The PCM in my 2005 911 works just fine in warm weather. In cold weather, though, the AM/FM radio just buzzes when I turn it on. The buzzing is uniform and does not change with increasing/decreasing engine revs. CD players (car has the single one and the 6-disc) work just fine. All other functions of the PCM work fine. Complicating matters is the fact that the problem is intermittent in cold weather but is prevalent enough to be annoying. I'm reluctant to send the unit out given the cost and wonder if someone here might have experienced this and have a solution. Thanks in advance.

 

Electrical buzzing through the speakers in the car? You can confirm by adjusting the balance/fade to see if the sound moves.

Mechanical buzzing? I'm pretty sure there is a small fan on the rear of the unit. Perhaps that's the source?

 

It's electrical buzzing through the speakers such as you'd expect to hear if an alternator was making electrical noise. It completely disrupts the radio sound so you can't hear the station at all. Just the buzzing. Not mechanical. Sorry, I should've clarified.

 

IIRC the tuner antenna wires are imbedded in the windscreen and assuming this is correct, I'm thinking maybe the thin printed antenna wire has a minor crack which moves with the temperature change. Not sure you would proceed with diagnosing this theory though short of replacing the windscreen ;-(

 

Could also be the amp. You could test this by waiting until it happens when it's cold then take a hair dryer and just warm up the amp in the front trunk and see if the sound returns. If not, it's probably the PCM itself, but until you troubleshoot the other components and grounds, I don't think you can determine that. Now if the screen was freak g out or it was cycling on and off, that's a different story, but if the PCM looks normal when this is happening, I'd do some further exploration.

 

Thank you and systech for the input. Both good ideas. I'm showing my lack of tech savvy but if it's the amp wouldn't that also affect the CD players' audio? Think i'll try the hair dryer idea anyway first. Thanks again.

 

Based on the information you've shared, I would guess the tuner section within the PCM unit is going bad from age. The fact that you mention it doesn't do it during warm weather or when a CD is playing, tells me that tuner components on the circuit board could be failing due to age. Normally, low freq. hum is associated with power/ground issues. If it's a ground loop (e.g. alternator), then the sound usually is not constant... it varies in pitch when you press the accelerator. Have you thought about purchasing another unit on eBay?

 

Thanks NuttyProfessor. I was hoping for a quick/cheap fix but certainly will explore the eBay idea. The car is bone stock and I'm inclined to keep it that way but am tempted by after-market installations especially when I compare the price of that solution to sending my PCM out for refurbishment. Thanks again.

 

I suspect the "amp" he's talking about is the RF Amp for the antenna signals (it's a "active" antenna I believe). Normally it should be located close to the antenna wires coming out of the windscreen so probably under the dash somewhere ...but I'm not sure where it's located.

 

I missed the part about it not doing it when a CD is playing. I had meant the stereo system AMP and it would do it regardless of the source. If it only does it with FM, that's a different problem. It could be the tuner section, or possibly something related to the FM antenna connection and when it's cold the male end from the PCM into the female antenna sleeve shrinks enough to not make proper contact. You know from the old Seinfeld episode on shrinkage. That seems pretty unlikely though and it's probably the start of a PCM failure.

 

Thanks to all who responded. I thought I'd write the final chapter in case someone else could be helped, too.

It turns out my original PCM had lost the AM/FM radio functionality thereby creating the buzz. All connections were solid and the amp worked fine. Here were my choices: Pay $2600 including a $450 core charge and get a rebuilt one from Porsche with a 2-year warranty, Send my existing unit to Becker in NJ for repair for about $1500, Buy a used one on eBay, or replace the whole thing with an aftermarket unit. Because I wanted to keep the car as original as possible, I opted to buy one from eBay. I found one that was well represented and bought it for $432 including shipping. Everything works exactly as it should except the part that records lap times, etc. from the Sport Chrono on my car. My dealer mechanic (whom I trust) said he was unable to program that part of it because the PCM had previously been coded to a different car. That particular functionality is not one that I use. In fact, I didn't even know it existed. So all seems fine for now, especially considering the cost differential with the other options.