Alternator Light and dead battery

[khamul02]

Well I checked the the ground from the battery to the car, the ground on the transmission, the battery, pulled the alternator to take it in for a rebuild and the guy said it was working perfectly. So... my alternator light starts coming on after about a15 minute drive and it the turns on completely. When I get home my battery is dead and reading anyway from 10 to 11 volts. And of course at that point the car lights barely work and the speedo is going crazy from lack of juice.

What is going on here (81 couple)??

[timmy2]

Did the test involve a load test or did he just spin it up and check voltage output?

An internal failing/shorting battery plate can wreak havoc as well.
If you put your battery on a charger will it hold the voltage or does it quickly bleed off with a meter on it?

Swap out the battery with a spare one and see what happens.

You should check alternator voltage output at the battery with a meter.
Should be about 13.5 volts with the car running. check your manual for exact number.

[khamul02]

The alternator guy thoroughly tested it... obviously he would have like to rebuild it but say it was good to go. Good idea about the battery. I will swap it and see but I got a feeling it is outside of these two components.

[304065]

Weak battery. I would replace it before it takes out the alternator and voltage regulator. How old is the battery?

[Justin@Athens]

I kinda have to agree with the other guys and say sawp the battery. I don't know if the VR can fail intermittently or not but, if it can, it might have fooled the guy at the rebuild shop. It may be that the alternator itself is fine just a bad VR?

[db_cooper]

Originally Posted by Justin@Athens

I kinda have to agree with the other guys and say sawp the battery. I don't know if the VR can fail intermittently or not but, if it can, it might have fooled the guy at the rebuild shop. It may be that the alternator itself is fine just a bad VR?

These old VRs can be fickle..if the battery is OK..the alternator ground strap is good (replace it if there is any doubt) and the rebuild shop says the unit is good..replace the VR..they have some history of failure.

[khamul02]

I actually pulled the alternator to have it rebuilt so it was tested out of the car. Where is the voltage regulator located? First thing is first I will be swapping the battery just in case.

[timmy2]

Here is your voltage regulator if it's the same as my '78 SC
Black box(s) in center of picture connected to engine harness by 3 prong connector that is disconnected in this picture.

[911pcars]

Light ON indicates the charging system isn't charging, not if the battery is good or not. As suggested, connect a voltmeter to the battery and measure the charge voltage (engine running). If the red light is ON, you can install 12 batteries and you'll have the same symptom - there's an issue with the charging system.

Sherwood

[timmy2]

It was the statement about running normal and then going red that indicates a normal component going bad as it works/charges/heats up from stress. Could be a shorting battery pushing the alternator or regulator into fail mode.
Failing battery banks will push chargers into an overload failure mode. (From my 25 yrs as Power System Electrician)
I'm thinking it's more likely the regulator but until he checks the charging voltage on a known good battery it's all speculative. Charging current and voltage readings at 1,2,3 and 4k rpm would be nice to know.

[911pcars]

Originally Posted by timmy2

It was the statement about running normal and then going red that indicates a normal component going bad as it works/charges/heats up from stress. Could be a shorting battery pushing the alternator or regulator into fail mode.
Failing battery banks will push chargers into an overload failure mode. (From my 25 yrs as Power System Electrician)
I'm thinking it's more likely the regulator but until he checks the charging voltage on a known good battery it's all speculative. Charging current and voltage readings at 1,2,3 and 4k rpm would be nice to know.

If the battery is shorted or creating a situation that causes the alternator to output maximum current, most likely the symptoms would include an overheated battery and probably the acrid smell of battery acid as it vents from the battery. If the battery cells are accessible, most likely one or more would be dry of electrolyte.

Yes. Could be the regulator.

[db_cooper]

1981 should have the external VR as shown by Timmy...however..it may have the VR mounted on the back of the alternator...if you don't see the external voltage regulator under the plastic cover on the left of the engine compartment.

I know of several 911s what were "upgraded" over the years. 81-82 was a change year too for the VR.

[Gunter]

+1

khamul02,

Do you have the upgraded internal VR?

I assume you would have because how else can a Tech check correct function of an alternator?

[911pcars]

Originally Posted by Gunter

+1

khamul02,

Do you have the upgraded internal VR?

I assume you would have because how else can a Tech check correct function of an alternator?

