Alternator Help !!!

[eubanksae]

1978 911SC...

I've got something happening, but I just can't afford to go replacing things that don't need replacing....

Here's what's happening...

when I turn key, I get no alt light, when I crank engine, I get alt light, after engine started, no alt light, unless rpm's drop really low, and engine is about to stall (but that's another issue when the car is warming up)

Here's what happened today.

At lunch I ran out to my friendly Interstate battery shop, and we:
Checked battery with engine off 11.6v
started car, and still 11.6v
reved to 2k, and still 11.6v
turned on everything in car, an it dropped to about 10.9v

So, I'm thinking it's the alternator...

It started fine that time, so I've got one more stop to make...
It wouldn't start again, but I used a jump box, and got it started, drove it straight home (about a 15 min drive in town).

I turn off the car, and get out, but then I thought... I wonder if it starts now. I turn the key, and this time I get the alt light, and the engine starts right up!

I was on my lunch break, so I didn't have time to check the votage again, but could this be something other than my alt or vr?
Like maybe some poor connection?
Is there an easy way to check output of alt right at the engine, in case there is a bad connection somewhere?

I know that I should have an external VR, but it doesn't appear to be where my service manual shows it, so I am guessing that the PO "upgraded" my alternator.

In the past months I have seen the flucuating dash lights, but it didn't seem to be in sync with the RPM's. They would just get brighter every once in a while, but were usually dim. Over the last week, they just remained what I would call dim.

That's all the info I can think of. If anyone has any insight, please help!

Thanks,
Andy E.
Sarasota, FL
1978 911SC Targa

[bell]

start by checking all the ground straps off the engine for corrosion or breaks.....these can cause a multitude of problems

[eubanksae]

I just got in from my further investigations, and found the following behavior (I will check the ground straps tomorrow after work):

I turned the key, and got my alt light before starting...
I started the car, and the battery was at 11.5v for the first 30 seconds or so, then the rpms dropped slightly, and the voltage jumped to about 13.9v
It stayed at 13.9v for a few minutes, and then it dropped again to about 11.6v and It stayed there...

I turned the car off, then turned the key back to the start position and didn't get the alt light.

I let it sit for about 15 min, and then turned the key, and got the alt light. I started the car, and it was at 13.8v immediately, I revved it to about 2k and got the voltage to about 14.1v.
I let it run, and a few minutes later I reved the engine again, but now it wouldn't get above about 13.9v when revved at 2k. I let it run at idle for about 15 more minutes, and it continued to be in the 13.8 range at idle.

Does this sound more like a VR issue or a ground issue? or something else entirely, what do I know... I'm new at this

Thanks again for the advise,
Andy E.
Sarasota, FL
1978 911 SC Targa

[Early_S_Man]

Andy,

It sounds like an issue with the brushes to me!

[304065]

Bad VR.

Turn key, get alt light = good continuity through warning light, through blue wire to alternator, through brushes to rotor and back, and then to ground.

Start engine, low voltage, rev engine, voltage increases, good!

You have intermittent low voltage output from the alternator. You could have a defective voltage regualtor, that's not varying the field strength to maintain constant output voltage.

Check the connections to the voltage regulator over on the "console" at the port-side rear of the engine compartment. Unplug the VR (carefully) and plug it in again, a couple times, to clear any oxidation from the plug, or follow Warren's advice on terminal cleaning (from the archives).

[Superman]

A set of brushes for that thing would be about $5 and you could clean it up while you have it apart. Or go for the full rebuild, about $100.

[targa80]

Yep! It sounds like the brushes are worn down to the point where the contact is intermittent. They are cheap to replace. If you have more time than money that is what I would replace first and see what happens. Naturally checking to make sure all the contacts in the alternator /voltage regulator circuit are clean and tight would be correct step also.

[RoninLB]

IMO
if internal regulator then get new brushes and new internal regulator, even if regulator is OK. save the old good [if] regulator for a back-up.......Ron

[ruf-porsche]

where can one buy new brushes for an alternator? Most car parts store just want to sell me a rebuilt alternator. Is the VR an item that Pep Girl or Auto twilight zone could order?

[targa80]

Look in the yellow pages for a starter /alternator rebuild shop or auto electric shop.

[ruf-porsche]

I did, they don't want to sell parts, they want to sell a rebuilt alternator or rebuild my alternator for anywhere $110.00 to $175.00.

[nostatic]

You can get the VR here at Pelican...$27. That is assuming that you still have the stock setup with external regulator (which you will unless someone "upgraded" to a new alternator). The conventional wisdom seems to be to keep the original alternator and just get it rebuilt. I changed the VR pretty easily on my car (it is external), and I'm not exactly Mr. Goodwrench. And I never played him on TV, either.

[targa80]

Ruf- Porsche:

Try calling this company in San Diego. I used them for all of the parts except the bearings. I use to live there so I just walked in the door and got what I needed. If I remember right they will ship by mail. A phone call couldn't hurt.

Good luck

N P Auto Electric Supply
4337 Sheridan Ln
San Diego, CA 92120
(619) 283-8998

[Lorenfb]

Don't forget about replacing worn slip-rings on the rotor,
old bearings, and wires internally if the insulation is gone.

Also, beware of junk regulators which cause intermittent
over-charging and damage electronic components. The
grey electronic replacement regulator has this problem.

Do the job right!

Have Fun
Loren
'88 3.2