6 ohms between the back of the warning light assy and the blue/red at the altenator. Should it be 0?

John Walker, we can abandon the PM chain and keep the discussion here if you would like.

What's the best way to trace the wire? Is the only break in the system at the voltage regulator (bulb to regulator then regulator to alt)?

In thinking about this more, should I just install a wire between the lamp assembly and the D+/61 terminal (to simulate what the blue/red wire does) and see if it lights? I suppose this could even be a long term fix...or is the regulator a required item in the path of the current?

Chris, in a stock 83 SC the regulator is in the alternator, so the "blue wire" runs straight from the bulb holder to the D+/61 terminal. It goes through a couple of connectors along the way which is probably where your problem lies.

So then this piggyback wire should light the bulb? I should have done this test when I checked the resitance but I didn't. Now I need to expose that back of the alt again.

What's the aluminum finned box next to the fuel filter? I thought that was the regulator and was assuming the blu/red wire goes into that and then onto the alt.

The aluminum box is the Bosch CDI ignition unit, it has nothing to do with the charging system. There are six wires going to it, none of them is blue/red. According to the wiring diagram, the D+/61 wire should be only blue.

Can you post some pictures? It seems like we are getting off into the deep end where a PO may have helpfully rewired with the wrong color making diagnosis like this impossible.

John, I haven't done anything with the CDI box.

My point with the CDI box was that I thought the wires went in there and I would check for break there...but clearly I'm wrong and the CDI box can be left out of the discussion now.

I have not checked any connections or wire colors anywhere but at the back of the lamp assembly and at the D+/61 terminal.

What are the connections that take place along the wires path from the back of the lamp assembly to the end that connects to the D+/61 terminal? Is it limited to the main harness connection or are there others?



I owe you several beers at the Glen in June.

John and John,

Any thoughts as to the best next step?

Chris, sorry to bring up an old thread, but I was wondering what your resolution was. You see, I have an 88 Carrera and need to start troubleshooting why my alt light does not come on.

My alternator does not excite until about 2700 RPM and then it charges fine, even at idle.

My alternator warning light does not come on, regardless of any variables. I pulled the VDO gauge and tested the light for continuity. The bulb is good (0Ω), and I also get 0Ω when I test the socket with the bulb in it. The connections are non corroded and the car is very clean, so I doubt a broken or corroded wire.

What should I test next? Should I just pull the alternator and check there after disconnecting the battery?

This has all the symptoms of an open in the warning light circuit.

The current through the light supplies the current through the field winding when the alternator is first started. After the alternator is spinning the diode trio provides the DC field current and the light goes out.

Usually in an alternator that has previously been used there is enough residual magnetism in the rotor that some current is generated at higher RPM and the diode trio starts supplying current to the field, just like it started up with currnet from the light.

To trouble shoot do exactly what JW suggested above. Verify 12 volts to the bulb, verify the blue wire goes back to the alternator. I like the step of grounding the blue wire and seeing the bulb light up.

Does the warning lamp bulb holder have a resistor across it per the bulletin to retrofit alternators with integral regulator to older cars?

If we assume a problem inside the bulb holder ... with the bulb not making good contact, then only the resistor is proving the excitation current, which would be lower than when bulb and resistor are in parallel, then the zero-Ampere speed would be higher than normal, and 2700 rpms would fit the pattern.

http://www.pelicanparts.com/catalog/..._tech_art1.jpg

http://www.pelicanparts.com/catalog/..._tech_art2.jpg

I had the exact same problem. I installed a volt gauge to check it all the time and noticed that it did not start charging untill I started to drive. I did all the tests and nothing worked. The blue wire was perfect and everything checked out alright. But the light would not work like it was supposed to.

I took the alt out and removed the regulator. I noticed my brushes were worn but not too far and my copper plates on the alt had grooves in it.

After months of working at it, nothing I did would fix it. Since my alt and regulatior were old anyways I bought a rebuilt alt and new regulator and installed it.

Problem Fixed! The red light and charging system now work just like they should. I guess I could have taken my alt to be rebuilt, but it was the type with only one wire on the regulator and not two.

I hope this expensive tip helps,

How old (miles) is your alternator? This may be a likely source of your problem also. Checking this requires removal of the alternator.

In my 88 cab the light worked OK but I had the flashing headlights syndrome at 60,000 miles.

Well, my alternator was rebuilt by the previous owners' mechanic in 2003, so I'm guessing that something is up at that end. Thanks, everyone, I'll start checking all these things out and report back.

murphy,

Well, worn brushes shouldn't show up in four years or less, but if they are marginal enough not to have continuity at rest ... sometimes vibration from the running engine will cause contact to start happening at the slip rings, and some Voltage output at higher revs.

Now that's interesting.

At the risk of hijacking the thread ... I went through charging light problems with my 73.5 a few weeks back

http://forums.pelicanparts.com/porsche-911-technical-forum/325072-73-5-charging-light-problem.html

and ended up replacing the mechanical voltage regulator with a solid state, and having the commutator on the (recently rebuilt) alternator turned.

It's all working great now, except that I've noticed the light doesn't go out at first start in the morning until I rev the engine to 2000 rpm for the first time.

I don't consider that a big deal, but since it's mentioned here ... the warning light comes on fine when the ignition switch is on but engine is off.

Perhaps the characteristics of the replacement solid state regulator are just different enough from the mechanical that I "should" replace the bulb or put on a resistor?

Chris,

Since I assume you are limited to the smaller 2 Watt bulbs, that is not a bad idea to install a resistor. <b><i>The Bosch Automotive Handbook</i></b> has a bit of info on the Zero-Ampere speed for alternators ... and says you can go as low as 68 Ohms at 5 Watts.

Yes, I'm limited to the 2W bulb. I'm just wondering if this is really worth it? The light goes out after the blip of the throttle.

Thanks!

The first try in my 88 cab I just replaced the regulator. All the usual excuses, I bought the regulator before I took the alternator out etc.

The slip rings in mine were in the condition described by rockaria . The new regulator fix, with new brushes, only lasted about 6 months before the headlights started flashing again.

The rebuilt alternator I bought the second time around seemed to have new slip rings and has worked fine since.

I wonder if your Previous Owner's rebuild was what I did or what the offshore rebuild company did. You might have to take it out and have a look. At least the wires will be easy to test with the alternator out.

Chris, from what I understand, the alternator must be "excited" to charge. On my 88 Carrera, the current flowing through the warning light bulb is enough to do this, so when the bulb is not closing the circuit, the alternator doesn't get excited until the alternator is spinning fast enough. So, really, it's normal for alternators to not charge until spinning fast enough.

Not sure if this helps, but it's interesting nonetheless.

Yup, thanks, I understand that. It appears that the alt gets plenty excited after driving off, just not at the very beginning. Or is that a too-simplistic interpretation of the charge light?

I admit to having odd turn signal behavior at stop lights (the signals work fine, the indicator in the tach doesn't). So perhaps the alternator does need a little more excitement in its life. (Boy, it's already in a 911, what more does it want?)

Yes, I should measure the idle voltage. It would tell me. I will.

Thanks, all.