P0621 Generator Warning / LIN Bus Fault - Alternator, Wiring, and DME Tested

Hi everyone, I'm hoping someone has seen this before because I'm running out of ideas. Vehicle 2007 Porsche Cayman S (987.1) 3.4L 6-speed manual ~114,000 miles Symptoms Battery/Generator warning appears during driving Typically appears after 15-30 minutes of driving Check Engine Light follows Car drives normally No rough idle No hesitation No limp mode No noticeable charging issues Voltage generally stays around 14V while running Main Fault Codes P0621 – Charge Indicator Light / Generator Communication Fault P0300 – Random Misfire P0301/P0302/P0303 – Misfire codes Various communication/body module faults The misfires appear to be secondary because there are no drivability symptoms. What Has Already Been Done Previous owner history Carfax shows: Alternator replaced around 96k miles Alternator replaced again about 1,500 miles later Belt and associated components were also replaced My troubleshooting AutoZone charging test passed Battery tested healthy Running voltage around 14V Battery/Generator warnings continue to return Independent Porsche Shop (Utah Imports) They: Scoped charging system Checked grounds Added additional engine-to-chassis ground strap Tested wiring Found LIN bus communication issue Saw constant ~5.5V control signal and no apparent LIN communication Determined charging output itself was okay Alternator Installed a used OEM Porsche alternator Same exact problem returned during road test DME Shop believed DME was the most likely cause. DME, immobilizer, steering lock module, and keys were sent to Specialized ECU Repair in Florida. Specialized ECU Repair: Bench tested the DME Installed and tested it in a similar Porsche Could not reproduce any fault Returned the DME as tested good Current Situation The Porsche shop says: Wiring checked good Alternator has effectively been ruled out Their diagnostics point back toward the DME ECU testing company says ECU is good So now I'm stuck between: Intermittent DME failure that isn't showing up on the bench Vehicle-side wiring/connector issue that wasn't identified Something LIN-bus related specific to the 987.1 charging system Questions Has anyone seen a P0621 fault with normal charging voltage? Has anyone seen a DME pass testing but still fail in the vehicle? Any known 987.1 issues involving LIN-bus communication between the alternator and DME? Any known connector, grounding, or harness failures that mimic a bad DME? Has anyone seen an alternator communication fault create false misfire codes without drivability symptoms? At this point I'm trying to avoid throwing another $1,200+ ECU at the car without stronger evidence. Any ideas, TSB references, wiring diagrams, or similar experiences would be greatly appreciated. Thanks in advance.

This issue isn't uncommon. Fixes (usually just cleaning the existing pin, but interesting that Porsche came up with a modified one) have been reported here multiple times. You mentioned the voltage seemed stable at >14V - where were you measuring it? And did you have a means to record it when it happened? As far as P0621: P0621 Battery Warning Light Diagnostic conditions Engine running Normal driving for 2 minutes Possible fault causes Open circuit in line between generator and DME Short circuit to ground in line between generator and DME Short circuit to B+ in line between generator and DME Generator faulty I'm guessing your shop did check for all these items - but they might not have thought of the power-distribution pin issue. That's appearing more and more as the cars age. A quick fix might be tried with a bit of sand-paper to remove the rust. Do not use "dielectric grease" on it - that is a non-conductive grease.. You want a conductive grease just on the portion that rusted, I suspect a good quality copper-anti-seize would work well, or: Link to Conductive Grease - at Walmart! I suspect one tub of the copper grease will last you the rest of your life. ------------------------------------------------------- A suggestion - the TSB above was borrowed from alldata who borrowed it from Porsche - visit AlldataDIY.com - and if you're serious about DIY, or even just want to make sure the mechanics are honest and doing a complete job, a 1 or 2 year subscription is very useful. They have a marvelous amount of knowledge for the 987.2 series cars (where that came from) and I suspect the same for the 987.1 series. Their wiring diagrams alone are worth the subscription cost. I was able to track down a wiring diagram for the 987.2 charging system in about 30 seconds on their wiring diagrams. I tried looking for it with the original Porsche diagrams and after 10 minutes of wasting time I gave up. Here is their charging system diagram: What's particularly useful with their diagrams - if you click on one circuit - it lights up in the diagram, making it very easy to see ALL the components a circuit might be connecting. The Porsche diagrams - doing the same would involve printing out all the pages related to the component/circuit - taping them together - and using a highlighter to trace out the entire circuit. I absolutely love the AllData diagrams! BTW - I really appreciate that you took effort to post a clear and complete picture of what the issue is, what's been done already (sometimes multiple times) and what help you were looking for. It's a perfect example of asking for technical help here. "Better questions get better answers" (me) - I've been trying to promote that for years here, and it's really appreciated when someone makes the effort. Please do come back with the results of cleaning that post.. I'm really hoping for a positive report.

I would still suspect the battery . Load testers aren't infallible... and voltage testers are even less so. How old is it? Has it EVER been dead or jump started? If it's older than 5 years, or ever been dead/jumped, just replace it. I have NO faith in battery testers, and have seen them say batteries are good when they aren't... Beyond that, Don, as always, has a ton of info provided... I admit I didn't read it all through, but that is a great place to start.

Hello there, this is amazing. Thanks for providing such a detailled info and advice. I'm pretty much lost on this but this message gave me some hopes again! The ECU is coming back to Utah from Florida where it was tested and they should put that back in the car. I will totally share this information with the shop to see if they can check or if not maybe try to do it everything myself. Is a bit frustrating that I went trough and alternator change, ECU test, and all that when the solution can be as "simple" as you mentioned. Again, thank you so much for you knoledge and for sharing, is great to see that there are people around like you.

This is to be fair, worth to try at this point! for the price a new battery should be easy to check...