Flat Battery and erratic idling

[Jeff4wd]

Hi,
Wonder if anyone can help. Just finished my wife's Dexta, hitched up new battery and it started first time. Went back in approx 1 hour and battery is flat! New loom, lamps, dynamo, regulator and switches all fitted. What could be causing it to run down the battery? Could it be something to do with the positive earth? Have looked on the internet, and it says that most electrical components are not polarity conscious and should work anyway, so should this make any difference? Should I change it to negative earth? Also, when it is idling, it revs up and then slows down to almost nothing, then revs itself back up again. Will settle down a bit when warm but is still not 100%.
Any suggestions greatly appreciated.
When I have sussed out how, I will post some pictures.
Thanks
Jeff

[Brian]

Jeff,

If you can run a battery flat in 1 hour you must have a pretty bad short. I would have expected to see smoke and the smell of burning.

Nothing wrong with your polarity. The tractor was designed to run with a positive earth and will work happily with either positive or negative earth. However if you have changed the dynamo for an alternator you would have to be negative earth as most alternators are for negative earth systems.

After all the electrical work and new parts, did you polarise the dynamo by fitting it and before attaching the wires, use a spare piece to connect the small field terminal to the output terminal of the battery?

If you did not, then you could have toasted the system. If you have fitted a negative polarised dynamo to a positive earthed system, running the tractor has possibly welded the points of the cutout together.

Then, when you switched off the engine, the dynamo would be "back-fed" and try to work like an electric motor. As the fan belt is possibly tight it would not be able to turn the engine causing heat and compounding the problem.

Because of the resistance in the dynamo you could have flattened the battery without setting fire to the wiring.

This is just a possibility and would account for a flat battery in that time scale.

Dextas always hunt! Its part of the beast. As you say, worse when cold but improves when warm or under load.