1985 911 electrical issue - alternator or regulator?

Brian_911 · 04-18-2007, 07:14 PM

About three years back I had to replace the regulator in my 911 because it was not controlling voltage (voltages up to 18 VDC). I replaced it, and "lived with" a new condition that my fasten seat belt and parking brake lights (FSB/P) would light up at idle due to low voltage. I just recently returned from a two month deployment and car was dead. Charged the battery up and drove the car around, and noted two new symptoms:
(1) I had to rev about 1500 RPM to keep the FSB/P lights off, higher than normal.
(2) At low RPMs the Battery warning light would come on, indicating the battery was discharging. This was new and different.
Note all three (battery, FSB, and parking) all cleared at higher revolutions. After a total of about 45 minutes of driving while dringin at highway speed (should be plenty of RPMs to support engine ops), the car stalled for lack of electrical power. My question is how do I differentiate between a faulty alternator or a faulty regulator? I'm leaning toward the regulator, but would like to be able to make a clean determination. Thanks for your help! My battery is on the charger right now so I can go "rescue" the car to get it home.

Brian

stealthn · 04-18-2007, 09:11 PM

Brian,

Yet your alternator refurb'd, cheap insurance, my opinion is this is your issue. It's worth it for the long run anyway...

Cheers

rusnak · 04-18-2007, 09:26 PM

hmm...if you have a voltmeter or multimeter, I'd check your battery condition to make sure you don't have a battery that is shorting out or only partially charging. Should read 13 volts with key off. If you're reading 14 to 15 volts at idle, then it's not your alternator or regulator.

A good shop should be able to test the alternator before rebuilding it.

ipacketeer · ipacketeer's Garage

How low is the low RPMs, if it is too low <500 RPM, I suspect the alternator warning light may come on. If the engine is running around 900 RPM light should not come on. Otherwise you have a situation that is drawing a lot of juice or Alternator/Regulator malfunctioning.

Brian_911 · 04-19-2007, 04:09 PM

Battery checked out fine at the garage. I figured that since it died at high RPMs, though I am curious as to why the low volt warning lights were not on. I'll admit, I'm still puzzled.

Keep the ideas coming!

John Leaman · 04-19-2007, 05:33 PM

Alt belt slipping???
Just a thought.
Next question, are you the Black SC with whale tale?? That's the only one I've seen on NSB other than mine. 89 Targa Black.

Cheers
JR Aka XO

Brian_911 · 04-19-2007, 10:28 PM

Okay, another piece of data. Fully charged the battery, put it in the car, 12.5 VDC before cranking. Started car no problem, 12.5 VDC at idle, 12.5 volts DC at 2000 RPM, 12.5 VDC at 3000 RPM. From my perspective that is definitely something in the alternator / regulator complex. I'm leaning toward regulator since I replaced that about three years ago.

Brian

niall · niall's Garage

Finally something I may be able to offer some advice on.

I recenly had my alternator rebuild due to similar read outs at the battery terminals.

Ignition off car engine stopped reading 12V
Ignition on engine running 900rpm reading 11.5V
Ignition on engine running 2000rpm reading 11.5V

Immediately I knew it was an alternator or regulator problem, so I removed the unit from car had it overhauled at shop and now I have a healthy 14V output when engine is running, no more problems.

Niall

jmohn · jmohn's Garage

Since you've had the battery tested and know it's "good" (I assume the shop "load" tested it and it's relatively new), the problem must be with the regulator/alternator (your '85 has an internal regulator) or the connections between the alternator and the battery. I'd start by ensuring all connections (including grounds and idiot light) are clean and secure if that doesn't help the next step is to remove and rebuild the alternator which should (you can insist) include replacing the regulator. You could be getting marginal performance from the alternator due to a variety of issues (bad diode, brushes, cold or cracked solder joints, slip rings, etc.) the end result is the alternator needs replacing or rebuilding.

