electrical trouble shooting

[Robfnk]

I have a little John Deere 790. Battery keeps going dead. I replace the battery a month ago with an Interstate battery thinking that was my issue but it still goes dead in a weeks time. I checked the alternator. It is putting out 3 amps at idle and 16 amps at the suggested running speed. My amp meter says there is nothing drawing off the batter when the tractor is shut down but something is draining it so it most likely is under .1 amp draw since that is what my meter goes down too. Can anyone give me any suggestions on where / how to start trouble shooting. If I disconnect the positive side of the battery to test I am just going to read through the alternator and starter with my ohm meter so not going to get an accurate reading. Any suggestions would be appreciated.
Rob

 

[Hoss710]

If it were me I'd pull the positive lead with the tractor shut off and key out. then measure the resistance between the positive cable and ground. it should be in the k if not mega ohms. if it's less. start disconnecting thinks until you loose the low ohm source. that will most likely be your issue.

 

[ROUSTABOUT]

Take a simple idiot light. Disconnect pos and put light one end on cable, one end on post. See if it lights up. If it lights up, start pulling fuses one at a time to make it go off. That finds what circuit it's on. If that don't find it, disconnect alternator, keep going until you find it. Because if the light lights up, the draw is there. You just have to find it.

 

[kenmbz]

Can you measure battery voltage not running and then battery voltage running and make sure running is higher than not running.
I like the lamp suggestion above too, make sure it is a very small 12v light, low wattage though or it won't get bright enough to see.
< .1amps means less than 1.2 watts.

 

[2manyrocks]

I would look at the positive battery cable as it passes from the battery under the tractor to see if the insulation has been rubbed bare.

 

[DonRybak]

Alternators have an internal regulating diode that goes bad causing a small drain on the battery. One terminal on the alternator is a direct connect to the positive on the battery. Try disconnecting that positive wire on the alternator and see if the battery is still charged after a could days. As Roustabpout mentioned, using a small test light would work too and be faster than waiting a couple days. If it's the diode in the alternator a repair shop should be able to fix it rather than replacing the whole unit. Good luck.

 

[MRTOES]

I can't offer you any better diagnostic advice than above BUT if all that fails be sure to check out all your ground connections to your engine block and anywhere else there is supposed to be a grounding strap. Bad grounds cause more problems and fires than people realize and they are very hard to diagnose.

 

[Robfnk]

Thanks, I will try the suggestions as soon as I can get back to it and let you know if I find anything.

 

[seville009]

All that I can offer is that I bought a new skidsteer in 2009.

Dealer told me to keep the battery detached when not in use so as to not drain down the battery. He was correct. About six years later, had to replace the alternator. I stopped disconnecting the battery just to see what would happen. The battery stopped draining.

So…in my case, the brand new alternator in the brand new skidsteer was apparently causing the drain.

Almost exactly six years after that, I had to replace the alternator again, as it ultimately went bad. I mention this second replacement just to point out that alternators can go bad.


How old is your alternator? Is your alternator belt tight and in good condition?

 

[ROUSTABOUT]

I can't offer you any better diagnostic advice than above BUT if all that fails be sure to check out all your ground connections to your engine block and anywhere else there is supposed to be a grounding strap. Bad grounds cause more problems and fires than people realize and they are very hard to diagnose.

I have fixed some heckuvamojo problems because the fuse box wasn't bolted on tight enough.