Bought a used car 3 days ago.. started and stopped about 6 times on my drive back. Was fine all yesterday. Today, needed a jump every time I've tried to start it.. tried to get the battery to charge to little success. Multimeter reading is 13.8. Any idea what could be happening here?

 

There's been a lot written on this in the forums. The battery voltage seems OK but you'll need to take the car up to Autozone and have them do a Free Battery test. They'll put a load on the battery and make sure it can handle the work. There are issues with the long cable running from the alternator to the batter and from the alternator to the starter. Last month I had my Alternator and Starter cable replaced under warranty. The starting and stopping part is odd. The computer doesn't check to see if you're getting voltage at the battery it checks to see if the alternator is putting out. You should have an engine light if the alternator is on the fritz. If you're car is under warranty then take it in. If not you may want to find an independent Porsche mechanic to check it out for you. You can ask here depending on where you live for a recommendation. You can also join you local PCA club and they will know who to go to.

Updated Battery / Starter / Alternator Harness

Per Porsche's technical bulletin.

Issue: Battery discharged and/or will not charge properly. Voltage drop due to alternator-starter-battery harness damage at starter terminal. This issue is often mis-diagnosed as a bad battery, bad alternator, or bad starter, and these parts are often replaced in multiple repair attempts before the harness is determined to be the cause of the issue.

Before ordering or replacing other parts, have the car professionally inspected. Check the voltage drop between the alternator and battery. Bring the car to normal operating temperature. Turn on as many power consumers as possible (AC, headlights, radio, defroster, etc.). Use the same grounding point for the following voltage measurements:
- Measure the voltage at the "B+" terminal on the alternator.
- Measure the voltage at the battery positive terminal.

If there is a voltage drop of greater than 0.5 volts between the alternator and the battery, and all of the connection points are tight, check the harness terminal at starter. It is possible that the crimps on the starter terminal where the two cables are joined together incorrectly, causing an excessive voltage drop between the alternator and the battery. This voltage drop does not allow the battery to charge properly, and as a result, the battery will continually discharge until it will no longer start the vehicle. There may be no visible damage to the harness, and more measurements may be necessary to determine which cable or part of the cable is responsible for the voltage drop.

Fits all Carrera & Carrera S models, 2005-08.

 

The harness itself is not that expensive at $100:

http://www.suncoastparts.com/product...ra_S_Engineexl

The installation however is a PITA if DIY.
Cheers,
=L=

 

new battery usually solves all problems

 

Worked for me!

 

Just replaced the battery in my 2007 997 C2 and have noticed a big difference. If the car is older (< 4-5 years) and on it's original battery it may be time for a replacement, especially if it's been sitting around a bit. I now keep my battery on a battery tender, as I don't drive that much. I picked up an original Porsche equipment battery for $150 from a reputable supplier in the NorthWest.

 

Yes - agreed. My 2006 C2S had the alternator to battery cable replaced under warrenty as the battery kept draining after normal use. Then this last year, as a 5 year old car I had the battery replaced. City driving with short engine runs will kill a battery faster.

 

just get a multimeter, connect it to the battery while car is not running, note the reading - if it is below 12v then you are obviously in trouble. 'normal' voltage should be somewhat around 12.5v. then start up the car, drive a bit and then connect multimeter while engine is still running to see if you get close to 13.8v or so there on battery terminals. if you still are barely at 12v with engine running then you have a problem with cable, if you do have close to 14v with motor on but after you shut it down in 3-4 hours battery shows under 12v again - replace the battery.

 

^ Good start but you need to measure resistance on all the cable to make sure. I did the above test and everything tested OK. On my second visit to the dealer for the same problem I asked the Tech to test the resistance on each cable and that's when they found the bad cable from the alternator to the starter.

 

Kronik,
What you measured is the alternator voltage, while the engine was running, the alternator does not supply anywhere enough current to start the car. That is what the battery does, to measure it's voltage the engine must be not running and it should be steady above 12 volts prior to ignition. How old is the battery, even at best they should be replaced every 5 or 6 years.

 

Our '08 C2S had all these symptoms - slow crank, etc. It was purchased in July '11 as a CPO car and had the original battery. After the key was left on accidentally, the battery dies and after I jumped it, the SRS warning light came on. Brought it to our local dealer here in Columbus, they diagnosed bad cells in the battery and suggested replacement (about $480, of course not covered by warranty). They also cleared all the codes that were thrown by the ECU (about 2 dozen codes) but the new battery still didn't cure the slow cranking problem so they dug deeper and confirmed the starter / harness issue as others have described. It took a day of labor and it's amazing how quickly the engine now cranks and starts. Covered by the CPO warranty, the service manager wouldn't say exactly how much it would have cost to replace but guessed "around $1200". That's for a rebuilt OEM starter and the harness. He did say it was mostly labor, but didn't go into detail about what was required to replace the harness. Hope this is helpful to others.