output would be marginal for headlights and battery charging.

This was a nice tractor with the heavy duty 6 speed transmission. The engine has a horizontal crankshaft. Since power is taken off both sides of the crankshaft, the blower shroud is difficult to remove for cooling fin maintenance. Cleaning cooling fins is important to prevent overheating and valve seat failure. The mower deck drive belts are covered with a sheetmetal panel that is supposed to keep the grass out. (grass still gets in there). The tractor has the garden style tires.

    Bookmark   November 1, 2010 at 11:02PM
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Bill and Bluemower, thank for the info. I have the system wired but havent checked it as of yet, been busy at work.

I will skip the ammeter unless I find one at the dump on a throw away mower. Strange enough that's how this one started out. I found it at the dump and just cant let it go. The mower deck is shot to h**l and I just saw that it needs a steering sector assembly, not to mention an ignition switch, battery, fuel pump, and has 2 tires that leak down. Now that I say it out loud I really cant figure out why I haven't taken it back to the dump? But zero dollars in right now and have a running driving garden tractor and many hours in the garage with the kids. Good times!

    Bookmark   November 4, 2010 at 10:38AM
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