I am now pretty much convinced that I have been chasing a problem that in reality does not exist, at least not on my car. I spoke with the Service Manager at Ruf in Dallas (Hans Hoffmann). To the best of his knowledge, as well as his experience as a previous owner of a 996 Turbo, the engine-compartment fan will run when the engine is running, and not just when the engine compartment temperature gets high. I believe the engine compartment temp sensor turns the fan on “high” when the engine compartment temp gets to 180 deg F. The fan will continue to run on high, even with the engine turned off and key removed, until the temp in the engine compartment falls below 180.
Here’s where I believe some of the confusion originates. The engine compartment fan relay and wiring on a non-turbo 996 is not the same as the relay for the turbo version, at least not between 2000 and 2003 model years. I was originally trying to trouble shoot my “problem” with a wiring diagram from a 2000 996. The relay for the 2000 996 is a simple “on” or “off” relay. In other words, based on the design of the relay the fan will either be running full bore, or be off. There is no intermediate or "low" state. The relay, and hence the fan, is switched on by the DME when the engine compartment temp sensor gets hot enough.
On my 2003 Turbo, the engine compartment fan relay has an “on” or “on” state that is also controlled by the DME. However, in the “normal on” state a lower voltage (7 VDC) is supplied to the engine compartment relay and hence the fan causing it to run at a “low” speed whenever the fuel pump is running (anytime the engine is running or when the ignition is first switched to on). In fact, the power source wired to the “normal on” pin on the relay is taken directly from the fuel pump and is bussed to the relay through fuse C5. The 2000 996 is not wired this way.
To cause even more confusion, this fuse location (C5) on the wiring diagram is labeled as “optional” and is shown as a 5-amp fuse. Why it would be labeled as “optional” I have no idea. It would however tend to make me think that certain cars came with the fuse installed, and perhaps others did not. Also, both of the fuse block diagrams I have found for my car show fuse location C5 as being “empty” and not even labeled. If I remove fuse C5 then my fan only runs on “high” when the engine compartment gets hot. If I put the fuse back in, the fan runs at a low speed at all other times whenever the engine is running. If you have never observed your engine-compartment fan running at anything other than full speed, my guess is that your car has no fuse installed in location C5.
The last bit of information that makes me think my fan is acting normally is taken directly from page 61 of my owner’s manual;
Warning!
Risk of Injury.
The engine-compartment blower always runs when the ignition is switched on. After the ignition is switched off, the engine compartment and coolant temperatures are monitored for approx. 20 minutes. During this period, and depending on temperature, the engine-compartment blower and radiator fans may continue to run or start to run.
Mystery solved . . . ?
And Loren, thanks again for all your help!