Intermittent Generator Light

[80SCBOY]

Hello,
I have a 1980 911SC with the updated intergral voltage regulator. Over a period of two years, I have rebuilt the alternator, throughly tested an checked the battery and all associated wiring, removed and retested the alternator and recently replaced the ignition switch. The light goes off for periods of time then will suddenly illuminate, along with the oxygen sensor light. The last time was while driving the car at freeway speed and hasn't gone off since. Now it is glowing at fast idle then gets brighter at lower rpm so I believe something is happening. Battery voltage is 12.34V - 12.54V with engine not running, same with engine running (higher voltage after using battery charger, then letting car sit overnight). I have pressed the reset button on the oxygen sensor numerous times seeing how it is in the circuit to no avail. Questions for you all:
1. There is a technical service bulletin #82-02 that recommends installing a resistor (91 ohms, 5 watts) across the generator light. Has anyone tried this with any success?
2. Has anyone dismantled the oxygen sensor mileage counter and found a problem in there that could cause the light to come on? Since the two lights are in the same circuit there must be some connection.
3. When the oxygen sensor mileage counter hits 30K miles, does it cause the generator light to come on also? Has anyone witnessed this?
4. I searched through past posts to try and find a similar problem but came up empty handed. Does anyone have a thread to recommend?
Thanks, Jim

[Ian Comerford]

Jim,
My condolences, I have a similar problem on my 79 SC (UK spec). The alternator light and the brake warning light glow dimly when the engine is running at any speed. I have had the problem for 8 months and the car still starts and runs well. At 3000 rpm with the headlights on the voltage at the battery is 13.57, within the limits of 13.5 to 14.5. There is no fault with the brakes, indeed the warning light glows bright when you remove the cap from the fluid reservoir. I have 3 threads to follow; faulty alternator, faulty earth (ground), faulty alarm. Having checked out the charging side and the earth and only found one minor fault, I am now leaning toward the alarm's built in battery being faulty. I'll let you know the result when I have checked it out, let me know how you get on.
Regards
Ian

[304065]

Jim,

There are separate circuits for the oxygen sensor light and the alternator warning lamp. If you are only measuring 12.54v at the battery with the engine running, it means your alternator isn't putting out voltage, which is why the charging lamp is on. This means that it's one of three things: alternator, VR or the "blue wire" circuit.

Ian, it sounds like you have a different problem. You have good alternator output voltage. The brake warning light is glowing for a different reason, it's not part of the alternator charging circuit. I would suspect a faulty warning light switch that is leaking current due to oxidation.

[Ian Comerford]

Thank you John,
This gives me another angle to investigate, but do you think that both warning lights are faulty duue to oxidation? Both the alternator and the brake warning light glow dimly all the time, nothing seems to change this.
Grateful for more ideas.
Ian

[Ian Comerford]

The latest attempt at solving the problem saw me completely disconnect the alarm siren unit, which houses its own re-chargeable battery, from the electrical system just in case the siren's battery was faulty. No luck and no change to the two dimly glowing lights that I have on my dash.
Any more ideas please?

[david914]

First things first. Unless you have replaced your gauge cluster with an '82 or later with the 4 watt indicator lamps, you need to install the resistor across the alternator lamp or you will have problems! Try the resistor and let us know if you still have a glowing indicator lamp.

[304065]

A 2W lamp is enough to excite the D+/61 circuit. If excitation isn't occurring then I would check the blue wire circuit, do a search on the words blue wire.

[Lorenfb]

How about voltage drops from the alt. to the battery?
The other problem could be that you got one of those junk Paris-Rhone
alt. used as a OEM part in '82 & '83 and as a replacement for the earlier
cars. That's the worst alt. Porsche ever used. Most of the time they're
never rebuilt properly.

