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This question came in from Ross, who had fitted the battery leads to the wrong terminals of his battery.

Tractor Battery leads

The Question…

Hello, I have very little experience with tractors and this is my first one.  It is a Massey Ferguson 148.  I have had it for about a year and it seems to be running well.

I took the battery out as I was going away for a while, and on getting back I put it back and maybe put it back in the wrong way round there where sparks and then the wire connected to the alternator melted.

I unhooked the battery and am now trying to work out what to do.  There is no – or + on the wires, but one is clearly earthed to the chassis and the other goes to the ammeter I think, and then to the alternator.

I would just replace the wire and put it back the other way round but have just read it could be earthed from the positive.  How can I tell?

I am sorry this is probably quite a novice mistake, but I really need to get it back up
And running as I use it most days.

Thank you for your time, Ross.

The Answer…

The general (almost foolproof) rule is that tractors with alternators are negative earth.

It will have probably have also burnt out the rectifier diodes on the alternator, so will either need repairing or replacing, as well as the wiring to be repalced if it has melted as you said. Even if it working OK, then replacing the wiring would be sensible given that it started to melt.  If the insulation has been breached then this could cause sort-circuits and cause a fire.

The battery terminals and the lead clamps should be different sizes.  The positive is larger than the negative.

The Vintage Tractor Engineer