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Voltage drop between alternator and battery


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In my 2005 997 manual Carrera S with Chrono pack, I have had an intermittant problem of the car

failing to start due to a battery with too low a charge. I drive this car daily. No dash battery discharge

warning indication has ever shown.

Recently I changed the battery, and then had a regulator failure requiring a new one. 4 weeks,

later, after a few starts during the day, the car failed to start due to low battery.

To shorten the story, I discovered there is a 1 volt voltage drop in the thick positive wire that

travels the length of the car from the alternator to the battery. Trouble shooting indicates the

problem is between the alternator, and the connector next to the gearbox. The generator/regulator

puts out 13.8V, but at the battery the voltage varies between 12.5 and 13 V.

I suspect it is where 2 wires are pre joined at the starter motor solonoid, since there are

no other connectors, and the wire is unlikely to have been damaged.

Before I go removing the air filter and fuel injection plenium chamber to get behind the starter

motor, I have a couple of questions.

Is this an uncommon problem, or is it a known design fault?

If anyone has had this before, what was the cause?

Why doesn't the battery discharge warning on the dash come on to warn me?

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I think your analysis is correct. The alternator voltage is correct, and the battery voltage is too low to charge the battery. The drop you measure is the problem, and is most likely occurring at the connection at the starter motor. Unless there has been a crimp or cut in the wire between the alternator and the starter, the connectors on the wire or, most likely, the tightness of the connection at the starter are the culprit. Good analysis, and good luck.

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The 997-2 cars have a bigger wire. I had this installed when

they replaced the motor. If you have a tip car, then it is almost impossible

to replace. A manual is easier because there is no huge transmission

cooler sitting on top of the transmission.

One thing I did that helped a little was remove the connection under the car

on the passenger side and sand paper the connection. The connection is behind

a plastic door that picks up alot of road dirt.

This is a common problem with the 997-1 cars.

Paul

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