A dead battery, a faulty alternator, ...?

drjdb · May 11, 2001

After a two weeks without use, I attempted to start my L2650, to no avail. Turned the key, and nothing. I boosted the tractor with my car to get it going, then took the battery to a reliable mechanic, and he reported it dead - 9V and not taking a charge. Got a new battery, put it in, and when I connected it, I got a small spark. This seemed strange to me, so I tried to isolate the problem - removed fuses, etc. - and it went away when I disconnected the alternator, reappeared when the battery cable was connected directly to the alternator.

I assumed the problem lay in the alternator - so I took it in to place specializing rebuilding alternators - he cleaned it up and replaced the brushes, but said there didn't seem to be any problem with it. When I put the alternator back in again, I got the spark when I connected the battery.

Now I'm in a quandry. Is the spark normal and to be expected, or am I likely to still have a problem with the alternator? Should I be looking for something else? I don't want to drain my new battery. Suggestions welcome!

dave

allmobile · May 11, 2001

It sounds like there is a current draw some where in the system. I would hook an amp meter into the system to make sure there is a current draw, then start disconnecting things until the draw goes away. I know this sounds vague but it is the easiest way.
Brian

drjdb · May 12, 2001

I measured the current between the battery and the alternator with everything else disconnected, and got over 250 mA ( the needle hit the end of the 250 mA scale on my little multimeter ). The measured resistance between the alternator case and the battery connection post is 250 ohms. With I = V/R, I should have measured 12 V / 250 ohms = 0.05 mA. I cannot explain the discrepancy. Does anyone have any idea what the correct values should be?

dave

allmobile · May 13, 2001

The way I measure for a draw is to disconnect the negative battery terminal and hook my amp meter between the battery and the cable. The mA setting is way too fine for this application. You shouldn't see any draw greater than 0.1 amp.
Brian

Kubbruce · May 13, 2001

Dave,
Sounds like to me that you don't have a problem at all. There is a constant 12 volts at the alternator. This is to keep it "excited" so when it starts to turn it will charge. Remember an alternator is making AC current and changing it to DC. Well anyhow I think you are fine and only had a sulfated battery. Hope this helps....Bruce

drjdb · May 13, 2001

Well, I have egg on my face! I went back and double checked everything, and discovered I had installed the battery backwards

The alternator has diodes that prevent it drawing current directly from the battery when it is installed correctly. You are right, Bruce, that it needs current to energize the coils, but if I understand it correctly, that current comes through the voltage regulator, which adjusts the current to make sure the correct voltage is generated to charge the battery when it needs charging.

The moral of the story is, if you get a spark connecting the battery with the tractor turned off, double check to ensure you have -ve on the ground and +ve on the bus. Fortunately there was no damage to anything, and I left the battery disconnected because I seeing the spark clued me into the fact that something was wrong. Thanks for the help Brian & Bruce.

dave.