The Battery Won't Charge or stay charged.
- Key switch doesn't turn off the system and the battery drains while it's parked.
- Bad insulation on the wiring somewhere and the battery drains down.
- The alternator is not putting out enough power or no power at all.
- The battery is bad.
Let's consider the battery first. This is an easy check if you have a cheap, simple volt meter. It's much better to have an old fashioned needle meter rather than a digital. The needle is more sensitive, much quicker, and makes a clearer diagnostic tool. But not to worry, for you fancy folks a digital still works (sometimes the results are not as easy to decipher).
- Set the meter to DC Volts.
- Attach the red and black leads of the meter to the positive and negative posts of the battery. Most modern day digital meters don't care if the polarity is correct or not. However, if you have a needle meter, best to put the positive of the battery with the red wire of the meter; otherwise the needle will not be happy with you.
- Note voltage. It should read near or above 13 volts if the battery is good and fully charged. If it reads below 12 volts the battery needs charged or it has bad cells. If after charging a few hours, the voltage is still below 12 volts then the cells are bad, replace it. That should solve your problem. If not...continue.
- "Load Test". Turn the key to crank the engine while keeping your eyes on the meter. Whether the engine cranks (turns over) or not, the meter should not fall much below 11 Volts. If it falls below 10 volts or worse yet below 9 volts, the battery has a bad cell or two. Replace the battery, charge it, and repeat the test.
- If the engine starts, rev it up and watch the meter.
- If the charging system is working the voltage on the meter should quickly rise above 13 volts.
- If it rises up strongly towards the 14 volt range this indicates the charging system of the machine is working.
- If it plays around down near 12 volts you are reading the recovered voltage of a good battery, but the charging system is not working.
- If it simply stays below 11 volts, the battery and the charging system are both suspect.
- Charge you battery.
- Repeat the test.If you get the same results...continue
- Replace the battery.
- Fully charge it,
- Repeat the tests before worrying about the charging system.
Note: if the battery falls below 9 volts the fuel cut off, a black cylinder on the bottom of the carburetor (if your model has this), will cut off the gas supply to the carburetor.
If the battery passes all the tests and you still don't get 13-14 volts of charge it's time to test the charging system.
Small engines have 3 different possible alternators, standard circuit, dual circuit, tri circuit. I would need the exact engine model numbers in order to precisely answer your question. Often mower models have choices for the engine they come with. In this situation the engine model is more important than the model of the mower.
Testing an alternator is fairly simple; all you need is the same multi-meter used for battery testing. Do take note: The settings for battery testing are DC Volts. The Settings for Stator or Alternator testing are AC Volts (in the 50 Volt range).
- Disconnect the connector from stator. The stator is right there on top of the engine under the plastic/metal fan guard. Take this off and look for a pigtail wire with a connector. Don't get the coil wire with the spark plug end.
- Set multi-meter for AC volts.
- Attach RED test lead to either pin on stator side of stator connector. (On single wire leads, attach RED test lead to the single pin.) On AC output alternators there is no positive or negative because both wires alternate from positive to negative, so either pin will do.
- On two wire lead models, attach Black test lead to the other pin. On single wire stator connectors attach the Black test lead to the engine or other ground.
- Prepare the lawn tractor for engine start (set parking brake). Start engine and run at full throttle.
- Check output. Output for engines of this general size run in the 30 to 50 volt range. For instance the B&S V twin 22 hp AC output at full throttle is 30 Volts minimum. Other B&S on these machines spec 40 Volt minimum.
If your voltages are not in the manufacturers range (too low), or non-existent, your stator is partly shorted or completely burnt out (open circuit). Either way you have to replace it.
A. In the picture is a single lead stator connector.
B .In the picture is a two wire lead stator connector.