Extruder heating on its own

On my Solidoodle I replaced the stock hot end with an E3D v5 hotend. It's got one issue: It's heating by itself as soon as I plug in the PSU. I connected to the board without 12V power and it was cooling, plugged it in and it instantly starts heating, disconnect and it stops. Heating button is off. I've tried changing polarities and to no avail.
What do? What check?

Just tried plugging it up to the bed heater slot and the same result, it starts heating on its own as soon and only as long as it has power. So I know it's got something to do with the heater.

Your MOSFET is burned out, I used to have this problem all the time when I used sanguinololu boards. Here's the replacement: MOSFET

Edited 1 time(s). Last edit at 05/15/2014 03:21PM by diytechshop.

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Through self sustaining actions comes freedom

DIY Tech Shop

Good guess, one that has not been said, but this was talked through and solved in another post.
Simple mistake entirely on my part; that wasn't the extruder heater output, that was the fan output..
So yeah, long solved problem, but thanks.

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Realizer- One who realizes dreams by making them a reality either by possibility or by completion. Also creating or renewing hopes of dreams.
"keep in mind, even the best printer can not print with the best filament if the user is the problem." -Ohmarinus