Dear Ole Mechanic;
I have a problem with my Dad’s riding lawn mower. It sat all last year, because it was so dry that nothing needed mowing. Of course, the battery was down, and the gas didn’t smell like gas any more. I took the battery out and put it on a trickle charger. I got the old gas out and took the float bowl off of the carburetor. Then I flushed about half a gallon of good gas through the tank, lines, and carburetor. The battery actually took a charge, and, with fresh gas, the mower started. It took a while, but it smoothed out and ran OK, so I mowed around the rough edges and tall weeds. Then, I shut the mower down to get a cold one. As I got ready to put the mower back in the shed, it wouldn’t start. The battery was dead. I left the battery in the mower and put the charger on it. After two hours of charging, it was still dead, so I pushed the mower back into the shed. I took the battery out, figuring that it was shot from sitting, but still put the trickle charger on it for a day. When I got around to going after a new one, the fellow at the auto-parts store checked the old one with both a voltage test and a draw-capacity test. He said that the battery was a little low on capacity, but that it should work just fine. I recharged the battery, let it sit for a week, and when I checked it, it still had a full charge. As I was putting the battery back in the mower, I noticed a spark as I started to hook up the negative cable. I thought the lights might be ON, so I made sure that the key was OFF, and pulled the key just to be on the safe side. I still got a spark! I was just barely able to reach the back of the ignition switch, so I pulled the electrical wiring connector off the back of the switch. I STILL got a spark when I tried to hook up the battery. I started disconnecting every wiring connector that I could find, checking for spark after I disconnected each one. When I disconnected a small wire that disappeared behind the starter and shroud, the spark at the battery went away. Where does that wire go? Why is it draining the battery? What's wrong?
Dear Too Many;
You are just full of questions, aren’t you? OK, here are some answers. That wire that disappeared behind the starter and shroud goes to the battery charging coils under and inside of the flywheel. Somewhere in the mowers recent past, someone has tried to jump the battery and gotten the jumper cables hooked up backwards. The wrong hookup undoubtedly caused some sparks, and it only takes a little one to burn out the charging diode. The diode is in the circuit to change the alternating current to direct current to charge the battery, and it also keeps the battery voltage from going back into the charging coils and discharging the battery. With the diode burned out, the battery can discharge back through the charging coils. At this point, the charging coils are probably burned up also. Why didn’t disconnecting the ignition-switch wiring eliminate the problem? Because the wiring from the charging coil goes directly to the battery. This is a very simple and cheap way of dealing with the situation, and most manufacturers do it that way.
Now for the "good-news/bad-news" part. A new charging coil with diode is under $40 for a Briggs & Stratton engine--add another $15 for a Kohler engine. To replace the coil, the flywheel must be pulled off, and unless you have a flywheel puller, you will either damage the flywheel, not get it off, or both. By the way, all engine shrouds must be removed to get at the flywheel, and to get the shrouds off, you must remove the hood or engine-cover off of many riding mowers.
Hopefully, you and my three loyal readers may have noticed that you forgot to tell me what make and model of mower you are working on. In this case, almost all mowers have a minor variation of the same system, because they follow the old adage of “if it ain’t broke, don’t fix it." Consequently, the system has been around a long time.
Herr Professor Nuzanbolts