Connect voltmeter leads to + and – posts on the battery. With engine running, should be at least 13-14 volts with loads ON. Of course, there's also the paths to/from the battery to inspect.

Sherwood

[khamul02]

Ok.. sorry for the delay. The alternator was pulled to let the tech rebuild it. He said the voltage regulator is on the alternator and is working perfectly. And sure enough there isn't the black regulator in the engine compartment fuse panel. So.. before when I tested the battery as Sherwood suggested it was reading about 11 volts. So I need to put the alternator back on and possibly swap the battery...

BUT before I put it on is there anything I should look at concerning the wiring to the alternator??

[Porboynz]

Well, that battery needs charging if you expect to use it to start the car. Charge it with a smartcharger and then let it sit a while. If the voltage drops under about 12.4V after 6 hours sitting I would suspect it and if it drops under 12V you have a faulty cell.

With a known good battery connected start the engine and again measure the voltage, with the engine at approx 1200+ revs the alternator should be charging and the voltage approx 13.6V-13.7V for a period as it tops up the energy used to start the engine. If the alternator light does not go out and the voltage stays at 12.4V then shut the engine down, that alternator is not charging. Having said that a good battery should run the engine at least an hour or so without other accessories like lights on.

If the light stays lit then check the wiring to the alternator, in particular if the original alternator was the seperate regulator type some wiring mods would have been done at the relay panel when the inbuilt regulator alternator was fitted and the seperate regulator removed. A possible cause for the light staying on and no charge when you do have a seperate regulator on the panel is if the relay panel earth is disconnected. ( the panel is rubber mounted.) There should be an earth strap from the panel to the earth stud in front of the CDI box. I am unsure where the integrated regulator version of the alternator gets its earth from, probably in the actual alternator itself through the mounting. That just leaves the wire to the warning light to be patched through at the relay panel and the big guage wire to the starter stud/battery.

[db_cooper]

Originally Posted by khamul02

Ok.. sorry for the delay. The alternator was pulled to let the tech rebuild it. He said the voltage regulator is on the alternator and is working perfectly. And sure enough there isn't the black regulator in the engine compartment fuse panel. So.. before when I tested the battery as Sherwood suggested it was reading about 11 volts. So I need to put the alternator back on and possibly swap the battery...

BUT before I put it on is there anything I should look at concerning the wiring to the alternator??

B+ is the red wire

D- is the brown..ground..should be two

61 is the blue wire

If there is a black wire..that was the old DF for the external VR..it should be clipped off.

When changing over to the internal regulator from external..the red charge indicator lamp may stay on because the exciter current is too weak with the 2w bulb in the instrument cluster.

There should be a resistor..911-641-981-00 91 ohms, 5 watt installed in parallel on the back of the warning lamp bulb.

Starting in 1982 they used a 4 watt bulb..which has a larger socket and cannot be installed in instrument cluster of the early cars.

[db_cooper]

Here is the product notice for the internal regulator change. Note the missing ground wire between the alternator and engine..on some models..and the resistor placement. Both are important if you have a charging problem

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

[Gunter]

Originally Posted by khamul02

Ok.. sorry for the delay. The alternator was pulled to let the tech rebuild it. He said the voltage regulator is on the alternator and is working perfectly. And sure enough there isn't the black regulator in the engine compartment fuse panel. So.. before when I tested the battery as Sherwood suggested it was reading about 11 volts. So I need to put the alternator back on and possibly swap the battery...

BUT before I put it on is there anything I should look at concerning the wiring to the alternator??

There should be 2 brown wires (ground), the heavier wire goes to the engine case per Porsche Tech Bulletin to ensure good ground.

Do you have this additional heavy brown wire to the engine case?

Also, one area that developes corrosion and rust is the negative battery connection to the body because it's out of view. The bolt usually holds multiple connections. it should be opened, all terminals cleaned and reinstalled with either dielectric grease or Vaseline. Scrape the connection area on the body a little to get good ground.

I suspect that your battery is kaput. Most shops will test it without charge.

[khamul02]

That is a lot of info. I haven't went through everyone's comments yet. As of right now the battery is good but alternator is producing only 11 to 12 volts when the battery is disconnected... That being said.. Now the battery and alternator are both suppose to be good.