Radiator fans are DC motors. Apply electricity and it rotates (runs). But when you turn off the electricity to the fan, it doesn't immediately stop rotating. At that instant, it is no longer a motor - it becomes a generator, creating electricity and back streaming it through the wiring. If the fan is wired directly to the ignition side of the fuse panel - very common if you want the fan to run whenever the car is "on" - that electricity will power the ignition system and the car will keep running until the fan slows sufficiently to stop generating current.
The fix for this situation can be done in one of two ways.
First, a relay added in the feed wire to the fan will assure that the feed wire disconnects as soon as the ignition is shut off. Simply use the ignition as the trigger for the relay with power tapped directly from the battery or another battery source. This does require a little bit of rewiring and some new connections. Check out our relay #L07-40 at http://www.watsons-streetworks.com/relays.html.
A second approach that needs less wiring is to add a diode to the fan power wire. But where do you find a diode of that size?!?!? Watson's StreetWorks, of course! We have sourced an 85 amp diode specifically for radiator fan applications. It even comes with butt connectors to do the installation. See part #L85A-DIODE at http://www.watsons-streetworks.com/wiring_accessories.html.
There are a number of problems that pop up with vehicle electrical systems that seem straight forward but aren't. The obvious cause is not always the correct cause. If you have run into one of these challenges, let us know so that we can cover it here in our blog. We'll do the same by adding ones that have come up.
What? You have the classic "hot start" problem but the usual fixes haven't worked? It may not be a hot start problem at all... ah, a future blog post!
Watson's StreetWorks http://www.watsons-streetworks.com
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