Porsche 928 GTS, 1993, alternator doesn't charges battery below 2000 rpm
 

Hi everyone,
my 1993 alternator is not charging battery below 2000 rpm, if I give push to accelerator than only it starts charging... If I leave accelerator( after push) it drops to around 800 rpm( idle rpm) & keeps charging battery(14v at battery)...... I have checked wire from alternator to 19 pin plug it is ok... At idle ( while battery is not being charged) I can't see any voltage in that line & while battery is being charged, there is around 12-13 volt...
Any idea what could be wrong...
Thanks...

What criteria are telling you the alternator is not charging?

With idling I can see 12 .6 volt which keeps decreasing slowly2 ..... Voltage at battery increases( around 14volt) only while reviving speed is greater than 2000 rpm....( Reading taken with multimeter at battery terminal )
Thanks

If you measure the voltage across the battery terminals with no load the voltage typically is around 12.5 volts for a healthy unit. The alternator is a machine just like the engine and it does not generate full output at idle speed or anything vaguely close to it. Thus there is a force balance- power consumed versus power generated. If you are running full accessories- lights, a/c and especially if you have a powerful aftermarket Hi Fi the alternator typically struggles to keep up with demand until the engine speed gets to around 3k rpms. The voltage regulator tries to keep the volts around 13.5 to 13.8 volts so that there is potential to drive the power generated into the battery..

If the dash voltmeter shows 12 volts then load is being taken from the battery. Typically the voltmeter should be showing 13.5 volts around 2500 rpms but that also depends on the connected load. My late 90 S4 had a powerful after market Hi Fi and at night with the a/c going full tilt and the Hi Fi blaring out the thing could not get full voltage until about 3k rpms. My current 928 with a puny Hi Fi fitted has no issues like that. The other consideration is the wiring harness. Some of these things [most of them] if the original item are seriously degraded especially on the 16mm2 feeder to the central electrics- when this happens the resistance increases and this taxes the system ever more.

Without knowing what load you are imposing on the system it is difficult to be specific- if you feel your performance characteristics are aligned to the above then what you are seeing is "normal". . If something else is going on it is what it is.

Sounds normal to me.albeit I do not understand what you mean by "decreasing slowly".. 12.6 volts is being driven by the battery- load it up and it will drop.- what accessories were running at the time?

My battery is original 12v, 70AH & new, it reads 12.6 volt generally.. if I start the car & allow it idling at 800 rpm, with no loads on, it reads 12.6 volt & it does decreases slowly 2..( 12.60 volt- 12.59 volt- 12.58 volt)......
Now if I give quick accelerator push( anything above 2000 rpm) voltage at battery terminal is 14.30 volt which stays constant all the time even if I drop the accelerator or give it a push... No loads on
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Yes it should be dropping from 12.6 volt, but at idle let's say at 800 rpm I see it going down slowly slowly ( 12.60-12.59-12.58........ it keeps decreasing) so alternator is not charging battery... Battery voltage just keeps dropping... I was expecting somewhat around 13.5 volt while at idle....
No load on during whole operation

Sounds like you are missing the external excitation circuit to the alternator. That circuit passes from the 15 bus (battery voltage with key in RUN or START positions) through the charge light bulb and resistor in parallel with the bulb, through the CE panel a few times for good measure, then out to the 14-pin connector at the right front corner of the engine bay, then the front-of-engine harness to the alternator. The quickie diagnosis is to look for that charge light bulb illuminating with key on but not started yet. If it isn't lighting, the wiring loop to the alternator is likely open. There are several recent threads here about diagnosing the problem, so I won't try and retype all that stuff here. Do a little searching on the subject, and refer to your electrical diagrams to get exactly the flow path. With care you can do almost the whole testing protocol with the car on the ground until you get to testing the wiring from jump post to alternator, then removing and testing the alternator itself off the car.

Do you get the battery warning symbol until you get a good reading?

No I am not getting any battery warning.. even if the voltage is low...

Have a look at the condition of the cables exiting the 14 pin connector in the engine bay going into the engine and as Dr Bob, suggests, trace and track the condition of the blue cable going from terminal 1 to the alternator [the exciter cable].

14 pin term.1, I believe this wire goes to alternator... It is perfect as it was new.. now if I check continuity between this wire to alternator... There is good connection

The Voltage Regulator brushes may be too worn down and bad now. Easy enough to remove it and see.

Thankyou for the input... This is what exactly my doubt is.. I don't see any battery warning in cluster ... My 93 cluster looks a bit different than the previous one ( especially the cluster dismantled in pics).....
But if the bulb is blown, alternator will not charge battery in controlled manner( voltage regulation), right ??

A blown bulb causes the alternator to not start charging until 3000 engine RPM or so. The fact that your bulb doesn't light does not mean the bulb has blow, although that's a [rare] possibility. There's a test protocol that will help you identify the problem. The circuit passes through the 14-pin connector in the engine bay on the right side. Lift the top (engine side) side of that connector. Pin 1 (blue wire that you can't see...) in the car side of that harness comes from the instrument cluster, including the bulb and resistor string. Test for voltage to ground from that point. You should see battery voltage or close to it when the key is in run position (2) or start position (3). The engine won't start with the 14-pin connector lifted. The charge light won't go on yet, but your meter should show voltage. If so, and without changing anything in the car (still in run position...), use a small jumper to ground that connection (pin 1 in the 14-pin car side, engine side still lifted), and then look for the charge light in the cluster. It should illuminate. If it does, remove the jumper, turn the key off, reassemble the connector.

Move to the alternator. The blue wire in the front-of-engine harness continues to the alternator field connection. Remove the cooling shroud from the back of the alternator, and respect the larger alternator primary connections (red) as they are unfused and at battery potential. Test the blue wire for voltage with everything connected, You should see excitation voltage there, less than battery voltage, with key in run position. If you see close to full battery voltage, it's possible the excitation in the alternator itself and the regulator has failed. If you saw voltage at the 14-pin end but none here, lift the blue wire and check the now-isolated terminal for close to battery voltage with key in run position. If still none, the wire in the FOE harness has failed. If yes, ground the alternator end and observe the charge light - it should be lit with key on. the matrix gets bigger than I feel like typing here, as it's been shared before. Post back what you find while testing and we can do more testing to exactly isolate the problem.