Mower headlights run on AC?

joshuabardwell · Jun 18, 2012

I just spent a little while troubleshooting a "won't crank" condition on my mower. After testing the battery, I went on to test the starter motor (good) and solenoid (good), only to find out that it was, in fact, corrosion on my battery terminals that was the problem. When I tested the battery, I touched the tester's leads to an uncorroded part of the post and got a good reading. When I actually tested the cables themselves, I got a bad reading--but only when the battery was under load, of course. The rest of the time, it showed 12.5 volts all day long.

But that's not my question. I just told you that so you could get a good laugh out of my dumb mistake. My question is, I noticed a while back that my headlight harness was disconnected, and when I hooked it back up, the headlights didn't work. So I tested the path from the headlights, back to the ignition switch, and found no voltage anywhere on the line. I assumed it was a fault in the ignition switch, and since I hardly ever mow after dark, I forgot about it. While troubleshooting the ignition problem, I found a wiring diagram for my mower, and it looks to me like the headlights run off of AC direct off the alternator. If that's true, that would explain why I couldn't find 12V DC :laughing::laughing::laughing: but it's surprising to me. Is that common? Am I reading the wiring diagram right?

I guess one advantage of doing it this way is you can't leave the headlights on and run the batteries down, but still... every vehicle I've ever worked on ran the lights on 12V DC. This is a new one on me.

The question will be answered once and for all tomorrow, when I go out there, hook everything back up again, and fire up the mower, but in the mean time, I'll post about it here.

not2old · Jun 19, 2012

I have a couple of dirt bikes that run the headlights on AC. Both Husqvarnas, neither have a key

JerryK · Jun 19, 2012

I just bought a new Craftsman LT3000 lawn tractor and it has the same setup. One side of the 'alternator' is rectified and goes to the charging circuit and the other just goes to a switch and the headlights. The lights brighten at RPM and dim at idle...... good enough for a lawn tractor light which is seldom if ever used .... I guess..... I don't ever remember trying to mow the lawn at night.

k0ua · Jun 19, 2012

I don't know about my current mower a JD, but I know several of the mowers I have owned in the past (MTD's) have worked that way also.

James K0UA

mjw357 · Jun 19, 2012

An incandescent bulb will burn with AC or DC of the same voltage, it's not polarity specific. It just lasts longer with DC. Yes, you are reading the diagram correctly.

If your battery voltage drops when under load, then:

1) Battery is not charged
2) Battery is bad and will not hold a charge

With the engine running, you should see 13.8v-14.5v across the terminals that would indicate that the tractor is charging the battery. With engine off, 12v-12.5v. Check that 15A fuse. With the key turned to 'START' battery shouldn't drop below 9v.

I would remove the battery, clean terminals, fully charge battery overnight and reinstall, check again. You might be due for a new battery.

joshuabardwell · Jun 19, 2012

mjw357 said:
An incandescent bulb will burn with AC or DC of the same voltage, it's not polarity specific. It just lasts longer with DC. Yes, you are reading the diagram correctly.

That was my understanding. Thanks for the confirmation that I'm reading the diagram correctly.

I would remove the battery, clean terminals, fully charge battery overnight and reinstall, check again. You might be due for a new battery.

Sorry I wasn't clear. That was ultimately what fixed the problem. I just missed it because, when I tested the battery, I touched the tester to a clean spot on the terminal, and so it tested good. I mean, given the amount of corrosion that was on there, cleaning the terminals really should have been the first thing I did, but you know how it is... can't do anything the easy way.

I slathered them liberally with dielectric grease, which will hopefully help keep the corrosion down.

joshuabardwell · Jun 19, 2012

JerryK said:
I don't ever remember trying to mow the lawn at night.

I have been finishing up at dusk occasionally and used the lights for the last 20 minutes or so. Especially if it was about to start raining.

DennisFolsom · Jun 20, 2012

My new Craftsman YT4000 has the same electrical setup. The lights run directly off the alternator. They dim a lot at idle, but you don't mow at idle, so it's not a big deal.