MoTeC Global Forum

the current factory fan setup on my XR8 uses 3 relays and 2 Aux outputs from the M800 to control them in a hi/lo setup. I installed a MOSFET PWM setup to free up one output and gain full regulation of the 2 fans. The original setup ran ground switched relays and i installed 2 p-channel MOSFETs in high side control in place. When there was power to the ECU, the fans ran as designed. when power was removed from the ECU, the fans ran at full speed. When i unplugged the gate, the fans stopped as there is a 1K pull-up resistor to +12v shutting down the MOSFETs. Does the M800 ground the outputs on shutdown or is there a small internal leakage (less than 1mA) enough to affect a MOSFET gate but not energise a relay?

I think that all outputs are somehow connected to Ubat and when the Ubat is down also all outputs are almost grounded. You can use an High MOSFET driver like TC4431 with voltage lockout function integrated.

http://ww1.microchip.com/downloads/en/D ... 21424d.pdf

The outputs are floating when the ECU is turned off. Some of the outputs have protection diodes on them as well.
Its possible that there is enough resistance in the output circuit to keep a FET on.

You could possibly select a logic level style FET to solve your problem or use a opto isolated FET relay