The $3.50 modification to have the ECM work both fans
(click on thumbnails for larger images)
I live in Northern California, and during the summer months the temps can sometimes reach 110F. On days over 95F, I noticed that if I was in stop and go traffic with the A/C off the temp would sometimes reach 190-200F. On the hottest days the temps would reach 195-205F!
I expected hotter coolant temps due to the ambient temperatures, but the odd thing that I noticed was that if I turned the A/C on the coolant temp would drop almost 15 degrees. After some observations of the fans and after a look at the wiring diagram in the shop manual I realized why. The ECM operates one fan only (the driver's side fan). Thus when the A/C is off only 1 fan (the driver's side) runs, and the single fan can't seem to keep up with engine temps at low vehicle speeds. The problem is compounded by the fact that the only fan controlled by the ECM is the low speed fan. If you turn the A/C on, the A/C pressure switch turns on the passenger side (high speed) fan to compliment the ECM controlled low-speed fan; this is why I noticed a reduction in coolant temp with the A/C switched on, because both fans were now running. The system is a little more complicated than this however. This is a basic run down of how the fans operate. (If I made a mistake, don't hesitate to email me.)
Driver's Side Fan (Low Speed Fan)
Passenger Side Fan (High Speed Fan)
The problem is that the ECM cannot trigger the high speed fan. The high speed fan only comes on if A/C is on and pressure is above 150psi or if the coolant temperature switch reaches 208F. While this may be OK if the car is completely OEM, this is a little hot for performance applications and the 160F thermostats which usually accompany them. 208F is a little hot in my opinion (although better for emissions) and the car is certainly down a good deal of power when coolant temps reach and exceed 190F.
I decided a little modification was in order so that the ECM could turn on both fans at the chip's set point rather than just the driver's side (low speed) fan. I did a little studying of the wiring schematic for the cooling fans in my Helm shop manual. There are two fan relays, one for the high speed fan and one for the low speed fan. The one side of each relay coil is fed +12v, and the relays are switched on by pulling the other side of the coil to ground. The relays can pulled to ground by the ECM, A/C switch, Delay Relay, and Coolant Temperature Switch depending on the conditions. By connecting the two fan relays together at terminal F (the ground side of the relay coil) this would cause both fans to turn on no matter what sensor/switch did the switching because both relays now share a common ground path. This way, the fans always run as a pair rather than individually. To accomplish this, I ran jumper wires from terminal F on each relay to a switch. This way, during hot weather I can turn the switch on, meaning both fans switch on simultaneously; during cold weather I can turn the switch off so that the fans run as per the original GM conditions outlined above.
Interested how I did it? Here are pictures and the procedure. I'm not an electrical engineer (I am a mechanical engineer however) so attempt at your own risk!
The high speed (passenger side) fan relay is the one on the fender just next to the coolant overflow tank. The low speed (driver's side) fan relay is located right next to the delay relay near the battery and radiator. Both fan relays have round corners and a gray weatherpak connector. The delay relay (more square than the fan relays) is located right next to the low speed fan relay. What we want to do is run a switch between the ground wires coming from terminal F on each relay.
Begin by removing your battery so you have room to work. Remove the split-loom from the wiring harness running from the high speed relay to the low speed and delay relay. You will be greeted by a small bundle of wires. You will see two splices near the high speed relay covered in tape-- these are the ground connections for the fan relays. My car came from the factory with these splices covered in black duct tape! Remove the tape from both splices, revealing the wires coming from the various sensors/switches. The splice is to reduce the multiple wires coming from the individual switches/sensors to one wire which goes to each relay. The pre-existing splices indicated by the red arrows are the ones we want to run a switch between.
The existing factory splices in the relay ground wires make things really easy, because you don't have to remove any insulation or cut into any wires. Thus, this addition will be very easy to remove later without leaving a hacked up wiring harness! Simply solder a jumper wire onto each splice. The red arrows indicate the splices which get soldered to and the new wires to go to the switch.
Insulate the splices by wrapping them tightly with electrical tape as the factory did. I use heavy duty 3M Tartan Vinyl hi-temp tape. Much better than the cheap stuff you usually find at home improvement and automotive stores. It's very thick, extremely durable, and doesn't unwind itself in a few weeks. Run the new wires back along the wire bundle to the low speed and delay relays. Cut the new wires to length (leave yourself a little extra) and crimp/solder (I do both, I like overkill) some spade connectors to the wires. Connect the wires to a switch of your choosing. You could run the wires into the car and mount the switch there if you wish, but I decided to leave mine under the hood. For a switch, I used a Radio Shack automotive rocker switch (part number 275-730, about $3.50) with mounting plate. It's all black and discreet looking. I mounted it using the screw which holds the delay relay to the body. Again, no new holes in the car. Place the split loom back over the wire bundle and replace your battery.
That's it! Now, for those hot days when the single driver's side fan can't keep the coolant temp reasonable, turn the switch on. The ECM will then turn on both fans rather than just the driver's side low speed fan. If you turn the A/C on, it will also turn both fans on. Both fans running means more airflow, and more airflow means more cooling especially at lower speeds. On colder days, turn the switch off and the fans will return to original operation.
How well does it work? This evening it was 90F outside. I took the car for a drive down the country roads (average coolant temp 164-165F, moving at 45-75mph the whole trip). I came home, opened the hood and let it idle in the driveway for a few minutes to cool the turbo before I shut it off. Idling in "GM Mode", the driver's fan remained on constantly but the coolant temp climbed to 180 and remained steady there. The single low-speed fan couldn't muster enough airflow to cool the engine any more than this. I threw the new switch, the high speed fan turned on as well, and within 3 minutes the coolant temp dropped to 165F and the ECM turned both fans off. Mission Accomplished!