Battery/alternator warning

[greig coutts]

Have an intermittent battery / alternator warning light, so far fitted new battery changed alternator to starter wiring loom, fitted rebuilt alternator all to no avail. Have checked voltage whilst driving and when the warning comes on the voltage is dropping so the sensor is correct the voltage drops to around 12.8 and then usually the light goes out and the voltage goes back up to above 14 volts. So it would appear to be a connection fault but where do I start to look

The car starts fine and once warm it will suddenly come up with fault warning which will then continue to reappear at random so to my thinking it must be linked to the charge circuit maybe a relay or a switch contact .

Any ideas or checks welcomed 

[wizard]

I'd check the voltage regulator/diodes first, then the alternator itself.

Edited August 21, 2016 by wizard

[greig coutts]

I thought it was an alternator fault originally so I have fitted a reconditioned replacement alternator and the car still shows exactly the same fault with the new alternator fitted.When the fault code appears the voltage drops but it disappears after a few seconds and the voltage returns to over 14 volts. If I switch everything on, lights, rear screen heater, air con, stereo, etc it will maintain the correct voltage at tick over so I think the alternator is fine. The problem seems to be with the energising of the charging circuit, maybe the ignition switch, a relay or a loose connection. 

Hoping somebody has solved a similar problem.

 

Thank you anyway

[JFP in PA]

Have you done a voltage drop test across the primary battery cables?  These cars are well known for having internal corrosion in the primary cables that leads to no end of electrical gremlins.

[wizard]

This may be of some help.

http://www.autohausaz.com/porsche-auto-parts/porsche-electrical-systems.html

[greig coutts]

Thank you for your suggestions, based on my research on the problems experienced I replaced the wiring harness from the alternator to the starter motor and through to the connection point by the gearbox.( my starting has never been a problem and the starter does not sound laboured) I thoroughly cleaned the connections and all seemed well but the fault is still there.

Because it almost seems to switch on and off stopping the alternator charging I think it must be to do with the charging circuit.

 

 Question:Where does the other wire connected to the rear of the alternator come from, is it a positive feed or an earthing function, presumably it has to be activated for the alternator to start charging ?

[JFP in PA]

The wire to the alternator is a 12V power.

 

The voltage drop I referred to comes from corrosion inside the primary wires, not necessarily the terminal ends, which is why you need to run a voltage drop test across the leads to find it.  It may not affect starting, but can appear as the resistance grows with heat in the engine bay,

 

You may also be encountering diode drop out in the alternator itself, or a voltage regulator issue, either of which could be seen on an alternator load test.

[greig coutts]

Thank you for the info on the wire to the alternator, I will do a voltage drop check on the wire from the gearbox terminal to the battery as I have already replaced the wiring in the engine compartment. As I said, I replaced the alternator but the new one has the same fault code appearing  

[JFP in PA]

I would still load test the alternator, it would not be the first time I have seen a defective replacement right out of the box, and it needs to be eliminated as a possible factor.