Alternator light

[exc911ence]

Some days my alternator light (red) goes out almost immediately after firing up the 911, other days, it stays on for 10+ minutes. I just installed a new battery 2 weeks ago (thinking that this was the problem... it sort of was, the old one was shot) but do you think that there's a problem with the alternator as well? Thoughts?

[singpilot]

Get a Voltmeter. Monitor the volts as the engine runs after startup. Will tell you how the alternator is doing.

11.9 - 12.3 = Batt at rest.
10.9 - 11.9 = Volts with engine running, no alternator.
12.7 - 14.3 = Volts with engine running, alt on line, not much loads.

There are variances for lights, A/C (if in Los Angeles this time of year), heater blower(s), mirror and rear window defrost, radio.

I use a VM that plugs into the cigarette lighter, you get to watch the system at work.

YMMV.

[exc911ence]

I did a quick multimeter test and got a little over 13 volts at idle with the defroster blower going and the headlights on. That's pretty normal I'd think. Some days, it just seems to take a long time for the alternator to start charging. Possibly the regulator?

[singpilot]

I have noticed that when cold, it will take something 'above' idle to get the VR to kick in.

[john70t]

Most alternators won't perform at peak at idle, check at a little higher RPM. It should be above 14 volts and less than 16 as a ballpark.

There might be a problem with the regulator or the connections. The random electrical problems are the funest to track down

[anh911]

It can depend on how you drive. I know but hear me out. The alternator won't start charging until it turns fast enough. It's possible, that you can be too gentle when cold and not be turning the motor fast anough to charge. Next time it doesn't go out just rev it to 2500+ or more and see if it goes out. Check the fan belt too.

[exc911ence]

The problem isn't rpm dependant, I always need at least 2000rpm to kick in the charging system but some times I can rev it past 5000 and the light stays on. When the light is on, it's definately not charging as the blower and wipers are slow and the headlights are dim. It's just odd that some days it's fine and other times the alternator just won't kick in for several minutes.

[Lorenfb]

Anh911 has mis-lead you & the others with hypebole as usual.

First, the DF terminal to the rotor slip rings should read 6 ohms
to the case when the shaft is turned and at rest. Those early
alternators with the red plate (Marchal 55 amps) should "start"
at less than 1500 RPMs and the light should be "off". Any higher
RPM indicates a bad alternator or a bad connection to the battery.
The Bosch (55 amp) alternators start at about 800 to 1000 RPMs,
and are much more reliable.

Those Marchal alternators if not rebuilt properly can be intermittent,
since the diodes are pressfit into the SMALL diode plates. This results
in a potential intermiitent connection. This alt. is also very "weak"
and prone to fail if the battery is a "monster" (928 type) and/or if
the regulator charges greater than 13.8 volts at the battery.

To improve the alt. startup RPM, solder a 50 ohm 3 watt resistor in
parallel with the light on the cluster unit. Just pull the light out and
add the resistor and then reconnect the wires. This will provide more
startup current for the alt., as the black relay for the defroster draws
some from the light besides the alt. rotor, which makes the startup
worse.

By the way, anh911 come up with some intelligent ideas other than a
loose fan belt.

[exc911ence]

Hmm, that's interesting. The battery I installed is a 1000CCA monster but it was acting up worse before the battery swap. I like the idea of adding the resistor, cheap and easy to do but if my charging system doesn't come online until 2000rpm or so now, the alternator is more than likely failing, no? I'm not sure what model the alternator is (it's the stock one for the 75 911S 2.7) but I don't look forward to the labour (and inevitable swearing) involved in replacing it...

[exc911ence]

I found an interesting troubleshooting table on Pelican (http://www.pelicanparts.com/914/914qa/914Q_electrical_alternator.htm) so the first items I'm going to attack are the ground straps and alternator connections. Luckily, as a laid-off aircraft mechanic, I have lots of time to fiddle with the car!

[Lorenfb]

It's probably a Marchal 55 amp. Those monster batteries, e.g. Interstate,
can basically "limit" the alt. voltage to 13.7 to 14.0. If the regulator is
set to 14.25, then the alt. can "self destruct" trying to supply the battery.
Those batteries can easily "sink" 40 to 50 amps when at a low charge
level.

Hopefully, anh911 doesn't have a monster battery in his 3.2 or those
Paris-Rhone 90 amp alt. can fail with a low charged battery. I've seen
those 3.2 alt. fail too, if the engine is revved after just starting with a
low battery. Those 3.2 are set to 14.6 to 14.7. We all know what happens
to a voltage source (an alt.) with no current limiting, lots of smoke.
He probably knows this already, though.

[exc911ence]

My car's got power windows, sunroof, air conditioning, etc... did Porsche supply a heavier-duty alternator for cars with high electrical option content?

Also, is the regulator adjustable?

[anh911]

Loren - I'm going to let the moderator handle it. I've forwarded all your insults.

"Have Fun"

Originally posted by Lorenfb
Anh911 has mis-lead you & the others with hypebole as usual.

First, the DF terminal to the rotor slip rings should read 6 ohms
to the case when the shaft is turned and at rest. Those early
alternators with the red plate (Marchal 55 amps) should "start"
at less than 1500 RPMs and the light should be "off". Any higher
RPM indicates a bad alternator or a bad connection to the battery.
The Bosch (55 amp) alternators start at about 800 to 1000 RPMs,
and are much more reliable.

Those Marchal alternators if not rebuilt properly can be intermittent,
since the diodes are pressfit into the SMALL diode plates. This results
in a potential intermiitent connection. This alt. is also very "weak"
and prone to fail if the battery is a "monster" (928 type) and/or if
the regulator charges greater than 13.8 volts at the battery.

To improve the alt. startup RPM, solder a 50 ohm 3 watt resistor in
parallel with the light on the cluster unit. Just pull the light out and
add the resistor and then reconnect the wires. This will provide more
startup current for the alt., as the black relay for the defroster draws
some from the light besides the alt. rotor, which makes the startup
worse.

By the way, anh911 come up with some intelligent ideas other than a
loose fan belt.

[Lorenfb]

For that year car, there was only the 55 amp. If you change the fan housing,
you can use the later 70/75 amp alts. You can also use the 3.2 alt. (90 amp)
(5mm) studs with the later fan housing (still different than the 70/75 amp).

If you have an ampmeter (clamp-on type), try and measure all the currents
your car has, as an example:

1. A.C. clutch - 5 to 8 amps
2. fuel pump - 6 amps
3. CDI - 3 amps
4. lights - 6 amps
5. sun roof - 8 amps
6. windows - 6 amps

These are all approximates and are presented as examples. Remember that
not all of the above operate at the same time. Have an excess capacity of
5 to 7 amps (after 10 amps to charge battery) when worst case current demand
is determined.

[RoninLB]

Originally posted by exc911ence
Some days my alternator light (red) goes out almost immediately after firing up the 911, other days, it stays on for 10+ minutes.

it's not impossible for ignition switch internal resistance to cause the light to glow. I changed my good ignition sw. at 100k miles for routine maintence.