ClassJ

The fans themselves seem to be integrated with the control electronics. They get control commands direct from the DME.

It seems per the SIT that if the fans lose communication with the DME that they run full blast to keep the car cool. My guess right now is that the fans could either be not getting correct signals. TBD.

https://www.dropbox.com/s/4jpmkhmjlupf6w9/991%20SIT.pdf

Electric fans for coolant radiators The coolant radiators with electric fans (1, 3) are installed on both sides in the front end of the vehicle. The variants intended for extremely hot countries are equipped with a third, center additional radiator (2) (Not for USA) without an electric fan.The functions of the electric fans are to• respond to cooling requests from the DME control (reduce engine coolant temperature, reduce air conditioner refrigerant temperature),• provide maximum cooling (emergency operation,n = 2,400 rpm) if there is no DME information available,• switch themselves off if there is a risk of damage from overheating.The fan motors are phase-controlled synchronous motors that are continuously actuated via separate control units (drivers). The fan motors are most efficient at n = 1,800 rpm. The drivers required for operation of the fan motors receive their commands from the DME control unit by means of PWM signals and form one unit with the fan motor.The 12V power supply for the fan motor is provided via a common line that also supplies the control unit mounted on it with current.

CSK 911 C4S

A few months ago my inside AC fan was on high even when the car was off and locked. Had to disconnect the battery until I got it fixed. Bad AC fan module.

I wonder this. If the radiator fans are on while the car is off I wonder if there is another AC module that runs the radiator fans that could be bad.

LSR

Wow. This is one great forum! Thank you for all this new information. Reading the SIT manual, it says the fans will operate full-blast in "emergency mode" if the CAN bus is faulty or if there is a loss of signal from the DME control unit. I never knew this. I now need to noodle about whether this loss of signal/faulty CAN bus failure mode could plausibly cause the fans to misbehave as they are doing. The fans incorrectly turn on after the car's been shut off only after the car has been driven lightly such that the engine is warm but not at full operating temperature, and do not turn on (ie, they function normally) if the car has been driven longer and is at full operating temperature. I need to think about how a loss of signal from the DME or a faulty CAN bus can cause this apparently temperature-dependent after the engine's off behavior. You'd think the problem would occur with the engine on as well, and irrespective of temperature. Gotta think about it.

BTW - where on earth did you find this manual? I wouldn't be surprised if there was a technical service bulletin (or whatever they're called) somewhere talking about this issue or something similar.

flgfish

When my battery was getting old, the DME was throwing quite a few CANBUS failures. These did show up as P codes however.

CanMikePlay

The voltage regulator might be built into the alternator.

pfan

The SIT also describes the functions of the radiator outlet coolant temp sensor, although if bad it most likely would have thrown a code.

Radiator outlet coolant temperature sensor
The radiator outlet coolant temperature sensor is located on the coolant pipe at the
radiator outlet towards the engine on the vehicle underbody. The temperature
information is transmitted to the DME control unit. The DME control unit uses the
plausible signal to control the thermal management system.
The sensor signal is used for:

• Actuating the coolant radiator fan

– as a function of the coolant temperature

– as a function of the setpoint engine coolant temperature

• Testing the function of the map-controlled thermostat/engine coolant

shut-off valve

• Diagnosis

ClassJ

No problem. The SIT has been released for every new Porsche model going back many years. It is not the full workshop manual but has a lot of good technical tidbits.

I still think this could be alternator related. I would check for AC ripple at the battery.

VII7

From your symptoms your fans are going into fail safe mode (full on) when the signal from the DME is gone but for some reason your fans are still powered. Don't have access to wiring diagram at the moment but if I had to guess sticky relay or faulty module.

Jack F

How much are fan relays?

LSR

Turns out I misspoke about the voltage. It wasn't low at 12.2v but rather 12.4 volts and later 12.495v. But both readings, the shop foreman told me, were unacceptably low and indicative of a charging system problem.

LSR

Not much money, but unfortunately the problem seems not to be with the fan relays. There are two fan relays, and if one was bad it shouldn't affect the other relay.

LSR

Yes - it's possible the fans are going into fail save mode due to loss of signal from the DME or because of a faulty CAN Bus. But the shop tech told me either scenario should have thrown a fault and the car shows no faults. That's what's making this so difficult to diagnose. Not only are there no faults, but the problem is intermittent - which makes it so difficult to diagnose.

The best clue so far as to what's going on seems to be the low voltage reading the tech saw last week after he drove the car, but all voltages have been fine since then. He said it's possible a too-low voltage after shutdown is causing the internal electronics of the fans themselves to falsely trigger emergency mode operation. Or, it's possible a too-low voltage could be causing the DME itself to issue incorrect commands to turn on the fans, or to stop signalling the fans causing the fans to go into emergency mode.

BTW, I was told the reason why the fans are still powered after shutoff is because when you shutdown the engine some things remain powered for a while before going to sleep. And one thing that still draws power and remains on is the DME, as it has things to do even after the car's been shut off. For instance, run-on of the engine compartment purge fans after the engine is turned off is completely normal if the engine is hot enough. You've probably noticed this yourself after driving the car on a hot day. The engine compartment purge fans are controlled post-shutoff by the same DME that controls the front radiator fans (different control signals, though).

ClassJ

Which is true. The battery would not be charging at those voltage levels and is approx 50% charged at best. Your charging system needs to be at 13.5V or better to even to charge the battery.

I think the charging issue on the car needs to be addressed first regardless of the fan problem. Whether they are related or not does not matter.

How old is the battery? Has it been load tested? What is the resting voltage after car shuts off? A bad cell or sulfated battery can also cause low charging voltage.

VII7

Nothing like your modern no fault no solution techs, the DME controls the fans just fine and will not report an error as it's going to sleep just fine. The problem is once the DME is asleep the PWM signal to the fans disappears, this should not be a problem as they should have no power at this point but since they do they start at full blast.
​​​​​​Module or relay.
​​​​​​

LSR

Thanks for your post. I was thinking today in line with your analysis, though I have a little more work to do to nail it. What you suggest makes a lot of sense. The problem could be the DME module itself (ouch - pricey!) or, possibly, something called DME 1 Relay. I'm becoming more and more of the view it's not the alternator, as I have been painstakingly documenting the condition of the car's charging system relative to how the car was driven (in normal mode or sport mode) and the engine's temperature when I shut it down, and documenting whether the fans do or do not turn on. And, I've measured the actual battery voltage across it's terminals with a multi-meter after the engine's shut down and that voltage is spot-on at 12.7v (it should be - the battery's brand new).

So I'm with you in now focusing on the DME 1 relay hoping that's the problem (and not the DME itself). There are several relays involved with the fans, but only the DME 1 relay seems able to influence both fans simultaneously. The DME 1 relay is not a fan relay per se, but rather a relay that energizes the two fan relays (among other things) whenever DME 1 Relay is energized. If DME 1 Relay is not energized, the fan relays can't be energized and so neither can the fans. But if DME 1 Relay remains energized when the engine is shut down, it seems to be possible for the fans to go into "emergency mode" and turn on hard if the DME turns off (or stops sending its control signals to the fans).

So, my current plan is to buy a new DME 1 relay and see if that solves the problem. Starting with the DME 1 relay 's the least expensive way to start playing "replace parts until the problem goes away."