The C5 Fan Trick
While working to improve your driving at the drag strip, you may have an opportunity to make several consecutive passes without stopping to cool down the engine. This routine is called "hot-lapping," appropriately so, because it causes the coolant temperature to soar above 200F degrees, often above 220F.
On a C5 motor with a stock engine management computer (ECU/PCM), elevated coolant temps cause the PCM to retard timing, which reduces horsepower. None of us want that.
By using this "Fan Trick" both fans turn on and remain on low-speed, keeping the coolant temperatures close to an ideal 194F while hot-lapping. As always, it's a good idea to keep an eye on the coolant temperature shown in the Driver Information Center (DIC).
This Trick is free, requires nothing more than one piece of wire and a white marker. And results in no permanent change to the car.
Here is the procedure for the Fan Trick.
You'll need one piece of wire, ideally 14-16 gauge, multi-strand copper. Cut a piece about six (6) inches long and strip the insulation from the last half-inch of each end. When not in use, I kept that piece of wire in the glove box inside my racing glove.
Pop the hood and locate the fuse relay center on the passenger side just forward of the battery.
Remove the cover and confirm the internal configuration is the same as the one pictured; if the car is a stock 1997-2004 C5, it should be. [Note: Smoke, once emitted, is hard to get back into electrical components. So check this out yourself; I provide no warranty and bear no responsibility for consequential damage arising from following this procedure. But it works for me and others without incident.] Now, identify the lower right-hand relay. Note the orientation of lettering on its face is the same as the others. Maintain that orientation when you're finished.
Remove Relay #45 by wiggling it and then pulling it straight up. Drop a dot to mark the terminals shown. Drag strip window marker or office "white-out" work equally well for this purpose. Once marked, you won't forget the correct terminals.
Now insert the ends of the prepared wire into the two marked sockets. You should hear both fans turn on.
Replace Relay #45, being careful to keep the wire-ends in place. Once the Relay is firmly in position, you should still hear both fans.
Once you verify the fans are still running, reinstall the cover. You are finished.
The fans will continue to run at low-speed until you remove the wire. Using this procedure, I have run the fans of my 2002 Z06 for six-hours without draining the battery.