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Discussion Starter · #1 · (Edited)
I have a lawn tractor with a Briggs and Stratton 16.5hp engine. It seems over the last few years I have had to replace the battery every spring, and when I do, I have to put it on a charger after a couple of mows. The ampmeter next to the ignition does not move at all when I do get it started. A few years ago, I did jump it off of a vehicle. Which I now know I possibly ruined the regulator. I found an alternator and a regulator, but would like to know how to go about troubleshooting and where I should start. The engine is Model#313777-0163-E5. I also put a voltmeter on the battery, and got 12.5 volts when I tried to start it. It pulled down to aroud 6 volts without starting and when i turned the key off it gradually climbed back to 11 volts. Any help would be greatly appreciated.
 

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Proud Wheel Horse Owner
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Coming out from under the top sheetmetal somewhere on that engine, you'll have a pair of wires. One red and one black. They'll terminate at a white plastic plug, and from there 2 wires continue on.

The black wire is AC voltage, for powering the headlights. With the plastic plug unplugged from the tractor's harness, you can test for AC voltage output with the engine at speed. It'll produce quite a bit, like 15+ volts AC, maybe even up to 30v AC.

The red wire is DC voltage that charges the battery. Just prior to the plastic plug that both wires go to, you'll see a black heatshrink protected lump. This is the charging system diode. This keeps the battery from draining backward into the stator windings when the tractor is parked. If this diode is blown, the battery will drain during storage. Having a battery under a constant drain will ruin the battery and the result is like you see when you go to start, the battery voltage drops off rapidly to 6v.

You can test for DC voltage at the red wire, just like the test for AC voltage at the black wire. You should see 13-something and it'll increase with engine speed....maybe as much as 15v.

These systems are "sensative" to battery voltage, so if you have a really drained battery, or a bad battery, and start the tractor by jumping off a car battery or jump box, the system voltage with everything hooked up, might be really low. The stator is only 2-4amps of charging power, so that's really low. In other words, if you test charging system performance, make sure the battery is fully charged, otherwise, you'll get a false low reading and be chasing your tail thinking you have a bad stator or bad external voltage regulator(if equiped).

The final test is of that diode. You'll need a diode test function on your electric test meter in order to do this test. The way I do it is to cut off the heat shrink to expose the diodes bare metal wire leads. I put my black test lead on the engine side of the diode, and the red test lead on the plastic plug side of the diode. In other words, will current flow toward the stator. It should not, and the meter should show "open loop" or "no continuity." Then I reverse the leads, and the meter should show "closed loop" or "good continuity."

If you find the diode is bad, you can purchase just a short "pig tail" repair harness that contains the diode and the plastic plug-in. Available on ebay and other sources for $14ish. Simply cut off your existing red and black wires just on the engine side of the diode, and splice in the new pig tail. 5min repair.....and it'll save you a $60+ stator that takes pulling the flywheel to replace.

Hope this helps.
 

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Discussion Starter · #7 · (Edited)
I checked the plug, and I'm getting about 11volts at idle and 15 at increased engine speed on the dc contact. Before I start cutting, I wanted to make sure this is the diode. I was expecting it to be bigger. Its the only thing on the wire between where it comes out and the plug. Its maybe the size of a pencil eraser. Thanks for all your help.
 

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I checked the plug, and I'm getting about 11volts at idle and 15 at increased engine speed on the dc contact. Before I start cutting, I wanted to make sure this is the diode. I was expecting it to be bigger. Its the only thing on the wire between where it comes out and the plug. Its maybe the size of a pencil eraser. Thanks for all your help.
Yes that is the diode, but if you are getting 15 VDC at RPM it is fine. At low idle there is not enuf output from the alt to bring the voltage up.
 

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Proud Wheel Horse Owner
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You have a good stator/alternator, so that's good.

Here's where my knowledge is a little lacking on these......

I don't know if diode is what "helps" the stator/alternator produce DC volts, or if the diode simply keeps the battery from draining backwards through the windings of the stator, and that that is it's sole purpose.

I know that when that diode (inside the black heat shrink) goes bad, the tractor's battery will go dead while sitting. But I don't know if the stator/alternator will still produce DC volts.

So do this. Take a sewing needle and poke into the red wire, above the black heat shrink. Connect the black lead of your ohm meter to the pin. Connect the red lead of your ohm meter up inside the connector, to the red wire's contact. With the meter set in diode test mode (you have to have a meter that has this function) you should see "OL" for open loop. In other words, no connection/no continuity. That means the diode is good. Reverse your meter connections, and you should now see continuity, because that's the direct voltage(current) is supposed to flow...."from stator to battery."

If the diode doesn't pass the test, they are available on ebay for cheap.

Also, a new ammeter is only about $8-9. Getcha one. Or at least check the connections on the back and make sure they aren't corroded.
 

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Well, you can't trust a gauge. You have to put a real volt meter on it and check stator output......which you've done. So you know you have 15V at running speed, and that's proper. Therefore, you have an ammeter that's dead, or has bad connections.

Your 2nd issue is the dying battery, so you'll do the diode test to see of it has failed.

Just FYI, automotive alternators have a diode pack also, and when one of those 3 fail, the vehicle's battery will discharge through the alternator. The alternator will still produce DC current, but when tested on an alternator test machine, it will fail the diode test.
 

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I don't know if diode is what "helps" the stator/alternator produce DC volts, or if the diode simply keeps the battery from draining backwards through the windings of the stator, and that that is it's sole purpose.

It does both. It converts the A/C alternator out put to DC, The igintion switch prevents battery drain when off, but if the alternater output falls below a certain level (low idle) then the diode prevents the battery current from feeding back into the alternator.
 

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Discussion Starter · #16 ·
Ok, I checked the diode and it shows good (open loop) Black wire on engine side of diode, red on connecter. I checked all my connections. Was able to start, but acted like weak battery. This battery has 1 mow on it, and I disconnected after mow. I put a meter on the battery when running and got 12 volts, after increased engine speed I still got 12v. Shouldn't this increase the volts if my stator is working?
 

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Ok, I checked the diode and it shows good (open loop) ?????.

Diode should read open One way, (reverse the ohm meter leads) and low ohms the other, 3-5 normally.

An open diode = No charge (most common)

A shorted diode = AC to the batt, and batt back feed to the stator (blows fuses (not so common).
 

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I set my multimeter to diode setting. Checked one way, I got a .OL reading, reversed leads and got .535 reading.
That is normal, was confused about the open statement above. Measure the voltage at the diode when running, and compare to the battery voltage. The charge current passes through the ignition switch and a fuse on it's way to the battery.
 
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