airkulr

Still working on my 95 Carrera 2 testing alternator issues. Does anyone know if on the Bosch alternators from this era that the Idiot Battery Light in the clock has to be functioning for the alternator to put out a charge? I know on earlier Bosch designs the light had to work so the alt. could be initially "excited" to start putting out the charge. I have checked my bulb and it is ok...swapped it with other bulbs and they all work. My problem is that I have no charge light with key on and a rebuilt Bosch alt. installed. Is there a test I can do at the alt. wiring harness while the alt. is disconnected...I am trying to manually light the bulb from that end just as a test to confirm my wires are ok.

Garth S

With key in 'run' position & knowing that the bulb is good, a meter should detect battery voltage at the "excitation" wire: alternatively, grounding that wire should light the bulb. Either approach will confirm the integrity of that circuit IIRC.

airkulr

So what you are talking about is the smaller blue wire that connects to the D+ terminal on the alt. I did some reading on the curcuits for Bosch systems and they were talking about this wire connected to the batt. bulb. It does energize with about 12 volts with the key on...but i hesitated to ground it with voltage there for fear of shorting something. But if I understand you correctly I should go ahead and ground it while it is hot with the key on?

Garth S

If it does show battery voltage at the alternator site with key on and disconnected - the circuit has tested good!
Grounding that wire is the second method mentioned; however, if I misunderstood your comment above, you can avoid the grounding test by simply using your DVM/digital multi meter as a bridge: look for battery voltage between that wire and any good ground with key on ( with this method, the bulb is unlikely to light as the meter is v. high impedance, so the current flow is insufficient - no prob as you have confirmed the bulb as good).

If that circuit checks OK, have your supplier either bench test the alt - or replace it.

airkulr

Garth, Yes it all checks out as you have mentioned....so looks like a faulty rebuilt alt. I also confirmed that all my wiring hookups were correct initially installing the alt. I guess my last question is do the Bosch alternators in my particular 993 still rely on that charge/battery light to put out a charge or can the alt. still charge if the bulb is burned out? I've heard that Bosch is the only manufacturer set up this way. Next I am having the alt. bench tested.

Thanks for all your helpful insight and help!

Garth S

Hey airkulr,
Honestly don't know if the 993 excitation circuit fails with a burned out bulb: while saying that, it is true that most vehicles rely on the bulb filament to complete the circuit, assuming that all other wiring remains intact; however, some cars have a resistor in parallel with alt. bulb to attenuate the current flow ( as in certain alternator upgrades for the 928, a resistor is added in parallel ....or the light never extinguishes: the corollary to that is that then if the bulb dies - there remains a circuit path to allow excitation current).

Here's a link to Bosch alternator function that describes the necessary excitation of the field coils to allow an alternator to "turn on" .... and why the bulb turns off once the requisite voltage is delivered

airkulr

Garth,
Finally got another alternator under warranty replacement...was also gone on vacation for over a week so could not install it right away. YES! The replacement alt. works as it should. Lesson learned is new or rebuilt parts are not always good. Thanks again for your help in this matter.

IXLR8

A local 911 owner installed a rebuilt alternator that was a dud as well.

Seeing how easy it is to test an alternator at the time of rebuild, its proof they don't even bother with this simple task prior to putting it in the box.

airkulr

Alex,
I totally agree with you. This is especially important on these cars because of the difficulty and time involved in the removal and install and also the time lost waiting for the replacement through the shipping. Luckily my 993 is a play car.

...and thanks again for all your wonderful info!
Airkulr