Weird relay idea. Is this possible?

[undefined0000]

Alright, so I was thinking about rigging up my radiator fan to be able to run alone. By this I mean it will still run after I take the key out of the car until it gets cool enough. So I was thinking to avoid draining the battery in the car, hook up a relay that will make the fan draw power from the second battery. Now that's where it gets complicated, and I can't think it out.

Constant power to relay would be + from ext battery. Signal would be + from car side of radiator harness, the problem comes with the ground. I'm sure it would cause a short if I grounded to both batteries. I'm also not having the ALT charge the second battery, so I was going to have a rocker switch put in so I can change if it draws power from the car or ext battery.

I really only need 2 things made clear:

Does the ECU actually turn on the fans?

What logic should be used with the relay setup

 

[wrencher]

It could be done, even an isolated charging to the axillary battery.
It would be involved & complicated circuits added to achieve this though.
Might be a lot simpler to run a timed circuit to run the fans of the primary battery.
But I have to ask why all this to run the fans engine off?

 

[undefined0000]

Heat soaking, and awesome effect. Using this on top of a turbo timer could probably help keep the hunk of metal cool, and I'm just bored. An idea was thrown at me to run dual turbo timers to get the same effect. Instead of having the second timer keep the car alive, it could keep the fan alive.

 

[Auto RS T]

The thermo sensor in the radiator controls the fan on the ground side. The ECU has nothing to do with fan control on a 1g.

I would think you could run constant power to the circuit rather than "key on" and let the fan function otherwise normally?

 

[undefined0000]

The thermo sensor in the radiator controls the fan on the ground side. The ECU has nothing to do with fan control on a 1g.

I would think you could run constant power to the circuit rather than "key on" and let the fan function otherwise normally?

Well that makes life easier, the only problem is switching power supplies, but I guess I could change the fans to constant power. Ill give that a gander tomorrow.

 

[DROOPY209]

If you use another battery that is properly isolated from your starting battery, you can do this:
Get a SPDT relay, wire pins 87 and 85 to (+), Pin 30 out to the (+) going out to the fan, The (-) wire on the fan will go to the (-) on the secondary battery, Pin 86 to the (-) wire coming out from the Thermo sensor on the radiator or a on/off switch that is wired to (-) or an aux channel on your alarm that has a latched output. If you diode isolate the (+) wire from the relay and the (-) on the secondary battery, your factory wiring should still work as stock with the car on.

 

[undefined0000]

Thank you thank you! Will follow that. All of these years of playing with electricity and I have truly never heard of "diode isolate". Sounds like a 1-way current, correct? What would you suggest using with this 12v high amperage application?

 

[DROOPY209]

I would recommend that you obtain the highest amp rating diodes that you can. I'm not sure how high up in rating they go but you could wire up (4) 5A diodes in parallel to obtain a 20A rating. Yes, a diode is to allow current to flow in one way only.

 

[undefined0000]

Alright, I'll be back with a theoretical diagram to see what you think since I never really messed with diodes before. Would hate to be working and KOOF.