Alternator light doesn't go out after startup

[kerthunk]

This is a tech note for folks that are having an issue with the alternator light not going out after the engine starts on 74-83 models. The typical symptom is that the car starts normally but the alternator light stays on until you blip the throttle to 1500rpm or so. After you blip the throttle, then the light goes off and all is normal. If you put a voltmeter in the system, you will see that you are running on battery power until the alternator light goes off. The symptoms usually follow the replacement of the alternator or voltage regulator.

There seems to be quite a few folks that are just living with this quirk, but for those of you that find it annoying, here is the solution. There is a Porsche Technical Bulletin that deals with switching alternators or voltage regulators to internal ones. Here is a link to a pdf of the bulletin:

http://members.rennlist.com/emcon5/Tech_bulletin_1.pdf

On the 2nd page, there is a little note referring to this issue:

If battery charge indicator lamp does not go out after starting the engine, the exciter current is too weak because of 2 watt lamp. In this case, supplied resistor, part number 911 641 981 00 (91ohms, 5 watts) must be installed parallel to battery charge indicator lamp.

Unfortunately, the parts referred to in bulletin are not available and the instructions are a little more than vague. So, I have taken it upon myself to document this process so that others that choose not to live with this quirk don't have to.

The first thing we need to do is find the parts. For this task, I went to Mouser Electronics. These are the parts that I grabbed:

TE Connectivity Terminals 250 RECP W/ DBL TAB
Part #571-726308-1
Link

Xicon Wirewound Resistors - Through Hole 91ohms
Part #280-CR5-91-RC
Link

You need 2 of the terminals and 1 resistor. The parts should end up costing less than $2.50. You may consider picking up an extra of each in case you screw something up.



Now, you want to solder the resistor to the outer terminals as shown.



I'm not a fan of having bare terminals behind the dash, so I hit it up with some liquid electrical tape (available in the electrical section of Home Depot, Lowes, etc).



While it is drying, we can go to the car and pull the alternator light socket out of the back of the gauge (be sure to remember which wires go on each terminal). We just need to slide our new doohickey onto the socket for the alternator light.



Just need to go slide it back into the gauge now. It seems to fit well with the resistor towards the bottom of the gauge.



Now, we just re-install the gauge and fire up the engine. Hopefully your alternator light will now go out right after starting!

[Porboynz]

Nice job

[Lorenfb]

The value is not that critical. One can also use a 100 ohm 2 watt (V^2/R, where V=13 before start). In some cases though, a 56 ohm 3 watt may be necessary. It's best to test the resistor before final
installation. Then simply just solder the resistor to the bulb spade pins directly.

[911T1969]

Will this mod
allow you to use an LED bulb?

[Tippy]

John, that is a clean write-up.

I would change your title to include that this is a write-up with a solution.

[Lorenfb]

"Will this mod allow you to use an LED bulb?"

Yes, but the resistor value may be different.

[kerthunk]

Originally Posted by Lorenfb

The value is not that critical. One can also use a 100
ohm 2 watt. In some cases though, a 50 ohm 3 watt may be
necessary. It's best to test the resistor before final installation.
Then simply just solder the resistor to the bulb spade pins directly.

Loren, you may be right about the different resistor values, but I decided to keep it simple and go with what Porsche recommends. I figure that they probably put some thought into it as a good general solution.

I would not solder the resistor directly to the socket spades for a couple of reasons. First off, the spades are not very long, so to get a secure connection, you are going to have to solder the resistor leads down next to the socket base and will still risk not being able to get the female end of the wires to securely lock onto the spade. Second is that the socket is made out of plastic and so you run the risk of deforming or melting it as you are soldering on the leads. I believe that the sockets themselves are NLA, so you don't want to mess them up!

[Charles Freeborn]

Sweet, I'll add it to my list.
Thanks!

[Lorenfb]

"I would not solder the resistor directly to the socket spades"

Done it many times to my independent Porsche repair shops' cars with no problems.
The spade connections do hold well after. And yes, it does require a little soldering skill and care.

[Brian Fuller]

I have for sale the parts to fix this issue. Could not find them on our hosts site and had to go to an electronics company. The sets I'm selling fixed my issue!

I’m selling 9 sets for $10 each plus $3 shipping (continental US) on a first come, first serve basis.
Please email me: 911-Porsche@comcast.net The company I purchased from sold them in bulk of 10. I'm not trying to make a profit, just trying to help out.

Alternator Light Does Not Go Out - Fix!

[Matt_911]

I had this issue after repainting my fan and cleaning up my alternator – turns out it I forgot to take the paint the surface where I reattached the ground wire ... rubbed it back to metal and that fixed it.

So I suppose it could potentially be caused by a bad earth on the alternator?

[Mr. Merk]

Could one not also run a 5watt bulb (if one existed?)