At 09:12 PM 9/8/00 -0700, Dan Shapiro wrote:
>Clarification: I meant the voltage gauge, not the amp gauge. The warning light that is melting its plastic sheath is the generator/alternator warning light. I understand from something that
was said to me years ago that this light is part of the voltage regulation system. Is this true? I am thinking of switching the light for the gas tank warning with this one unless it is a different spec light. I am checking the wiring diagrams for this data now.
The warning light is part of the charging system.
An alternator such as is used on the 928 is not self-exciting, which means
that there must be a small current available to kick off the field so that
the alternator can start generating. This current is furnished thru the
charge indicator light and a 5 watt 68 ohm resistor mounted in parallel to
the bulb. I doubt that changing the bulb type would significantly change
the excitation current - but that is speculation on my part.
Incidentally, a moment's thought about this arrangement will explain how
the charge light works. A simplified explanation - You put 12 vdc from the
battery on one side of the bulb, and hook the other side to the field of
the alternator. When the alternator is not charging, the bulb is grounded
thru the field, and glows. When the alternator is charging, the bulb has 12
vdc on both sides, so no current flows, and the bulb is not illuminated.
Wally Plumley
928 Specialists
================================
>The warning light is part of the charging system.
<snip>
>This current is furnished thru the charge indicator light and a 5 watt 68 ohm resistor mounted in parallel
>to the bulb.
<snip>
The Porsche Parts & Technical Reference Catalog, in the Tech
Bulletin #11, page 166 mentions replacing the old resistor with
a 68 ohm 5W part if the charge indicator lamp does not go out
after starting the car. Stating that: "Exciter current is too weak"
This leads to a minor conundrum: If the alternator is self excited
once spinning, why would the size (value) of this resister which is
already in parallel with a small light bulb (known to have a very
low cold resistance anyway) make any difference?
Side trivia question: What was the value of the old resistor that
they suggest replacing anyway?
I ask because I'm now on my third alternator this year. :-(
The first one lasted 18 years, the second 4 months and the third
seems to have erratic low output level. (As did the second.)
Every connection from the battery to the alternator, to the resistor
and gauge up in the dash, including all related grounds has been checked
and cleaned several times now...
I have the newer style alternator with the 68 ohm resistor installed.
-Larry Ware
=============================
At 09:25 AM 9/9/00 +0000, Lawrence R. Ware wrote:
>At 09:01 09/09/2000 -0400, Wally Plumley wrote:
> >The warning light is part of the charging system.
><snip>
> >This current is furnished thru the
> >charge indicator light and a 5 watt 68 ohm resistor mounted in parallel to the bulb.
>
>The Porsche Parts & Technical Reference Catalog, in the Tech Bulletin #11, page 166 mentions replacing the old resistor with a 68 ohm 5W part if the charge indicator lamp does not go out after starting the car. Stating that: "Exciter current is too
weak"
>
>This leads to a minor conundrum: If the alternator is self excited once spinning, why would the size (value) of this resister which is already in parallel with a small light bulb (known to have a very low cold resistance anyway) make any difference?
The value of the resistor is of interest ONLY when kicking the alternator
off - once the alternator starts producing current, the value of the
resistor is of no importance. The reason for replacing the resistor per the
service bulletin is that if you don't, the alternator won't start producing
current until the engine revs are over about 1800 - 2200.
>Side trivia question: What was the value of the old resistor that
>they suggest replacing anyway?
Damifino, but I would assume higher than 68 ohms.
>I ask because I'm now on my third alternator this year. :-(
>The first one lasted 18 years, the second 4 months and the third seems to have erratic low output level. (As did the second.)
>Every connection from the battery to the alternator, to the resistor and gauge up in the dash, including all related grounds has been checked
and cleaned several times now...
>I have the newer style alternator with the 68 ohm resistor installed.
Somewhere in there, you have a problem - no new news, eh? Check the two
heavy gage wires from the alternator to the starter - these have been known
to break inside the insulation. Also check the engine to chassis ground
cable, and the wire harness at the jump start terminal.
Wally
==========
The alternator should put out 14.1 volts. Less than that indicates diodes
going south.
D
==========
John McDermott wrote:
>The low voltage light on the dash came on this week on Tuesday. The car was starting fine and I measured 11.86v at the positive terminal in the engine bay with the engine off so I concluded it must be a faulty connection in the pod. Today the voltage gauge dropped into the red zone and then it failed to start. It jump started fine and I drove it home without a problem. I have noticed that the voltage always runs low when the temperature is over 95 which I have always attributed to slip in the alternator belt. I maxed out the adjustment on the alternator but the belt still had more deflection than Wally has suggested should be there. I will put a new belt on tomorrow and see if that cures it but if not, how can I check out the alternator? I have a basic multimeter and some electrical knowledge. The alternator is original to the car and has done 77,000miles. Is it about time for the
alternator leave this mortal coil?
