Quote: Why not use the ground as the trigger with an OE type switch? Why trigger with the +?
That's how Mercedes designed it. You don't want to interrupt the current by removing the ground. 50 amps is a lot of current to be switching on and off. The fan always has power, just like your headlights. It only turns on when you flip the switch. The fourth cavity in the plug is for a feedback wire for the engine controller. It obviously isn't used in our applications. The internal fan relay grounds through the fan motor to chassis ground.
The fan relay I used to fire the old electric fan now powers the trigger. My computer grounds the relay to complete the circuit. Since most folks here don't have EFI, all they need to do is find a way to put 12 volts to that trigger lead. Do whatever makes you happy. Since it's powering a relay there will be no appreciable amperage on the lead.
It's my understanding that supplying a lower voltage signal to the trigger will run the fan on low speed but I don't have a good way of testing that.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind. - Stu Harmon
