Cooling fan switch.

Hi Gentlemen, I've read a post on another forum about fitting a switch to turn the radiator fans on. Apparently if you connect a switch to the earth on the fan relay and then run from the other connection on the switch to an earth on the car body this will allow you to turn the fans on and off. Does this sound feasible? I'm not electrically clued up so I'm scared of frying something. Any thoughts?

I am clued up electrically but not familiar with the wiring of the 996 per se. Lets assume that the 996 is wired up in the conventional way then that said in theory you can fit a switched earth to the relay. Let us pray that you do not intend to remove the connection to the temperature control switch, and rely solely on the switch to operate the fans (please tell me you aren't); you can turn the fans on, but luckily for you you wont be able to turn them off if they are on (conventional wiring notwithstanding) without a more complicated circuit than the one you describe, because the relay earths through the body of the switch with no connecting wires as far as I know, but the 996 could be different to every other water cooled Porsche I have worked on, and I welcome the opportuniy to learn from someone if I am incorrect. Imagine if you could turn them off and in a traffic-jam you forget to put the switch back to "normal operation", which brings me to this question, why?

Why do you want to do that? If you want to lower the coolant temperature, wouldn't it be better to fit a lower temperature thermostat? I can't really see the advantage of running the fans permanently, especially as it will take longer for the engine to warm up, and the fans will kick in automatically when the temperature gets high. Maybe I might think differently if I lived in Arizona though.....

I wasn't looking to run the fans permanently, just to stop the temperature rising in traffic really. I take your point about the low temp thermostat but this switch seemed a relatively cheap mod. According to what I've read this should be able to turn the fans on and off as and when you require but I am very wary of forum experts so I appreciate your replies.

As John says, if you connect a ground to Terminal 85 on one of the two high speed fan relays, both fans should run at high speed (T85 is linked between both relays). Look for a yellow wire with grey tracer. Should be easy enough to do. You'll just have to remember to switch it off when you don't want it, and it will revert to normal operation.

A bit of science now. In a pressurised cooling system the water will not boil at 100deg C. Secondly taking my 996 as an example of cooling capacity, even in traffic the temperature gauge does not get as high as 85, so if yours is going into the 90s I would at the very least get the coolant changed if it hasn't been done recently. Coolant mix contains corrosion inhibitors and I think I am correct in saying that it has other additives that help to break the surface tension of the water substrate, making it a more effective heat conductor. These chemicals break down over time and become less effective. Let's be honest; with the millions of Deutchmarks invested in 996 development, you would have though that the Porsche engineers in their wisdom would have specified bigger radiators if the cooling was marginal. I smell snake oil.

Thanks for your advice Richard and John. In reply to John the coolant has been recently replaced so that's ok and the temperature the engine reaches isn't extreme but the car has now done 105,000 miles and still runs like a good un, I just try and keep an eye on everything to keep it that way, having said that I'm not paranoid waiting for something bad to happen. On a lighter note John you say the Porsche engineers made it this way, could they be the same engineers that designed the RMS and IMS? Again thanks for the replies, I will let you know if I try it.

Probaly a different set of white coated individuals for engine internals "jack of all trades master of none." So maybe I have defeated my own argument and the coolant specialist did have a got at the IMS and RMS. Incidentally I read than there was much myth and panic about rms leaks when infact it was a build fault, and important thread sealer was left off the stud behind the flywheel which seeped oil that ran down the crankcase and made it look like the seal was leaking and why it proved so difficult to effect a repair. Did you hear that one Richard?

I think the big problem is as baz said in his article "the temp gauge is so inaccurate and when it reads 185' its more like 200'. " On that note has anyone seen these, seem very reasonable. Wahler low temp

Well if that is in deg F thats still 12 Fahrenhiet degrees below the boiling point of water at atmospheric but water at 15 psi won't boil unitl it reaches about 248 F or 120 c so the system is well away from getting a vapour lock, and a well maintained system should hold 15 psi. The system runs at about 7 psi under normal operating conditions all the time.

Much cheaper than the MotoRad one I bought from Hartech. That was £85-ish without the housing. Not that I see any reason to change the housing though.

No agreed, just makes changing it a bit easier. I think the OEM are wahler too. And at that price who wouldn't.