Biblio-Porsche: Engine - Alternators

From rickydik@ix.netcom.comTue Apr 23 01:07:03 1996
Date: Mon, 15 Apr 1996 22:48:38 -0700
From: RD Rick 
To: Dave Darling 
Cc: porschephiles@tta.com, 914@giza.estinc.com
Subject: Re: [all] Octane Rating, Alternator Problem

DD wrote:
>Older Porsches have fuel requirements listed in RON, the Research
>Octane Number.  My local pumps are all listed as being (R+M)/2,
>which>would be the average of Research and Motoring Octane Numbers.
>Is there>any rule or rule of thumb as to the relationship between
(R+M)/2 and RON,>or of MON to RON?
>
I believe even my early '73 914 listed:  ROZ*RON/2 = min. 91 Octan.

>I think my alternator might be dying.  When the car is cold, I
>see 13V across the battery.  When warm and loaded (no, not me, the
>batt! Lights on, etc.) this drops to 12.3V.  I replaced the voltage
regulator,>which cured the problem for one day only.
>        How do I check the alternator?  I know I can check it for
current>output, but how?  Anything else I can do?

Are you using electronic Voltage Regulators, or the old mechanicals?
I toss the mechanicals.
The VR is supposed to regulate to a slightly higher voltage when cold,
then reduce to 13.8 when warm/hot.  Mine sits at 13.5, and I am happy
with that.  Check this with minimum load.  Are you using an
independently calibrated digital meter?  Are you reading directly
across the batt terminals?

The alt. is supposed to put out the voltage demanded by the VR.
Regardless of the regulated voltage, the output voltage will not change
with load such as headlights and blower motor, if the alt. is good.

Make sure the belt is not slipping:  With the engine off and in
neutral, try turning the alt pulley with a 7/8 wrench.  If tight
enough, the belt will turn the engine over and not slip.

Does your radio whine, and not just your SO?  If so, a diode is open
inside the alt.

RD

To: "John Rodriguez" , <911@PorscheFans.com>
From: Dave Christiano 
Subject: Re: 911 gauges - little red light. LONG

At 09:11 PM 3/29/97 -0500, John Rodriguez wrote:
>light has a " G " on it. Is this for G - for "GAS" What does it mean?

The red light is connected between the alternator field winding and the
alternator output. Electrically the alternator is a three phase a-c
generator. Each of the three phases generates an a-c voltage that is 120
electrical degrees apart. These are fed into a three-phase bridge rectifier
composed of six diodes (called a six pulse rectifier). this is done for
electrical efficiency as well as generating a smoother dc output. The
alternator also has a simple three phase rectifier that serves to provide
the d-c field excitation for the alternator. The voltage regulator senses
the alternator d-c output and increases the field excitation when it is too
low. If the field voltage is less than the alternator output (or the
battery voltage if the alternator is not putting out any voltage at all)
the red light will come on.

Each of the rectifiers is composed of individual diodes (9 in all). I have
had a similar problem in my 76 911 and suspect that one of the field diodes
is open. If so, the field voltage will always be a little low, and hence
the alternator voltage also. At lower engine speeds the problem is
exagerated as the alternator output can only be raised so much by
increasing field excitation. My alternator seems to keep the battery
charged but at a lower voltage, hence I really don't have a fully
up-to-snuff battery.

As to what you do about it it depends on if you want to feel secure (I do)
and how good you are mechanically. Removing the alternator is a pain.
You definately have something wrong with the charging circuit. As was
advised by another list member, the first thing to do are the easy things
like checking connections and voltage regulators.
Good luck,

Dave Christiano   -   Springfield, Missouri
'74 914/6 3.0 liter   -   '86 930

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Last Update: 15-Mar-1998 1:30 PM