Yes, from what I can see "Relay Support 2 (ROW B) (Behind left rear seat well) [Hot w/ DME 1 Relay energized]" Terminal 2 (Fuse1 10A) provides power to the fan control units (inside the fans) and to "Fan Relay Support" 1&2 which are used to provide the power to actually run the fans. So if you have a sticky DME 1 Realy the fans would remain powered even after the DME is off.

 

Absolutely, I agree. The only caveat here is, what else does the DME Relay energize? I‘m going to trace that through the wiring diagrams. If it provides power to the DME itself, that might suggest the problem is not with the DME relay. If the DME remains energized along with the fans, presumably the fans would not go into emergency mode because the DME was not asleep. Also, it’s entirely possible the DME relay is supposed to stay energized after engine shutdown so that the DME itself remains powered at least to monitor and control the engine compartment purge fans. I wish I knew the answer to the question of whether the DME relay is supposed to turn off when you shut down the engine or is it normal operation for it to remain energized for a period of time after shutdown? That, to me, is a key question. But I’ve seen no documentation discussing this. One way perhaps to get to the answer is to pull one or both of the fan relays and probe the relay socket contact for the positive side of the relay coil, which would be the wire coming from the DME relay. If that’s consistently hot for a while after the engine’s been shut down, that might suggest the DME relay (and, hence, the fan relays) are supposed to stay energized after shutdown for some time period.

 

It is amazing, and disappointing, that PCNA has not furnished tech support to resolve this problem. Makes long-term Porsche ownership questionable at best. If owners are expected to trouble-shoot post-warranty electronics problems themselves with no PCNA assistance, I am not interested.

 

Changing out the fan relay fixed mine. The dealer couldn’t find anything troubleshooting it, but I complained until they changed it out. This is on my ‘16 Cayenne, but it’s very much the same electrical/computer system.

 

The DME relay is actuated by the DME (terminal 30 unfused) to provide power to a variety of consumers, so yes if this is your issue the other consumers on relay support 2 should be awake when the fans are running full blast.

 

This is encouraging to hear. Thanks. A faulty relay in my car might not cause a fault to manifest. Another reason to suspect a relay.

 

Yes. That's right. Many consumers. I'm going to trace the circuit to identify as many of those as possible. If the devices energized by the DME relay do not need to be on after the engine has been turned off, you'd think the DME relay itself should drop out when the engine turns off. And if I can find in one of the footwell fuse boxes a pin connected to the N.O contact of the DME relay (pin 2), I will be able to tell what the DME relay is actually doing when the engine shuts down. That's a job for tomorrow.

Thank you, VII7, for your insights. Can you imagine a dealer's shop doing any of this? I tried to engage PCNA's help, asking their tech support folks to contact my dealer to assist in diagnosing the problem because the dealer admitted to be "guessing" to an 80% confidence level it's the alternator. But PCNA declined, saying in their email to me: "When a question of vehicle repair exists, we do rely on our authorized Porsche dealerships as our technical resource. With factory trained technicians and technical support from the factory, the Service Department would be in the best position to properly diagnose your vehicle and advise on recommended repairs. We do stand behind our dealers, and would be unable to provide a different resolution than the recommended resolution ..." (emphasis mine). In fewer words, in response to my request for assistance the answer was "no."

 

I thought I would post what the problem was that caused my 991.1's radiator fans to turn on furiously when the engine was off. The cause seems to have been a sticky DME relay. The DME relay is not a fan relay per se, but the car's circuit diagrams show it directly actuates a pair of fan relays that, in turn, send power to the fans. When I pulled the DME relay from the car and put it on the bench to test its operation, it appeared to work fine. However, I could hear a faint rattle when I shook it. Relays aren't supposed to rattle! So, employing a form of "kick the thing and see if that fixes it," I shook the heck out of the relay and then slammed it a number of times against the workbench. After a few rounds of these abuses, the rattle disappeared. So I reinstalled the relay in the car and since then, the car's been perfect. If the fan problem recurs (it might, because whatever caused the relay to stick in the first place may do so again), I'll invest $37 and buy a new one.

Thanks again to all of you who pitched in with ideas and suggestions about possible causes and solutions.

 

Now take a bill to your delaership for all your wasted time. Make the bill at least half what they wanted to charge you for a **** poor diagnosis and alternator installation.

 

It's tempting. What I am going to do, though, is go back to the case I opened with 1-800 PORSCHE, explain what the real problem was, and complain mightily about how they refused to involve a zone tech to assist the dealer. I used to believe Porsche was a customer-oriented company. But this experience with my 911, especially coupled with several truly miserable experiences I've had trying to resolve brakes and transmission problems with my 2019 Cayenne, has changed my view 180-degrees. Porsche - in the guise of PCNA - has proved itself to be one of the least consumer-friendly companies with which I have ever dealt.

 

Nice call young man. Simple logic dictates breaking the system down into simple blocks. If the DME isn't reporting any failures it means it doesn't know anything about what's happening after power has been cut to it. That leaves only the physical circuits in play. Smart man.

I'm not a technician but have been working on my own stuff since before the days of all the wizz bang computers. My first car still had a points based distributor in it. No dummy lights or messages for ****. If it quit running you had to figure it out. Amazing how many go directly for the diag tool when a scope on rope,a volt meter, and a wiring diagram would be all that's required to find the problem.

 

I wonder how PCNA and the dealer would have handled this situation if the car had been under warranty.

 

Just so the community knows, it turns out power to the DME is not cut when the engine's off and the key's out of the ignition.

In trying to figure out the ultimate cause of my problem - whether due to the DME actuating the DME relay at the wrong time, or the DME relay itself being faulty - I needed to know whether the DME remained powered with the engine off and key out of the ignition. And as it turns out, the circuit diagrams show the DME receives power at all times from an unswitched fuse that is always connected to power. The service manual confirms this in describing a diagnostic test for current draw, noting that the DME draws a small quiescent current (a few ma) even with everything turned off and the car cold for at least 8 hours.

 

I’m glad this will be here as a resource and record for future problems. Thanks for sharing!

 

hey LSR, did you finally figure out the root cause ?