[80SCBOY]

Here's the latest update. I took voltage readings at 3 locations; A: Ignition Switch side of Gen Light, B: Blue wire side of Gen Light, C: At old External Voltage Regulator Plug (yes Lorenfb, I have a Paris-Rhone replacement alternator with the internal VR).
With the key on but engine not running I measure 11.8v at all three locations (remember battery is not getting charged so it's a tad low)
When the engine is started, I measure 11.7v at loaction A, 4.3v at location B, and 4.25v at location C (this seems to be the problem).
Next, I take a jumper wire and connect one end to the rear window defroster fuse holder where 12 volts is available and touch the other end of the jumper to the blue wire connection at the old VR plug, location C. Guess what! the alternator kicks in and is now putting out some current and voltage. I disconnected the 12v jumper from loacation C while leaving the engine run and take my measurements again, noting the red light is not lit. I get 13.9v at location A, 14.4v at location B and 14.38v at location C. The alternator is working properly.
I have been unable to find the 91 ohm 5 watt resistor that is recommended to be put across the generator light, apparently no dealer in the united states has one on stock. I have found a 100 ohm 5 watt unit at my local electronics part house but haven't installed it yet.
John, I think I may have some minor problem with the blue wire circuit, perhaps a bad connection somewhere, Other than the connection at the bulb, the 14 pin connector and the one on the back of the alternator where else would the blue wire have a connection? There must also be a splice or solder connection where the old VR connector comes off in the engine compartment.
Does anyone know the exact route of the blue wire or where to look for potential trouble spots where the insulation may have gotten cut or anything like that? I'm still puzzled by the voltage readings (good when key on and engine off, bad when engine running).
Jim

PS I am doing a "blue wire" search in the meantime to see if I can find additional info. If any of you know of a thread, let me know.
Thanks for the help, this is a great forum!

[304065]

A= red with black stripe before bulb holder = 11.8 > 11.7
B = blue wire after bulb holder = 11.8 > 4.3
C = blue wire (I assume) at cannon plug = 11.8 > 4.25

OK, you should be seeing 12 and change with engine off and ZERO volts with the engine running.

So you jumpered the blue wire circuit and got the alternator to boostrap. Bravo, not many guys have ever done that and been successful. (I'm serious) So now you know you have good alt and VR, you just need a good blue wire circuit.

You know you have a good circuit from the old VR cannon plug, into the engine wire harness, out to the alternator and to ground. That's what your successful bootstrap test showed.

From the cannon plug the blue wire goes into the body electrical harness through another 14 pin connector. That's a good trouble source: it's hidden out of sight under the electrical console and gets pretty dirty. Take pictures of everything and extensively diagram where all your wires go. You seem pretty thorough so this shouldn't be a problem. Then remove the electrical console by disconnecting the 14 pin connectors and the bolts. Don't forget the ground strap sandwiched between the wall of the engine compartment and the console. With it out, you should be able to thoroughly clean and inspect the wires and connectors. Then you need a continuity test on the blue wire circuit-- using a long jumper and your VOM, test for a high resistance between the blue wire side of the bulb holder and pin #11 of the 14-pin connector. Clean all the oxidation off the bulb holder terminals with an ink eraser and some electrical contact cleaner.

You can get the resistor online from Radio Shack. Shipping cost to your location will be many times higher than the price of five of the resistors.

Good luck!

[80SCBOY]

John,
Thanks for the response but I'm confused on a couple of items.
1. Are you saying that with the engine running I should have zero volts at location B and C? Wouldn't that cause the bulb to illuminate? During my testing, when the voltage at B and C were lower than at A, the red light was on. When voltage was higher at B and C than A, no red light.
Am I understanding you correctly?
2. I am aware of the 14 pin connector on the relay mounting bracket in the engine compartment. Where is the electrical console? Are you referring to the fuse block / relay unit in the front compartment where the spare tire is located?
3. What has happened to other people who have put 12 volts externally to the blue wire connection? The blue wire is connected to the flow side of three diodes so the current should flow into the alternator. If one or more of the diodes was bad, wouldn't that just cause the inline fuse I used to blow?
Thanks, Jim