Alternator warning light issue

[iansdad]

Long time reader, first time poster. My recently purchased 86 911 targa has an issue with the charging system.
It all started with a bad battery (dead cell). I replaced the battery with a new unit due to a no start scenario. After replacing the battery I noticed that the alternator warning light was staying on all the time. Not knowing how old the alternator in the car was, I decided to bite the bullet and replace the alternator with a rebuilt Bosch from Pelican. I installed the new alternator today being very careful with my wiring connections, but the warning light is still on.
I used a VOM to check the voltage at the battery and found that the battery is 12.6v when the engine is off and 14.2V when running. I assume that this means that the alternator is charging, but the light persists.
From looking at the wiring diagram in my Bently manual it appears that the blue wire from the alternator goes thru the anti theft unit and then to the warning light. Could the anti theft unit be causing an open circuit that would allow the car to start but not allow the indicator light to see equalized voltage and go off?
Am I going in the right direction or is there a more logical explanation for this issue? Any help would be greatly appreciated.

[Bob Kontak]

Welcome to the forum.

First thing that pops into my mind is why, right after a battery replacement does the light go on?

Is the light constant in intensity? There are some pretty involved threads/posts that talk about if the light gets brighter with rpm it means one thing and if it gets dimmer it means another (and so on)

Suspect if you have been lurking you know how to search. If not use the search / advanced search function to find some of these threads and narrow it down.

[Joe Bob]

Multimeter on the battery posts. FRESHLY charged battery....

at rest, 12.5v+
idle 13.0
2500 rpms, 13.5-14v

Report back. Good luck.

[djcdmv]

Had this problem last fall. I had readings of 14 at idle, but 17 at 2500 rpms. It was the voltage regulator. My light would only go on during acceleration.

[Bob Kontak]

Originally Posted by djcdmv

Had this problem last fall. I had readings of 14 at idle, but 17 at 2500 rpms. It was the voltage regulator. My light would only go on during acceleration.

iansdad - did you install the original V reg? Check the voltage as djcdcm says across a range of rpms. With my limited new v reg experience (maybe three) I usually see 13.9 and it does not vary but a few hundredths of a volt. I wonder if the 14.2 is a bit high even if v reg is ok. Don;t know for sure.

Edit - See this link - seems 14.2 is a-ok.

how to test if your alternator is healthy

[iansdad]

I did not check the voltage above idle. I will do so this evening and find out if the voltage increases above the 14.2v that I saw at idle yesterday. I was under the impression that the voltage regulator was built onto the back of the bosch alternator (valeo). Is this not the voltage regulator?

[Bob Kontak]

It is the voltage regulator. Was not sure if your alternator came with a new one.

[iansdad]

I checked the voltage at the battery. The readings are as follows:
Static 12.6volt
Idle 900rpm 14.2v
2000-3000 rpm 14.3v-14.4v
Headlights on = 14.0-14.1 from idle to 2000rpm

The indicator light does not change in intensity with the rpm's. It is the same brightness as the emergency brake light and seat belt light. It seems to me that the alternator is charging and the voltage regulator is controlling the output voltage but the light is still staying on. Is there any possibility that the blue wire from the alternator to the indicator light is the source of this problem? I noticed in Bentley's that the 85 and earlier cars had a diode but the schematic on the 86 only shows the anti theft device in the circuit. Thanks again for any input.

[Joe Bob]

Does your e-brake share the same warning light? I have an SC and not familiar with the Carrera dash lights.

[iansdad]

Yes the e-brake and the seat belt warning lights share the cluster with the alt light.

[iansdad]

The e-brake light is functioning correctly as well as the seatbelt light. They both go out after the car is started and the e brake is released. It's only the alternator warning light that is staying on.

[djcdmv]

Originally Posted by iansdad

The e-brake light is functioning correctly as well as the seatbelt light. They both go out after the car is started and the e brake is released. It's only the alternator warning light that is staying on.

Is it the light directly to the right of the brake light? In my 85, it has a circle symbol with some dashes around it. If so, that is the brake pad indicator light.

[djcdmv]

Originally Posted by djcdmv

Is it the light directly to the right of the brake light? In my 85, it has a circle symbol with some dashes around it. If so, that is the brake pad indicator light.

[iansdad]

Imagine my embarrassment! It is indeed the brake pad warning light. It came on at the same time that I replaced the battery and I "assumed" that it must have been related to the charging system. My owner's manual is in bad shape so I was not aware of the brake pad warning light. You guys are great! I truly appreciate everyone's input. Now on to replacing the brake pads.... any recommendations?

[djcdmv]

That's what we are here for. I'm just glad I could actually be some help. I'm usually asking all the questions. Before you worry about the pads, try doing a search for brake pad wear indicator (or something similar). It may be a simple adjustment or sensor issue.

Also, now that I think about it, the voltage regulator problem lit up my seatbelt light...

[djcdmv]

Brake Pad warning light will stay lit

Brake Pad Wear Warning Light

Not 911, but may help

[iansdad]

Thanks again guys. I will continue to research the cause for the brake light illumination. I did get a Bentley manual recently since my experience with Porsches is very limited and the owner's manual is toast.... the heat and humidity in Fla is not kind to paper nor anything else for that matter. I did take a look at the brake pads and they are getting very thin. Any recommendations on replacement pads for a street car?

[lonewolf]

Least you know your charging system in bang on
14.2 at idle is perfect ,little blip and you see 14.4 and up ..... perfect.

Bob. with all due respect. idle at 13v .somethings not right. if that's a true reading.
13.5-4 at a rev ,either you have grounding issues or a worn charging system.
Car is not running 100%

If you put a car with those readings on a electronic engine analyzer you will see your spark plug readings are all the same but they are all not as high up the screen as they are suppose to be .

[dshepp806]

I run Mintex, just for street use.....they do fine for the money,..do plan to change each pad sensor with the pads' changeout (I always do on my '89,..usually changing out my pads when they get get over 50 %, installing fresh sensors, too.

MAN,..you had me going on the alt light!!!!!!!!

BEST!

Doyle

[Art Graves]

I have exactly the same problem with my wife's 89 911. Everything checks OK but the alternator light is flickering or on. Originally the problem was a bad voltage regulator. I had the alternator rebuilt which included a new reg. I've done all the tests in the Bentley shop manual and all is OK except for the light. I am currently looking at a bulletin I found on the Pelican site, Technical Bulletin 1, "New Alternator has Integrated Regulator", Model 1974-83 911/T. It mentions putting a 91 ohm 5 watt resistor across the bulb. I'm going to get a similar resistor from an electronics shop and give it a try, before ordering the Porsche part. The common denominator between this bulletin, for an earlier car, and my problem is the new regulator. Suggest you ensure you are testing with a digital meter with a high input impedance, not an older analog volt-ohm-meter.


Art