Alternator warning light ON

[wdesbrow]

My alternator warning light (at the bottom of the oil temp/pressure guage) is now faintly on when I drive the car with the headlights, etc, on. It tends to flicker weakly when idling. Also -- although I don't have much experience with seeing them operate at night -- seems like all the dash gauges show unusually dim (They do NOT seem to fluctuate in brightness at idle).

I realize the problem could be a loose or broken fan belt, but that is probably unlikely (the car recently came out of a complete engine re-seal from an experienced mechanic).

Any help is appreciated. THANK YOU.

[71gold]

Was there any issue with warning light before the engine rebuild?
Was Alternator rebuilt as part of engine work.
Check all ground wires to body bolt at drivers side engine compartment.
Tension correct on belt with spacers correct etc.
Could be many possible faults but always start with areas related to most recent work

[304065]

What is the voltage measured at the battery with the engine OFF?

What is the voltage measured at the battery with the engine running AT IDLE?

What is the voltage measured at the battery with the engine running AT 3000 RPM?

Post the answers to those three questions along with a photo of the engine electrical console and we'll have your answer.

[wdesbrow]

71 Gold and 304065,

Thanks much for your quick response. I had to wait to get to a technician to answer some of your questions:

Batteries with the engine off: 12.2 v.
Batteries with the engine at idle: 11.8 v.
Batteries with the engine at 3000 RPM: 11.8 v.

Given the above -- The technician tells me I have been slowly LOSING power (charge). I just returned from that testing -- and the car now refuses to turn over!

I don't know what you mean by the "engine electrical console. (?)"

There was no issue with the warning light prior to the engine rebuild.
The alternator was not involved with the recent work.

At present I am trying to get into my Porsche mechanic's shop so he can check for lose connections and belt tensions, etc.

Do you have any conclusions n the meantime? THANKS AGAIN.

[john rice]

Clearly it is not charging. You probably have either a bad alternator or voltage regulator.

Without knowing what exact year, etc you have we can only guess about specifics. If you don't have an owner's manual for the car you should get one. Older 911s have good and detailed information that can be very helpful. BTW it is unlikely a loose fan belt. If it were loose enough to prevent charging it would make lots of noise. It if were broken your engine would be DEAD in short order. The fan belt drives the cooling fan- which is attached to the alternator [You should also have a decent tool kit as well as always carry a spare fan belt.]
Good luck

[wdesbrow]

Clearly it is not charging. You probably have either a bad alternator or voltage regulator.

Without knowing what exact year, etc you have we can only guess about specifics. If you don't have an owner's manual for the car you should get one. Older 911s have good and detailed information that can be very helpful. BTW it is unlikely a loose fan belt. If it were loose enough to prevent charging it would make lots of noise. It if were broken your engine would be DEAD in short order. The fan belt drives the cooling fan- which is attached to the alternator [You should also have a decent tool kit as well as always carry a spare fan belt.]
Good luck

John, Many thanks for good input and advice. Hopefully, with some goo help today, I can whittle-down the problem and fix it. My car is a 72S Targa. Does anyone know if there are any choices/recommendations when it comes to replacing alternators and regulators. I am trying to keep my car as period-correct as possible: are there any choices to make between period-correct parts and more modern, technically improved versions that might be more much more dependable or stronger/faster-charging?

THANKS, all.

wayne

[edmayo]

If in fact your alternator is bad, either get that unit rebuilt or a replacement of like kind. There is a later alternator that is considered a supersession, however it has the regulator built in with the alternator and requires some minor changes to wiring and duct work,,,,and it is more prone to failure.

[wdesbrow]

If in fact your alternator is bad, either get that unit rebuilt or a replacement of like kind. There is a later alternator that is considered a supersession, however it has the regulator built in with the alternator and requires some minor changes to wiring and duct work,,,,and it is more prone to failure.

Great information -- and extremely timely. THANKS ED! BTW do you have a preferred source (s) for refurbished alternator or regulator replacements of this type? Should I assume that if it's the regulator, replacing it with a new one is the preferred way to go?

[edmayo]

I am fortunate in that we have an old shop here that does excellent rebuilds (Ft. Worth Starter and Generator, that should tell you how long they've been around!) although they sometimes have problems getting parts for the Marchal alternator which is what you could have. Depending on what regulator you have they can sometimes be adjusted to raise the charge rate, but yes, new is usually the better choice. I don't have a source for an alternator for reasons I mentioned above, so it will probably take some phone calling and/or internet checking to find a good source for rebuilt units. You won't find these new anymore.

[304065]

Ok W, I'm going to tell you how to tell whether it's your voltage regulator or alternator that is defective. Before proceeding with this diagnosis you need to understand that if you do this wrong, you could start a fire that could burn the car down, or electrocute yourself, or damage the alternator, the CDI box, the radio or other electronics in the car.

If you don't feel confident in your electrical skills or are concerned about the risks, either consult a qualified professional or use a "parts changing" approach which involves buying a new voltage regulator for $20 and connecting it to see if it's the VR that is the problem. Modern solid-state voltage regulators such as this one made by Beru are inexpensive and durable and it pays to have a spare in the car at all times.



Now. . . with that disclaimer out of the way. . . to the real diagnosis.

As I have written here many times before, the charging circuit takes current from the battery and passes it through the warning lamp bulb back to the voltage regulator through a blue wire.

This blue wire is connected to the VR at terminal D+, and it continues to the D+/61 terminal on the back of the alternator. The VR has two other terminals-- DF (for Dynamo Field) and D- which is the ground.

The alternator creates power through the rotation of the Rotor which contains a few windings of electrical wire that act like an electromagnet. The rotor produces an electric field which induces an alternating current in the windings of the Stator. This alternating current is rectified by three large diodes into a pulsing DC voltage that charges the battery.

Also in the alternator there are three additional diodes called the "Trio" which are smaller versions of the main diodes. They are connected to the same three stator windings and they produce a voltage at the D+/61 terminal.

When you turn the key on the rotor is not turning, so the voltage regulator connects D+/61 (the blue wire) internally to DF-- so that current flows from the blue wire into the rotor's field windings and then to ground. This completes the circuit and the bulb illuminates.

Once the rotor starts turning, +12 volts appears at the D+/61 terminal on the alternator, which balances the voltage from the battery. With equal voltage on both sides of the bulb no current can flow and the bulb goes out. When the output voltage from the trio drops below a certain point (which is set to produce a 14.4v output from the B+ on the alternator's main diodes) the VR turns the field on and the output rises until +12v is present at D+/61 again, at which point the field is disconnected.

By rapidly switching the field on and off the voltage output of the alternator is maintained at a constant level through "pulse width modulation." It's literally an on-off switch-- old voltage regulators, like the one your car came with, were solid state, basically a calibrated relay that would switch DF on and off to maintain the set-point.

Ok. . . that theory is important because it explains what to do next-- you are going to "full-field" the alternator or connect D+/61 on the female plug on the harness to DF. This will send the full voltage to the field, which will immediately cause the alternator to go to maximum output. You can then measure the voltage at the battery to verify that it is above 13v or so, which tells you that the alternator is working, and the voltage regulator is defective. You should only connect it for a couple seconds then immediately disconnect it as you don't want to overheat the alternator.

If after the full field test you don't see the voltage rise, it means the alternator is defective, you should remove it and have it rebuilt.

I just did this very procedure with my '66 to confirm a defective VR. . . but my car has no modern electronics and kettering ignition. So again, for a lower risk alternative, just buy an inexpensive Solid-state VR, connect it up and try . . . I am not a fan of "parts changing" as a form of diagnosis but it works well.