Dual fan hack

hey folks, I believe there is a mod to link both high and low speed coolant fans together so they come on in sync. Does anyone have a link or pics to share on this? I think it's a jumper between relays. i searched and can't seem to find it

I think you are confused, the low fan speed is achieved by dropping the voltage to the fan across a high power resistor (They are prone to failing, another 996 gotcha). So when the fan is in high speed mode the nominal 12V drives the fan directly, and when it's in low speed mode it's feed in series with this resistor.

From an older archive... Originally Posted by 1999Porsche911 View Post If you want your engine to remain cooler (190F rather than 220), simply do my fan switch mod. No need for heaveier 3rd radiator. Radiator Fan and Engine Compartment Fan Switch Mod The 3.4 996 runs extremely hot when driving around town in temperatures higher than 75 degrees. This mod is a way to reduce coolant temps back to a safer level, helping to reduce oil temps and improving performance. The mod consists of installing a switch that will allow you to turn on the high speed radiator fans at any time, while maintaining the automatic operation of the fans. We do the same for the engine compartment fan. Radiator Fans Connect a wire to the ground of the 2 high speed fan relays located on the relay carrier located in the driver�s side foot well. The relays are #20 and #22. Feed this wire to where your switch will be located. Connect the wire to one side of the switch. Connect another wire from the other contact on the switch and then connect the other end of this wire to a chassis ground point. With key on, this switch will now immediately turn on both radiator fans providing maximum cooling. When you turn the key off, the fans will continue to run for about 5 seconds until the relay shuts down. Your fan operation remains normal when the switch is off. Engine Compartment Fan Connect a wire to the ground of the relay located in the relay carrier behind the passenger seat, driver�s side. Relay #8. . Feed this wire to where your switch will be located. I simply fed the wire along the driver�s side of the car and under the door sill. Connect the wire to one side of the switch. Connect another wire from the other contact on the switch and connect the other end of this wire to a chassis ground point. With key on, this switch will now immediately turn on the fan drawing air into the engine compartment. When you turn the key off, the fan will continue to run for about 5 seconds until the relay shuts down. Your fan operation remains normal when the switch is off. This mod helps keep the engine compartment much cooler when driving around town and is particularly beneficial to any engine that draws its intake air from inside the engine compartment, such as the EVO SC. If you are adventurous, you can automate the operation of these switches so they will switch off at a specific speed, or operate when vacuum is present only, etc. You can choose any style switch you want and even use Porsche switches. I simply used 2 toggle switches that I installed in the batwing, Jim

Thanks everyone, I also found out about the cracking of the low fan resistor, I will have to check mine out and replace if broken

I think it is a pretty nice safety feature to have that switch available to turn on the fans to highspeed if you think it is required. I have been sitting in the car wondering when finally those fans kick in; not so nice. Having said that, first and foremost check if the cooling system is working as designed; I had one of the fans not kicking in at low speed, and that explained why I saw the cooling temp rise to 225F before anything happened.... In my case it was not the resistor but a relay issue.

Quotes source post: 14326441

FYI it's pretty easy to check the low and high speed functioning of both fans by just jumpering each of the relays (19-22) one at a time. Turn the ignition key on, and then attach a jumper wire between terminals 30 (+12V) and 87 (fan) on each one (one at a time!) You should hear the fan run as long as the jumper is connected. 19 left low speed 20 left high speed 21 right low speed 22 right high speed If a low speed doesn't work, but the high speed does, it's probably the resistor, or wiring. If neither speed works, it could be the fan, fuse, or wiring. If all this works, but then it doesn't work in practice when you turn on the AC or the motor gets hot, then the associated relay is bad. I performed this test the other day and found my right low speed not working, but everything else is good, so I'm reasonably confident I just need to replace that resistor.

Quotes source post: 14325946

would the same work for a 996.2?