>Kind regards,
>John McDermott
>1982 928 Pacific Blue Metallic
An alternator converts rotary energy into electrical energy by moving a
magnetic field (the armature) thru the coils in the housing. The magnetic field
MUST be present for the alternator to generate electricity. It is initially
created by the excitation current that is supplied thru the charging warning
light and the parallel resistor. If the excitation current is too weak (bad
resistor, bad bulb, bad connection), but present, the alternator will start
charging ONLY at higher RPM levels. If the excitation current is not present at
all, the alternator will never charge. The charge light must come on with the
ignition switch on and the engine not running.
Once the alternator starts charging, it supplies its own field current.
Since there is now voltage on both sides of the light/resistor combo, the light
goes off. Once the alternator starts charging, the light/resistor combo has no
effect. The light must go off with the engine running.
The power to excite the alternator has an elaborate route on your '82. You may
need to check and make sure that the excitation current is getting to the
alternator.
1) Battery to ignition switch.
2) Ignition switch to bus 15 (general switched power bus).
3) Bus 15 to terminal H7 on the central power panel. (Connectors are A-Z, left
to right on the bottom of the panel).
4) H7 on a black wire to terminal 3R on the instrument pod.
5) Terminal 3R to the Generator light, and to a resistor mounted in parallel to
the light. If the light is burned out, the alternator won't receive enough power
to generate. Bad connection or bulb in the pod is a likely problem if the light
never comes on.
6) From the light/resistor to terminal 11R in the pod.
7) Terminal 11R on a blue wire to terminal H8 on the central power panel.
There are three blue wires here - power goes to the central warning computer,
and to the mileage counter under the cover on the passenger door sill.
8)Terminal H8 to terminal Z6.
9) Terminal Z6 outside on a black wire to terminal Z1.
10) Terminal Z10 inside to terminal O8.
11) Terminal O8 on a blue wire to the alternator.
If the charging warning light is on when the engine is running, there is voltage
on one side of the bulb and not the other, and there is a problem, usually in
the alternator. If your charging light was coming on and staying on, but is now
going off after tightening the belt, you may have already fixed the problem. If
the light is still staying on, you may have a problem in the alternator.
We sell rebuilt alternators, and will be happy to sell you one - but that might
not be the best way to handle the problem.
My suggestion is that you find a good local auto electric shop - not a general
repair garage, not an auto parts store. This shop will probably do nothing
except for repairing auto electrics - starters, generators, alternators, motors.
They can repair/rebuild your alternator, starter and motors, often at a lower
cost than buying a rebuilt. An advantage - you get YOUR alternator back. If you
still have the correct alternator on your car, you will still have the proper
housing, the cooling cowl will still fit, etc.
If you have a shop install a rebuilt alternator, make certain that the regulator
is also changed - some rebuilders furnish units with no regulator, and the shop
installs your old unit - which may have been the problem in the first place.
It is important to have the cooling cowl and the hose fitted - otherwise, the
alternator is "cooled" with oily, very hot air off of the exhaust manifold. It's
not unusual for crummy shops to "forget" this, and your cowl is in the county
dump.
Wally Plumley
928 Specialists
www.928gt.com
===========
At 01:32 PM 8/20/2005, Bob Foster wrote:
>A few months ago, I did something that I hesitate to admit but... due to a low
battery, I hooked up jumper cables under the hood with positive to negative and negative to positive. I know, I know. When I turned
the key to start, smoke bellowed from the area of the alternator. I had cooked parts of the alternator which I have since had rebuilt
for $154. The alternator is back in, the car started and I get a 13 volt reading on the dashboard voltmeter gage but I have to rev it to
over 2,000 RPM to get it to move off of 10 volts AND the red light at the bottom of the voltmeter gage remains lit regardless of the needle
reading. Thoughts please?
>The oxygen sensor light (OSX) also remains on.
>Bob
>
>'82S 5sp Blk/Blk
>'88 S4 Auto Blk/Blk
>
Bob,
Having to rev the engine over 2,000 rpm to start charging indicates that there
is little or no excitation current at the alternator, or that the alternator is
still faulty. The red light staying on while the alternator is charging usually
indicates a problem in the alternator. One possibility is that the shop may have
missed a bad diode in the diode trio. Another possibility is that there is a
problem in the excitation circuit. The resistor on the back of the voltmeter may
have failed. It takes both the light bulb and the resistor (which is mounted in
parallel with the bulb) to excite the alternator at low speeds - but I don't see
why that would keep the light on while the alternator is charging.
I would discuss the situation with the shop that repaired the alternator.
You paid a premium price - I would demand premium service.
The oxygen sensor light is meaningless. Porsche didn't know how long an oxygen
sensor would work on the street, so they put a counter circuit in the car that
turns the oxygen sensor check light on every 30,000 miles. It has nothing to do
with anything - it's just a mileage counter. There is a box under the cover
between the passenger seat and the door, and there is a semi-hidden switch in
the box that turns the light off.
Wally Plumley
928 Specialists
www.928gt.com