Mike1999

Question about 1999 Porsche 911 carrera 996. I am new to Porsche ownership and mine has a switch under the dash that turns on the front fans. I can't find any information about the operation of the fans or why someone would put in a switch. Should I be driving with the fan always on?

cdk4219

The fans have 2 speeds low and high, with a separate relay for each speed and each side fan, four relays total. The fan motor has a resistor close to each fan that reduces the voltage to the fan, effectively slowing the fan motor, that’s low speed, High speed is direct 12v through the.high speed relay to each fan.

The relays are triggered by the sending a ground from the ecu at either a certain temperature or when the A/C is running. Sounds like someone has put a switch to ground to the low or high relay to bypass the ecu, hopefully the ecu is still turning the fans on as well.

It could be that the low speed resistor may have failed in one or both fan motors,which is common, causing the initial cooling not to run, maybe someone then jumped the high speed relay to manually control the fan. The 996 temp gauge is not accurate and always seems to stay at 180 even though the actual temperature is much more. Since I have 2 ls1 996s I can verify this with the scan tool hooked up to the ls1 ecu while the 996 sender is piggybacked on the the ls1.
Regardless this should be looked into and make sure this switch isn’t a bandaid fix, to a fan or other problem. If it just adds manual control of working fans you can turn it on around town on hot days in traffic.

Mike1999

Thank you greatly for this in depth explanation. I have only had the car for about 2 months and the previous owner had no knowledge of the history. I am a garage mechanic at best and only know enough to get in trouble. I will get one of those live ECU readers to keep an eye on the temperature. You are correct, the temp gouge has always been pretty solid and I wasn't too worries. However, sometime I would come back from a long ride and it seemed to me that the engine bay was way too hot. I don't think its overheating. I will also do some research about the whole fan setup now that I understand that mine is not normal setup. After a quick search about the relays, I can see that I have a problem of some kind because my heat is not blowing hot in the cabin.

De Jeeper

Cabin heat us an unrelated issue. The foam has come off one or more of your hvac doors inside the heater box. Many threads in the repair as it can be done without pulling the box. Do a rennlist google search as its better then finding threads then the seach feature here.

Engjne bays also get hot in these cars but u may want to double check that that the trunk fan lid works and the temp sensor, top of passenger side intake plenum, is in the rubber gromett.

A good shop scanner and turn the fans on both speeds.to check operation. The resistors can be relaced for cheap with amazon partz. Again do a search.

pesuazo

Probably the resistors died and the PO just hardwired them....
I replaced mine with these:
There are plenty of "How to do it" posts....Google search or Youtube will come up with the DIY

Here is one of the threads: https://rennlist.com/forums/996-foru...-with-a-c.html