Good luck,

Jerry M
'78 SC

304065 · 04-20-2007, 07:05 AM

Brian,

A couple tips before we begin: hit the "user cp" button on this page and modify your autosignature with the year and model of your car. That way everybody will know what electrical system you have. Also on that line, there's a search button, hit that and enter a few key terms and you will uncover a whole bunch of threads, like 100, on this very topic. Hope you don't mind me mentioning this, I am trying to be helpful.

And now to electrics. First of all, I would be very concerned that someone has been monkeying around behind the gauges of your car, and inadvertently wired the warning lamp circuit to the fasten seat belt and parking brake lights. The ONLY lamp that is supposed to come on is the alternator warning light, which is located in your oil pressure gage to the left of the tachometer.

The rotor of a 911 alternator has very little residual magnetism, which means that it is incapable of inducing a current in the stator on its own to begin the charging process. It has to get current to start charging from the warning lamp circuit. You turn the key and current flows out of the battery through the ignition switch into the bulb holder for the warning lamp through the bulb (lighting it) back through the blue wire to the voltage regulator. The voltage regulator senses that the voltage on the blue wire is only about 12V, which is less than the 14V "set-point" where the regulator stops charging, and therefore it sends 12V to the alternator rotor, which magnetizes and induces a current in the stator, which is rectified by diodes and fed back into the blue wire as a 14V output, which then "balances" the current flow through the bulb and the warning light goes out.

So anything less than 14V output measured at the battery is a result of one of the following items:

1) Problem with charging circuit. Blown bulb, improper wiring, high resistance due to corrosion of terminals in the "blue wire" circuit.

2) Problem with voltage regulator;

3) Problem with alternator, either brush wear, diode problems or something worse; or

4) High resistance due to corrosion in cable from alterator to starter, starter to battery, or battery clamp.

All diagnosis must be done with a voltmeter. You have one, and have measured 12.5V, which indicates an almost fully charged battery, so you can eliminate that as a problem. (a battery showing 12.2V is only 25% charged, and less than 12V MUST be returned to 12.6 or so with an external battery charger, NOT the car's charging system)

So now it's time to look at the blue wire circuit. What year and model of car, and any electrical add-on's? Need that to tell you what to look for.

Brian_911 · 04-20-2007, 11:05 PM

John,
Thanks for the advice, I'll add that to my signature, though I did put that in the thread title. It is a 1985 911 Targa. While a miswiring issue could be a cause, it would have to be in conjunction with something else going wrong as well as the car has worked fine for about three years since I replaced the regulator in the alternater.

I'm pretty sure it is in the alternator / regulator, as I measured voltage at the battery while my son ran up the idel for me. Prior to start: 12.5 VDC at idle, 2000 RPM and 3000 RPM. I think I'll pull the alternator and regulator and have them rebuilt.

Brian_911 · 06-23-2007, 11:07 AM

Finally got back home and had a chance to get into this. Problem was alternator, copper slip rings worn away, so I'm replacing alternator.

Wil Ferch · 06-23-2007, 03:31 PM

Worn slip rings and commutator brushes are indications of a garage queen...typically...a car that isn't driven much and the "load" put on the alternator is much more than the "topping-off" capability of the car's alternator. No surprise many Porsches need alternators at unusually low mileage ( 40-60k miles).

Suggestion...have your alternator checked, and if OK have it rebuilt with new slip rings, brushes, etc....should cost you less and you "know" what you have.

See my tech article on alternator/regulator R&R...

http://www.pelicanparts.com/techarticles/911_volt_reg_replace/911_volt_reg_replace.htm

- Wil

Lorenfb · 01:03 AM

"Worn slip rings and commutator brushes are indications of a garage queen...typically...a car that isn't driven much"

DO WHAT???

Absolutely not! Those are ONLY a result of RPM X TIME, i.e. the how much time
the engine is run and RPM or basically the total miles (for a 3.2 ~= 80K-110K).

"http://www.pelicanparts.com/techart...reg_replace.htm"

Avoid step #11, as this is a Mickey Mouse (butcher mechanic) approach to
a 911 alt. removal.

USE A BLOCK OF WOOD TO "SHAKE" THE ALT OUT OF THE HOUSING.
(The block of wood - 2X6 is placed on the ground.)