new but alternator light is on

[johndaroza]

My new alternator is in the car but the light is still on in the car.

My question is were do I go from here?

I,m stumped what should I do.

The engine was out for rebuild just after I bought the car 2 years ago.

What do I look for now?

Thanks in advance

[timmy2]

Year of car, type of alternator, internal or external regulator?
Ground connected, all wiring installed correctly, have you revved it past idle?

[johndaroza]

Year of car, 1986 911 type of alternator, internal regulator.

Yes I like to revv it up it sound so good!

[john walker's workshop]

either the wires are on wrong or it's dead on arrival.

[Ronnie's.930]

The first step would be to measure the voltage, at the battery, while the car is running - "good" would be 13-14 volts.

[rick-l]

Does your car have an anti theft control unit?

Unfortunately it might be time to get Mr Voltmeter out and remove your alternator and see what voltage is on the blue wire.

[rhh4]

I'm in somewhat of a similar situation. '83 SC. My symptoms are classic alternator: Used multimeter and alternator isn't charging the battery. My alternator dash bulb never comes on (not even when iginition is in "on" position), but the bulb definitely works. I've taken the alternator out and getting it tested today.

Wiring is getting pretty old. First thing I noticed was the thick brown ground wire that was bare. Could this be the answer to my problem?

If the alternator test comes out good (which I think it will - it's 7 years old but only has 3,000 - previous owner didn't drive it much) my plan was to simply replace the ground wire and put it all back together.

But before I put it all back together I wanted to check the conductivity of the lines from the alternator to the dashboard bulb and the battery. How do I do this? I know I'll be using my Ohms meter, but which wire do I need to be testing? The thick red wire? I assume that goes to the alternator, and the blue goes to the dash bulb.

[Ronnie's.930]

This past thread is worth a read:

New Alternator - Now Battery Light is on at Start-up Idle...

[rick-l]

I have never had the need to do this but it seems a reasonable test would be: after you have the alternator flopped out reconnect the battery and measure the voltage on the disconnected blue wire to chassis (brown wire). It should be about 12 volts and the light should be out. Switch you meter leads over to current and use it to bridge the blue wire to the respective terminal on the alternator. Should read more than zero but less than an amp and the light should go on.

I am not sure why the blue wire goes to the anti theft unit but I would guess it is just a pass through to see if the engine is running.

[johndaroza]

I had bought this car with a bunch of crap on it after market alarm , auto roll up windows , a huge huge no I meen extra HUGE stareo ..... I had it all puled out to make it stock.

I think this is were the problem is,,,,,, So any one know some wiring pro in the sacramento area?

Thank for the help as it made me think about what happed in the past.

[derkpitt]

So what’s the reason the wiring from the warning light goes to the anti-theft control module before it gets to the alternator on an 86-89?

[x98boardwell]

Pinging an old thread...

Removed an alarm from my 87' as well. From what I can tell 87-89's don't have the diode which I can't find either.

This is what I've done:
- verified the alternator is working after bringing back to rebuild shop, all spins well and works fine
- turned on ignition key with alternator out (no light) and then grounded the blue wire (light went on). So, that means it's making its way through the anti-theft control module correct for it to light up?
- Verified all fuses are good
- brand new ground strap to engine case behind alternator, new ground strap from transmission to body and also new wiring harness.. so all of these are good.
- bulb is good on back of gauge. I did an LED conversion but DIDN"T replace that bulb so it's still factory OEM bulb and works.. just won't go out.
- revved car up to see if that helped and nothing
- verified at battery while running that it isn't charging and still showing 12.2 while running on brand new battery which is fully charged.
- I have 12.7 volts to the red wire at the alternator when detached from alternator which is same voltage as charged battery up front so no drop.

Question: What can I test when the alternator is out or when it's connected but hanging out so I can still access leads to see what's happening...? I've searched all threads and no idea where to go at this point from testing..

Otherwise, I'm about to reinstall Alternator and start looking for other things which could be messed up from alarm removal, etc.. but figured before I do maybe there is some tests I can run with Alt still out to determine if wire communication is in order.

pic for reference of wiring

[x98boardwell]

Update:

Put the old alternator in from a spare 3.2 and worked perfectly... No idea why an alternator would be rebuilt and work on the bench but not in the car.

Question: Is there a way when the alternator is rebuilt that the output from the exciter wire or voltage regulator is different where it won't communicate with the dash to trigger the alternator to start working..? Seems odd but it's fixed. Now to take alternator back to shop who rebuilt it and ether have them do it again or get my $$ back..

[spuggy]

Originally Posted by x98boardwell

Update:

Put the old alternator in from a spare 3.2 and worked perfectly... No idea why an alternator would be rebuilt and work on the bench but not in the car.

Question: Is there a way when the alternator is rebuilt that the output from the exciter wire or voltage regulator is different where it won't communicate with the dash to trigger the alternator to start working..? Seems odd but it's fixed. Now to take alternator back to shop who rebuilt it and ether have them do it again or get my $$ back..

There's a factory advisory that you might need to fit a resistor to drive the exciter circuit hard enough to work on older cars updated with newer bits. One possibility. It's both mentioned and reproduced in this thread:

https://forums.pelicanparts.com/porsche-911-technical-forum/989178-electrical-wizard-help-needed-alternator-light.html

[x98boardwell]

Spuggy, I had looked through that thread but figured it was mainly for older cars. I don't have an LED light which has been replaced nor do I have a diode in place like the earlier cars so still stumped to some extent as to why.

I do agree that there is a resistance problem which is probably the cause for the issue but wishing I knew why or what could be causing it.. maybe the VR..? I'm guessing because the voltage or resistance is off the light won't turn off or allow the alt to start charging..

[spuggy]

Originally Posted by x98boardwell

Spuggy, I had looked through that thread but figured it was mainly for older cars. I don't have an LED light which has been replaced nor do I have a diode in place like the earlier cars so still stumped to some extent as to why.

I do agree that there is a resistance problem which is probably the cause for the issue but wishing I knew why or what could be causing it.. maybe the VR..? I'm guessing because the voltage or resistance is off the light won't turn off or allow the alt to start charging..

Well, my thinking is that you fitted another alternator with your re-jigged wiring, and it works. Which would seem to rule that out. I don't know anything about the alarm module, so I'll just conveniently gloss over that..

Being a later car, you should already have the 4w warning bulb, rather than the earlier 2w, so shouldn't need a resistor either. But clearly there's something odd going on.

I'd lean towards talking to the shop and explain what you see; maybe they can suggest something, or a penny will drop at their end...

Or you could just try feeding switched 12V temporarily into the exciter via a 10 or 15w bulb; see if that makes it suddenly work. Your year should have an internal VR, right - so was that changed or checked?

[x98boardwell]

No re-rigged wiring.. It is a new harness but thats it. Same wiring as before.

I think you are right when it comes to resistance and the VR as this one is a different company and may not do what is needed for our 911 with resistance even though in most cars/environments it would work.

I will take back and let them know.

[timmy2]

Make sure the terminal designations on the new alternator are the same as the old one for B+, D+ (61) and D- (ground)
It almost looks like the D+ and D- are swapped in the photo? Have the shop mark them for you if no markings on the alternator.