So, new idea on this general topic. First, I have already ordered two center console switches so I can manually turn on the front cooling fans and also the engine bay fan. I am also thinking that replacing the ballast resistors in the front cooling fan circuit with resistors that have 25% less ohm rating (and appropriate watt rating), will increase the low speed fan speed by 25%. This would make the low speed fans run 25% faster and in my mind, this may reduce temperature spikes in slow or stop and go traffic. Maybe even would test with a 30% less ohm rating... Should be easy to try and I am thinking a low speed that is faster than stock but still a lot less than high setting would not be too noisy... Thoughts?

You're on the right track regarding your warm 996. I don't know about messing with the resistors, though. It sounds like a good idea, but personally, it sounds like a lot of work to me. I also put my high-speed fans on a switch to help prevent coolant temps from rising too much. You're right about them being noisy, but I have gotten used to them. If you don't have a low- temperature thermostat, I would recommend you install one: With the high-speed fans running, I've found the coolant temps refuse to dip below 93 degrees Celsius (with the original thermostat), because the thermostat would begin to close. The low-temp thermostat allowed the high-speed fans to cool the coolant down to 80 deg C.

Quotes source post: 20558441

I like this idea/line of reasoning, but the fan flow rate is power law function, so may have less effect than expected. Worth looking into though. It�s sort of surprising there isn�t a more modern two speed fan solution to this problem actually. The resistor isn�t very efficient burning up amps either.

Yeah I agree. Also, just thinking that getting a newer design resistor in there with some extra watt capacity may reduce resistor heat / make resistor last longer. Let me try this manual switch install and see how that works. The engine runs about 180 all the time when moving, unless I am standing on it at the track in lower gears, or sitting in traffic for a long time, so it's not a huge issue. The max I have ever seen is just under 215 hash mark on the gauge, which is within reason for this car, from what I read. ... Just something to make the car less of a mechanical worry.

Quotes source post: 20558441

996hobby - Did you do this hack? If so how did it work out? Did you see temps drop? And if it went well can you link the parts you used (resistors/switches,etc) and last question what gauge wire did you use? TIA.

Realized my low speed resistors arent working. Ordered a couple new ones, but in the process found this thread. Installed this while waiting for the new resistors and see good results so far. More or less, it helps you combat heat soak earlier and on demand. It doesnt alter the cooling capacity but when combined with a low temp thermostat enables running in the 80-90C range far more often than a standard car. Grounded here Switch mounted by the OBD receiver. Need to refine it and probably seal the one connector, but works for now.

I figured I�d chime in with my own experience and maybe a question for others. I ordered this switch off of amazon which also has an LED on a separate circuit. I jumped the pin 85�s on relays 20 and 22 using the switch throw to chassis ground, and also tied the LED ground to the relay pins so that it turns on whenever those pins are grounded. That doesn�t necessarily translate to only illuminating if the switch is on because the DME will do what it wants and ground at will, but it�s good enough. This was a fun little experiment. I�ve had a lot of �round town driving, stop and go, highway, and 1 track day that consisted of 20 min sessions each. The rest of my cooling system is in pretty good working order with no leaks, new low temp thermostat and water pump. I still need to clean out my radiators. I don�t have a solid temp read-out or metrics (yet) so I am guessing by the needle. I know that gauge is buffered as hell and probably delayed, but it�s what I have for now. 1. Town driving: stable temps with needle in the middle, great. 2. Highway driving, leads to the left a little bit, great (I assume this is roughly 185�) 3. Stop n go: the needle will tend to wander off from middle of �8� to �0� and back depending on how idle I am. Once you hit speeds of 40mph it drops quick within a minute or so! 4. Track day: toward the end of each 20 min session in the latter part of the day (when ambient temps hit 80�F+, I�d get the needle to the right edge of the �0� on the gauge. This might just be the cooling capacity limit without the 3rd rad, or my rads are just dirty. My guess is 220� Overall I am pretty happy with where I am with street driving but I�d hope I can drop track temps down a bit with radiator clean out, or 3rd radiator addition. I am wondering - is it expected that my temps would be that high on track with the fans running constantly or should I be doing better than that?

Sounds like your car's cooling system is healthy. You can get temps through the A/C lcd screen if you'd like to get precise readings: https://rennlist.com/forums/996-foru...ater-temp.html Like the fan button/location.