Fan Override

[Racincc85]

I've got a 1999 nissan altima. The other day I was reading an article that talked about a fan override switch so that the driver could control the fans at will. The car was a 1993 nissan sentra. I understand the concept of how the ECM controls the fans. It grounds the relay for the fan speed it wants to use ( The altima I believe has two speed fans). The article said to use a relay and tap into the ground wire going to the ecm. The way it would work would be power is sent to the relay via a driver controlled switch and the relay closes grounding the fan relays. My question is does a relay have to be used? I was just going to get a pinout of the ECU connection, find the high fan speed relay ground, tap into the wire for it then run in through a toggle switch then ground it to the chassis. Basically it is creating an alternate ground for the fans and allows the ECU to still control them. Why am I doing this? Because I enjoy doing useless things to my car .

[curtis73]

There should be no reason to add a relay other than if the ECU is sensitive to voltage potential spikes on that pin. For that reason I wouldn't tap in at the pin, I'd tap in at the relay. Wire from the blade on the relay to a switch, then to ground. But I would still find out why they chose the relay route for your application. There may be a wise reason to use one.

[Racincc85]

http://www.nissanperformancemag.com/march04/fans/


That is the website that has the article. The reason I was leaning towards this method instead of going directly to the relay under the hood is because the ECU in inside my car. This eliminates drilling through the firewall and running lots of wire. That is a good point about voltage spikes curtis. If I don't want to use a relay could I install a diode in the wire going from the pin tap to ground so power can only flow to ground and not back up the ground into the ecu? Also, how would a relay prevent voltage spikes? A relay would basically act like a rocker switch except that is uses an electric coil to turn it on.

[curtis73]

Unfortunately the finer points of delicate electronics are not my strong point. I don't see the need for all the other stuff he has put in there, other than the relay to carry enough current to ground the fans.

I might contact the author of that article. I would personally ghetto it with some 10-ga wire from the fan relay, back through the firewall (you should be able to find multiple places to snake it through beside or with an existing wire bundle) and switch it directly to ground. It would be fine to relay it and run a much smaller wire as well, but it looks like he's trying to combat the voltage reversal/spike with the diode in his design.

Anyone else have some advice on the finer points of electronics theory?

[Racincc85]

thanks for the help curtis. I'm just going to tap it at the relay. Cheaper since I don't have to buy many extra parts and seems safer.

[CBFryman]

what is the use of being able to turn on your fans at will?
i mean i know yo uare doing it because you like doing useless things (lol)

but why would one write a whole article abotu something like that?