Are there any electrical experts out there who can verify my Mark 8 fan wiring diagram? Instead of getting a variable speed fan controller I want to try to build my own system. I thought this might be a creative way to get the single speed (variable fan) to still work, firstly, with using the stock wiring, and secondly, to maintain two speeds and still have the car's ECU in control. Not to mention the already build in A/C toggle.
A lot of aftermarket fan controllers need a separate temp. sensing unit to be installed, adding a layer of separation from the ECU to the cooling system. I know they all work perfectly, and possibly may even run better depending on where the temp probe/screw is placed. I just thought I'd think out of the box and see if I could whip my own up.
Anyway here is the circuit I drew up:
I have the low speed signal running through a 75amp N/C relay. When the high signal kicks in it will trigger the relay and also provide full power to the fan. I’m not sure if this is a good design; allowing the relay trigger pins to get the full amp pull of the fan. I have no idea if when a relay is rated for a certain amperage if it also includes its trigger pins??
I also am wondering if I will need an additional fuse on the fan power lines, like I have on the direct line to the battery on the switch circuit below. Or perhaps I can simplify it a bit and put it between the relay and the fan?
have a stare and let me know what you guys think!
Reason for diode across the motor terminal:
FREEWHEELING DIODE
Since the motor is an inductive load and an inductor is an energy storage device, when the contacts of the relay open, the energy that is stored in the motor has no place to go. The voltage increases (negatively) and a spark is created across the relay contacts, greatly reducing the contact service life. If a diode is placed (reverse biased) across the motor, it will not conduct in normal operation but will conduct when the relay contacts are opened, thereby suppressing the spark. The current is said to "freewheel" through the diode and the motor after the supply current from the battery is interrupted. This voltage spike can get unbelievably high, hundreds of volts for a 12V system. Use a 1N5404 which is rated at 3A, 400PIV.
A lot of aftermarket fan controllers need a separate temp. sensing unit to be installed, adding a layer of separation from the ECU to the cooling system. I know they all work perfectly, and possibly may even run better depending on where the temp probe/screw is placed. I just thought I'd think out of the box and see if I could whip my own up.
Anyway here is the circuit I drew up:
I have the low speed signal running through a 75amp N/C relay. When the high signal kicks in it will trigger the relay and also provide full power to the fan. I’m not sure if this is a good design; allowing the relay trigger pins to get the full amp pull of the fan. I have no idea if when a relay is rated for a certain amperage if it also includes its trigger pins??
I also am wondering if I will need an additional fuse on the fan power lines, like I have on the direct line to the battery on the switch circuit below. Or perhaps I can simplify it a bit and put it between the relay and the fan?
have a stare and let me know what you guys think!
Reason for diode across the motor terminal:
FREEWHEELING DIODE
Since the motor is an inductive load and an inductor is an energy storage device, when the contacts of the relay open, the energy that is stored in the motor has no place to go. The voltage increases (negatively) and a spark is created across the relay contacts, greatly reducing the contact service life. If a diode is placed (reverse biased) across the motor, it will not conduct in normal operation but will conduct when the relay contacts are opened, thereby suppressing the spark. The current is said to "freewheel" through the diode and the motor after the supply current from the battery is interrupted. This voltage spike can get unbelievably high, hundreds of volts for a 12V system. Use a 1N5404 which is rated at 3A, 400PIV.
