Alternator Trouble Shooting
 

I have a 73.5 911 T with a Marcel alternator and regulator. The previous owner had the alternator "rebuilt" by a local shop 6 years ago. I have had no problems except the red light would glow dimly at idle but go out at speed. Very recently and suddenly, it glows red at idle but only dims a little at speed. I have checked the connections, grounds, bulb socket, etc, as noted on prior posts. There is no resistance between the blue wire and the VR. I feel it could be a bad VR or alternator. My next step should be to test the VR. Can someone tell me how to do this? Thank you

If you suspect a bad voltage regulator but are not sure whether it is the regulator or alternator that is causing the problem, disconnect the plug on the bottom of the voltage regulator. Connect a multimeter set to DC volts to the battery terminals. The reading for a good battery should be at least 12.2 volts. A fully charged battery is 12.6 volts. Run the engine at ≈ 2000 RPM and momentarily connect a jumper wire from the positive side of the battery to the field wire (black and marked “DF”) in the regulator plug that you disconnected. The voltage reading should increase to ≈ 14-16 volts. Anything less indicates a bad alternator. If the reading is more than 16 volts, it is most likely a bad regulator.

Just knowing the battery voltage at 2000 RPM would be helpful. It sound like one of the small diodes that provides field current is shorted.

Over 17 years ago I had the charger light glow at idle and go out as RPM's were increased. Troubleshot the problem down to the three small exciter diodes in the alternator. They disintegrated as soon as I touched them from years of high heat. Bought a pack of 1N4000 diodes from Radio shack and soldered them into the mounting circuit board in the alternator. The 1N4000 are a general purpose diode and they are still working today. while I was in there I changed the bearings and brushes. For me it was a very cheap fix vs buying a new or reconditioned Marcel alternator.

Hell, just go out and buy a new solid state regulator...

Unfortunately unless you have an oscilloscope there isn't a good way to test for a bad diode. Measuring voltages only gives you the average reading but won't tell you if all three phases work properly.

A bad (short) diode in the diode pack causes ripple on the alternator output and this can make the charge light glow. I had this on my original 74 engine and it showed up on he oscilloscope.

Maybe time to get a new diode pack.

Ingo

Any DMM can be used to test for a shorted diode within the alternator, an open diode does not necessarily result in adverse operational indications.

The quick and simply test of the alternator for the '74 is to disconnect the regulator and jumper
the D+ and DF pins on the connector. With the key "on", the alt light should glow and then go out
when the engine starts. The engine RPM may need to reach 1000 to 1200 RPM before the light goes
out. Once the light goes out, slowly raise the RPM and monitor the voltage, i.e. it should reach > 16 volts.
Avoid allowing the voltage to exceed 16 volts by revving too high. A glowing light may indicate
an open diode.

Here's a test posted on Pelican:

Using the alt. light as a diagnostic tool:

1. no light with ignition "on" not running
a. bad regulator
b. bad light
c. open slip rings
d. bad brushes

2. light stays "on" with key "off" - shorted diode/diodes

3. light glows brightly while running
a. open diode/diodes
b. shorted rotor/stator winding
c. shorted alt. light wire to ground
d. bad regulator

4. light glows dimly & gets brighter with higher RPMs - voltage drop,
e.g. @ alt. B+ post or @ starter connection

5. light flashes at low RPMs - bad slip rings

6. light glows dimly at all RPMs - open diode/diodes

here is a diagram (ignore the short line). #6 above (and possibly #4) would be one of the diodes attached to D+ bad.
D+ and battery should be at the same potential (two diode drops, one in reverse)
if you measure the charge voltage and it is normal it would narrow it down.

I just removed the DF wire (mine was RED), ran a jumper to the + terminal of the battery,
and measured the voltage:

At rest 12.8 V Light Off
2000 RPM 13 V Light Bright
2500 RPM 15 V Light Dim.

I think this tells me the alternator is going bad?? Car seems to run fine, not losing cranking power for cold starts. Trouble is I am driving this car 600 + miles to Porsche Parade in 4 weeks. Don't want to chance it. Seems like the "rebuild" done 6 years ago not holding up??

Does the alternator light lit with DF disconnected mean one of the diodes connected to D+ is shorted?

Did you measure 15 Volts at the battery?

The above were measured at the battery

We do have an alternator trouble shooting article here that you can take a look at. Maybe it'll be helpful to you for figuring out your issue. Good luck!

Every porsche should have a voltmeter... These alternators are flaky...

Go to you local auto store, buy a voltmeter that plugs into your cigarette lighter...

Cheap, and let's you know what going on...

Yes, a voltmeter is a good idea. I found one that displays voltage as well as temperature. It's round so it blends in well with the adjacent knobs. I think I paid about $10 for it.

I've been chasing a similar problem for months now. Everything works well at first but after about 15 minutes the voltage drops from 13.7V down to the battery voltage, 12.2V. I checked the wiring, connections, voltage regulator but, the problem being intermittent, couldn't identify the source of the problem. Getting frustrated, I pulled the alternator out and brought it to a specialized shop. They told me the alternator was 'working' but was in bad shape, needing new bearings and new brushes. They said the brushes were probably the cause of the problem, loosing contact with the slip rings once they warmed up.

What really surprises me is that, in all the charging system troubleshooting threads on this forum, I haven't seen anyone mention brushes! Seems obvious now; if the brushes stop conducting, the rotor looses its excitation and the alternator stops working. I'm installing the alternator back in the car next weekend - hopefully all will be good and I can stop looking at the voltmeter every second when I drive!

Brushes initially can get intermittent. They stick and vibration shakes them loose to make contact. Excessive carbon prevent them from sliding in their slots. It's an easy fix when you get a replacement assembly. Some even replace the individual brushes and clean up the slide mechanism. Do a search here - I remember having seen pictures here.

Ingo

Carbon doesn't buildup at the point of the brush's contact, and there should not be any
"slots" for the brushes to "slide in". "Slots" indicate badly worn slip rings which need
to be machined (turned) in a lathe or replaced and then turned.

Correct! Two more tests:
1. Measure the field resistance of the alt (DF to case) SB 4 - 6 ohms.
Then spin the rotor and again measure the resistance while spinning.
2. Remove the reg and jumper DF to D+ on the reg connector.
Light should remain out (no dimming) at all RPM above 1200.

If either of the above fail, alt needs rebuilding.

I sent the alternator out. They replaced 4 diodes. Red light still on brightly at all RPMS. One battery was bad. New one on order. Checked all grounds. What next??

Does the rebuilt alternator charge the battery?

how was this test performed?