Cold start alternator problem- really strange

[bigmac]

Hello,

 

I have a 02 996 tip and it is has a really weird problem. So when you first fire up the car when it is cold to will run rough (rpm around 600). If you turn the key off and back on it runs fine. Checked fuel pressure before the first cold start and it is in spec. I have found that the battery was low. Changed it out. Still same problem. I noticed that the alt was only kicking out 13.2v at the alt and 13.0v at the battery. Replaced the alternator, it was making some bearing noise when I spun it up.  Charged the battery and fired it up with the new alt and its good for about 2 days. I have noticed that when the its running rough the battery voltage to the dme and the battery is 12.2 (When I start it the starter only pulls the volts to 10.6 so that's ok). After about 10 seconds of running poor the voltage jumps up to 13.8 and the car runs great. Its like the alternator is not getting voltage to energize on cold start. Any ideas? No codes stored and the DME has been reset, The battery has been tested and it's fine but I am going to replace it anyway. 

 

Thanks! 

[JFP in PA]

Hello,

 

I have a 02 996 tip and it is has a really weird problem. So when you first fire up the car when it is cold to will run rough (rpm around 600). If you turn the key off and back on it runs fine. Checked fuel pressure before the first cold start and it is in spec. I have found that the battery was low. Changed it out. Still same problem. I noticed that the alt was only kicking out 13.2v at the alt and 13.0v at the battery. Replaced the alternator, it was making some bearing noise when I spun it up.  Charged the battery and fired it up with the new alt and its good for about 2 days. I have noticed that when the its running rough the battery voltage to the dme and the battery is 12.2 (When I start it the starter only pulls the volts to 10.6 so that's ok). After about 10 seconds of running poor the voltage jumps up to 13.8 and the car runs great. Its like the alternator is not getting voltage to energize on cold start. Any ideas? No codes stored and the DME has been reset, The battery has been tested and it's fine but I am going to replace it anyway. 

 

Thanks! 

 

Do a voltage drop test across your primary battery/starter cables.  These cars are well known for having resistance build up in the cables which otherwise look fine, and need to be replaced.  Porsche has released new cables designed to eliminate this issue.

[bigmac]

Thanks for the quick reply. I did a volt drop test and it was only .2 from the alt to the battery. I am getting 13.75 at the battery and the DME.  This car is killing me! 

[JFP in PA]

Thanks for the quick reply. I did a volt drop test and it was only .2 from the alt to the battery. I am getting 13.75 at the battery and the DME.  This car is killing me! 

 

What kind of voltage drop do you get on the starter cables?

[bigmac]

It's about a .3v drop. No too bad. 

[JFP in PA]

You may be experiencing a dying voltage regulator (inside the alternator, but can be replaced).

[bigmac]

So with the car running and hot (super hot today) the alt is only putting out 13.3 at the battery and at the alt (no volt drop). What would cause this? The alt was just replaced so there is a small chance that its a bad alt but I doubt it. 

[bigmac]

What would kill a volt regulator in the span of two days? The old alt was putting out around the same voltage, that is why I replaced it. 

[JFP in PA]

What would kill a volt regulator in the span of two days? The old alt was putting out around the same voltage, that is why I replaced it. 

 

The correct output voltage should be 13.5-14.5V at idle (checked using a digital multimeter, not the dash display).  If all the primary cables pass voltage drop, and there are no problems with the car's grounds (resistance), there has to be a problem with the alternator itself (diode) or the voltage regulator inside the alternator.  You've basically run out of other possibilities.

[bigmac]

I think you were right with the cable. I am not checking the volts at the alt but at the postive junction box in the engine bay. The only thing that makes sense is the leg of the cable that goes from the alt to the starter. If that cable leg was smokes that would cause the low volts at the box and at the battery. I am using a dmm