82 SC - alternator light is on, voltage is low, alternator tests OK

[perry roubay]

Hi,

Driving the other day and about 15 minutes into the drive the tach started bouncing around. First only with the clutch in, by the time I got home it was consistently all over the place. Parked the car, shut it off and tried restarting and nothing. Starter wouldn't turn over. I checked the battery voltage and it was low... 11ish volts.

Charged the battery and the next day the car started fine. I checked the voltage at the battery and in the car (cigarette lighter). Both were 12.5 +/- at first and started dropping pretty quickly. After a few minutes both were down under 11V the alternator light came on. It hadn't been on before.

Seemed like an alternator issue to me (it's a 3 month old Bosch remanufactured version) so I pulled it and took it to the local auto electric shop. They bench tested it and said it's fine.

So I'm not sure where to look next. Don't want to reinstall the alternator and just have to pull it again.

Any help or thoughts would be very appreciated! Car is an 82 SC 108k miles... if that makes a difference.

Thanks!

[timmy2]

Have the battery tested. Could be a bad cell in it if it wants to drop to 11 volts.

[7783911]

I had the same issue and a new battery addressed it after I confirmed I had no parasitic drain and that I was getting correct charging voltage. I replaced my clock with a VDO voltmeter using an old clock housing so to look "standard" and now I can easily monitor charging status. I do also know if you change bulbs on the ALT lamp in the dial you can screw up the exciter circuit in case you ve done that recently?

gl

Bob

[perry roubay]

Thanks, hadn't considered the battery causing issues like that.

It keeps falling down to almost zero volts after 5-10 minutes... if that makes a difference.

Either way getting the battery tested is an easy one.

[perry roubay]

So the battery checks out as good.

Any other suggestions? Is it possible the alternator is bad but looks OK on the test bench?

[911obgyn]

Clean all the grounding points, battery ground at chassis, ground strap on transmission, alternator ground if you still have it out. Also the wire that runs to the starter from the alt may be crispy. And the wire from the batt to the starter could be loose or have a compromised connection.

[911obgyn]

Resting battery voltage should be 12.5 or therabouts.

[kimlangley7]

Did you measure the voltage of the battery while the engine is running about 1000rpm = alternator voltage regulator doing its thing. voltage should be 13.5v - maybe low 14 v.. // voltage should not be over 14.7v = voltage regulator overvoltage - which will destroy the battery.

kgl

[perry roubay]

Thanks, I'll definitely give all of the grounding points a good look.

@kimlangley7 I did measure the voltage at the battery around 1k RPM. It starts off around 12.5 and slowly drops. The voltage in the car (multimeter at the cigarette lighter) starts off around 12.5 and eventually drops to almost zero.

[911pcars]

Originally Posted by perry roubay

So the battery checks out as good.

Any other suggestions? Is it possible the alternator is bad but looks OK on the test bench?

If the battery checks out good (pro checked?) and the battery rapidly discharges while running, something is amiss with the charging system (circuit wires included). If it rapidly discharges while OFF, that indicates a parasitic discharge (partial short), or perhaps a circuit is ON while sitting (e.g. glove box light ON, faulty power window circuit, etc.). A faulty alternator could also do that. It was inspected, yes? Did you reinstall correctly?

Sherwood

[perry roubay]

Hi Sherwood,

Thanks for the input!

I had the battery checked at 2 different places (just to make sure) and had the alternator tested at the local auto electric shop. Both the battery and the alternator tested Ok. The battery holds a charge when it's parked so no parasitic discharge.

I haven't reinstalled it just yet. Wanted to make sure there wasn't something else to check before I put it back in. It was new/installed a few months ago and has been fine up until a week or so ago.

The guy that checked the alternator was wondering if it could be an alternator/heat related issue because it starts out fine but starts dropping after it's been running for a few minutes. He said he had no way of simulating/checking that out.

[911pcars]

Ok. Check for open voltage after the battery (not installed yet) sits for a few days. Open voltage shouldn't drop significantly.

Good luck.

[87maniac]

Had similar experience.
Recheck your alternator.
Test with a known good/new battery.

In my case, a new alternator
and a new battery solved the issue.
(discharging rapidly from 13V to 11V while driving).

[targa80]

You mentioned a rebuilt alternator but you did not mention if the alternator has the built in voltage regulator or is it the earlier version with the regulator mounted on the left side of engine compartment.

[Trackrash]

Originally Posted by targa80

You mentioned a rebuilt alternator but you did not mention if the alternator has the built in voltage regulator or is it the earlier version with the regulator mounted on the left side of engine compartment.

That’s my thought as well.
Sounds to me like either a wiring problem or bad regulator.

[theiceman]

i think 82 is internal.

[mysocal911]

Originally Posted by targa80

You mentioned a rebuilt alternator but you did not mention if the alternator has the built in voltage regulator or is it the earlier version with the regulator mounted on the left side of engine compartment.

It's '82 911SC, which came with an alternator with an internal regulator.

The alternator voltage needs to be checked at the starter when the voltage at the battery starts dropping.
The actual alternator voltage can also be checked by measuring the voltage at the alternator light (assuming all 9 alt diodes are OK).

[GH85Carrera]

When the alternator and the battery have been tested and you feel they are working properly, it is pretty much a connection. Take apart, clean and verify the battery cables at the battery, and the grounds at the ground to chassis. Take off, and clean the connects at the ground strap at the transmission. Check every connection even the ones that look just fine.

Disassemble, clean and tighten ALL the connections to the battery. Do the same thing for the alternator connections. Assume they are bad, and scrape and get the surfaces down to nice shining surfaces.

[theiceman]

id also be interested how long they bench tested it for , you said yourself it starts out fine then slowly dropped off.

Also did they check it for ac ripple ? they probably did if they are an alternator shop, but could indicate a shorted diode.