how do I wire a fan so that it only turns on when the alternator is on

[catfive]

I have an application where I hook a DC fan up inside the body of a delivery style truck. Normally, I hook the fan relay to a "ignition hot" circuit, where the relay closes only when the ignition key is turned to make sure that the fan doesn't kill the battery.

For various reasons, that isn't possible this time. Instead I wired the fan to battery hot & installed a switch in the cab so that the driver must remember to turn the switch on when the truck is running & off when he turns the truck off. For obvious reasons I'm not entirely comfortable with that solution.

I also thought of routing the circuit through a normally open oil pressure switch, but that isn't practical in this case. I thought of putting in a zener diode that would be sized so that it prevents the relay from engaging when the voltage is below 12.8V. As I've had little electical experience & little knowledge, I'm not sure if this would work or how exactly to do it.

Unfortunately the truck needs to be delivered to the customer immediatly & there's another to be delivered in several days... I'm looking for something that I can do/buy quickly & relatively cheaply. Any help would be greatly appreciated. Thanks.

 

[WiringGuy]

You could use a latching relay that energizes when the key is turned to the start position and de-energizes when the ignition is turned off. The only way that the fan would drain the battery would be if the driver bumped the starter, it didn't start and then he left the key on. Even if this did happen the other electronics in the truck would drain the battery anyway.

 

[rmw]

I don't know any particulars about your electrical system, but I can tell you of a system that I am familiar with, and maybe it will give you some ideas.

I take care of an Eagle Bus (Trailways, and Greyhound type in the USA, formerly made by Kassboher in Belgium) which takes a point off of one of the fields of the alternator. (I think, I am a mechanical engineer, so forgive electrical misstatements) to which a 6 volt relay is connected. It is a 12 volt system. This 6 volt relay picks up when the alternator begins to produce voltage, and acts as a latching relay to enable the 12 volt circuit relays that supply power to the big power users, such as the air conditioning equipment fans, (big current draws.) This does two things, (1) it protects the entire electrical system from rapid discharge upon alternator failure, and (2) it removes all this electrical load from the starting system as it brings the engine to cranking speed.

The more modern alternators do this in the voltage regulator, as it does not begin to produce until after starting, and the engine is revved up to some speed higher than idle speed.

I hope this gives you a place to look.

rmw

 

[tonywiddows]

Cat5, if you can't get at the ign switch itself what about getting at the cigarette lighter or radio that requires either IGN or ACC to be on? If your fan is small just connct via additional fuse located close to where you tap in. If the fan is large use a separate relay... Tony