Alternator bad? - Please confirm my diagnosis

[tedder]

So on the way home from work a week and a half ago, the porsche started stuttering and then completely died (86 911 carrera btw). Dead battery, would not turn over at all. After charging the battery it runs again but;

Voltage across the battery terminals

13.4v before starting, right after trickle charger comes off

12.25v idling in the garage

12.2-12.4 volts from 1000-4000rpm

12.1v @2000rpm with lights and fans running


I am thinking that the alternator is shot. Are there any other tests I should run before I send off the $$$?

Also, is there an easy DIY test I can do to check whether my battery (2 years old) is salvageable or needs replacement?

Thanks

Ted

p.s. Of course it died just 2 days before the Porschefest at Larz Anderson park here in MA. I had to go to Porschefest in my Honda. On the plus side, the owner of the one Carrera GT that showed let my son (8 yrs old) sit in his car before the crowds got too crazy. He said he was the founder of the Yankee Candle Company but I didn't catch his name. Very friendly guy and if he is reading this, thanks again from my son.

[ischmitz]

Well, the alternator is not charging the battery. Now you need to find out what exactely went bad. It could be as simple as a broken cable or bulb. Is the charging light coming on when you switch the ignition on. If not maybe it is a broken bulb that prevents the alternator from kicking in.

It sounds like your battery is O.K. given that it takes a charge, is able to start and run the car. I wouldn't worry too much about it.

[jclporsche]

Had mine rebuilt for $160, replaced worn contacts, brushes & diodes, total refurb. Check your local auto electrics for references to shop in your area who rebuild alternators. Be sure to torque the fan housing strap properly. Mine was not tight and the fan housing cracked, NLA, had to go reproduction at $360 See my other posts for photos. Will post more once it's done. Good luck with yours

[DRACO A5OG]

That is exactly what mine did on my way home from the Gym.

Screaming down the boulevard my radio died like it was losing current then my baby stumbled and died in the middle of the road.

Tried to restart but failed to do so. It just drained the battery.

I towed it to my parking lot (2 blocks, what luck eh?). jump started it and left it charging. Started fine next day the same thing while I was going to dealer to check out my alternator and/or battery. Died 1 block from the dealer, Doh! what luck eh!!!

Anyways, it was the alternator. I replaced it and a new V belt while I was at it. No more issues.

Sorry for the bad news but you already tested it by being able to re-charge it then the battery draining while you are driving. According tot he mechanic, only the battery was igniting the spark plugs, once drained it will kill your baby since the alternator is not regenerating the battery.

$160 Rebuilt, Nice Find JCL

[ischmitz]

Hm, it could be as simple as DF not getting power. So a blown alternator bulb is all it takes to prevent the alternator from charging. Check the simple things first before spending $$$......

Contrary to a generator an alternator needs an initial power source to energize its rotor coils. Without that it will never produce any electricity no matter how fast it spins. This is why the regulator (internal part of the alternator on your model) gets power from +15 (switched power from ignition) through the charging light and a diode. A lit charging light indicates that power is flowing from the battery to the alternator. If the light bulb or the diode is blown it won't work. Once the alternator is producing electricity the regulator used a tiny part to keep the rotor coils energized just to the level to keep the output at the 14.4 V level.

Ingo

[jclporsche]

When I took mine out I pulled off the brushes and the copper rings on the rotor had worn down a good 3 or 4 mm per side, I think the brushes were dancing over it toward the end.

[jclporsche]

PS, she has 159k mi. on odo and I have no paper showing previous repair to alternator. Is that possible?
DRACO, yes that was a good deal, but the new fan housing was a disappointing requirement.

[tedder]

its not the alternator bulb. When I start it it comes on briefly (2-5 seconds) and then goes out. Already tightened the fan belt so thats not it either. So it looks like it is time to pull the alternator.

Glad to hear that the battery is still probably good though...

On a side note, I stupidly left the oil fill cap on the bumper after checking the oil for my last drive and now it is gone. I zip-tied a plastic baggie over the spout for the moment. While running in the garage the baggie pulses with the engine. Not enough pressure to tear the bag (glad sandwich bag) but I am wondering if this is just the pulses from the oil pump or if it indicates some blow-by.

[tedder]

One last thing, 126k miles and no previous alternator repair. I think Its due

[ischmitz]

If the light works then I agree. Pull the alternator and have it bench-tested. Maybe the brushes or the internal regulator need replacement.

Get a replacement the oil filler cap soon. The pulses you are seeing are normal. The crankcase breather delivers slight pressure to the oil tank and the intake scavenges it. Usually, there is a slight vacuum in the oil tank from the intake connected to it. You will note a slight change in idle with an open oil tank because the intake can get more air through the connection to the oil tank.

Make sure the plastic does not get sucked into the tank. That would be really bad!!!

Ingo

[dshepp806]

Just recently replaced my alternator/reg ,...'89 Coupe. Had Pelican overnight me a Valeo reg, with the intent of replacing prior to departure since my Passport displayed voltage alarm (16.5 VDC),..I verfied with a second meter,...truly a REAL event,..had visions of blowing a DME or something...THEN it went away...heading back to it's resting (yet a bit elevated) 14.7 - 15.0...WTF. Schedule didn't permit my attending to the matter and it was faithfully handed over the my P-wrench to install a new Bosch alt/reg and load test the battery,..all so that I may take her ON our vacation (it's HER [[[911]]] vacation, too?) Point is:

He found the ORIGINAL alternator/reg upon removal....yep,.I noticed my PO records indicated no ALT service at all.....WOW! Glad that's put to bed....pretty good to have lasted all those years. No time to look at brushes (and so on) but I'm SURE they were worn,..not to mention the reg. suspiciousness.

Anywaze: she's 14.2 - 14.4 ,..steady.

Yep: she's due!

Best of luck,

[1911Guy]

Check all your Grounds too. Make sure they are clean and tight.

[450knotOffice]

Well, dammit, add me to the list of possible alternator failures. I was out to lunch with PatrickB today and when we tried to start the car, no go. Patrick pushed, it started right away and it ran fine all the way home. When I got home I checked voltage and it's around 12.2 to 12.6.

[DRACO A5OG]

Originally Posted by 450knotOffice

Well, dammit, add me to the list of possible alternator failures. I was out to lunch with PatrickB today and when we tried to start the car, no go. Patrick pushed, it started right away and it and it ran fine all the way home. When I got home I checked voltage and it's around 12.2 to 12.6.

Strange, if it was the alternator you would have stalled out way before you got home.

Reason I say this is that you push started your baby and did not jump start it nor were you able to keep cables connected long enought to regenerate the battery but you were able to drive home delivering sparks to your plugs. If the battery was drained and alternator was dead, your baby would not have enough current to hit the plugs.

Question were you able to restart it when you got home? I am thinking it may be the notorious DME Relay for us 84-98r's

[450knotOffice]

Yea, the car starts fine (right now). The no-start issue was due to a depleted battery not having enough power to turn the motor over. Now, what caused that, I'm not sure. I hear you about the fact that the car ran fine all the way home. It's possible that there was enough potential left in the battery to power the DME but not enough to turn over the starter, which is a high draw component.

I'm going to check all of the big grounds tonight, including the one near the transmission, and make sure they're good.

[jclporsche]

Before I realized my alt was gone I was having many off/on elect probs too. No start traced to no fuel pump turned out to be the DME relay. That problem was intermittent for a couple of weeks before it totally would not start. I replaced the DME and it was all good for about 4 or 5 days then the alternator light started coming on. My theory is as the brushes & rotor were wearing, the voltage variations were contributing to the other issues. is that possible?