Is my alternator going bad?

[wild man]

I have to put a charger on the battery every couple of weeks or so, because it seems to slowly get run down, until it will no longer start. In trying to figure out what the problem is, I took some voltage measurements. Here is what I got, after taking it off of the charger, and starting the car:

13.7 volts, with no accessories turned on. it did not go up at all when I gave it more revs.

13.4 volts, with lights (high beams), blower motor, and rear defogger on. Also did not go up when I gave it some revs.

I don't think it should ever go below 13.8, even when loaded down. So I'm thinking that the alternator is dying (alternator light is not glowing, even slightly), but could it be just the regulator that needs to be replaced?

[Vicious]

id say its already gone, once you start your car the voltage should jump to about 14.4~6 due to the alternators charge coming in. take it off and have autozone/pepboys test it for you its free and rather simple.

[matthewb]

I think these cars are a little too old style to be expecting 14.5 out of. It seems like his 13.7 number is close to right but maybe the regulator or some of the windings are going bad and it can't keep up under high load. What is the voltage when the car is turned off. Maybe you have a bad battery and the alternator can't get it to go above the 13.6.

[bearone2]

it probably needs a vr, brushes are worn.

pepboys, checker, napa can check it on the car.

[VINMAN]

I agree with MattB 13.7 does not sound bad at all. it is still charging the battery. I would have the battery tested. That sounds like your problem.
Also, did you check to see if you have a drain on the battery?

[wild man]

Originally Posted by matthewb

I think these cars are a little too old style to be expecting 14.5 out of. It seems like his 13.7 number is close to right but maybe the regulator or some of the windings are going bad and it can't keep up under high load. What is the voltage when the car is turned off. Maybe you have a bad battery and the alternator can't get it to go above the 13.6.

The battery was reading 12.5 volts when the car was not running. I apologize for failing to post that info. The choice I am faced with is either "some of the windings are going bad and it can't keep up under high load", or "maybe the regulator". I would definitely prefer the latter. I do have a spare alternator, but that is a card that I would rather not play, unless unless I absolutely have to. I can try swapping the regulator from my spare one into the car to see if that is it or not.

[legion]

Yes, the alternator is probably bad. You should be at 14 volts, maybe a little less when blasting the stereo with the headlights on. I put an aftermarket voltage regulator on my old 951 and set it at 14.5 volts when I had it rebuilt. This solved a lot of quirky electrical problems with the car.

You probably also want to replace the main grounds along with a new/rebuilt alternator.

I'd also check for voltage leaks. Do a search for posts by Iceshark. He had good procedures for doing this.

Also, just because a battery reads 12.5 volts doesn't mean it isn't bad. A battery can still produce good voltage without delivering enough amps. The simple way to test this is to buy a device that tests the density of the fluid in the battery. You can pick one up at any auto parts store for under $10. Assuming you do not have a maintenance-free battery (this won't work if you do), you just pop the cover on the cells and suck the fluid up into the testor. A little dial with tell you if the fluid is good (can deliver enough current) or not.

[bearone2]

a high rate discharge test is the best.

[wild man]

When I stated that the battery showed 12.5V with the engine off, I actually meant that in a "good" way. It probably would read a quarter volt or so higher if it is given a full charge and then left to stabilize for 2 hours. It is a NAPA professional line (The Legend) 75 month battery with CA rating of 815, and a CCA rating of 660. Although at first glance, it does appear to be the "unmaintainable" variety, it really is not. The caps are removed as 1 rectangular piece of plastic covering half the battery. And I do maintain it well, keeping it (and the terminals) clean, and always making sure the electrolyte levels are up to the split rings. Specific gravity testing comes up good on all cells.

I agree that "a high rate discharge test is the best" way to test a battery. And there is no more convenient of a way of conducting such a test, as cranking over the engine. It is not too often that VR's start to fall short on their regulating capabilities, so most likely some turns in the alternator coils are starting to short out. But I still see where there is a slight possibility that it could be the VR. That is because the brushes that feed the VR juice to the alternator's field winding are attached to the the VR, itself. And if they are worn down to the point where they are starting to loose connection with the alternator's commutator rings, I think that could possibly be making it show that symptom's that I am seeing. In summary, the question I was asking in this thread was, what are the chances that is the problem, and not the alternator itself?

[Rasta Monsta]

I think 13.7 is fine. I also think that if you disconnected each ground strap, brushed their ends and mouting points well with a wire brush, and reinstalled them with conductive grease, you might get a couple more tenths.

[bearone2]

he's getting 13.4 with lights/blower/rear defogger and that's too low.

[wild man]

Originally Posted by Rasta Monsta

I think 13.7 is fine. I also think that if you disconnected each ground strap, brushed their ends and mouting points well with a wire brush, and reinstalled them with conductive grease, you might get a couple more tenths.

If anti seize compound qualifies, then it has already been done (except for the bellhousing one). I really don't think that you should never see less than 14.

[HondaDustR]

Maybe the alternator wire to the battery or the starter to battery wire is internally corroding and needs to be replaced. Could be bad battery clamps, or bad connections at the auxilary ring terminals at the battery positive clamp, which supply juice to pretty much everything on the car. IIRC I got around 13.8v with a brand "new" rebuilt alternator and it still did dip a little under load (mostly the radiator cooling fans). The idle does get a little rough when the cooling fans come on, and a new battery didn't do anything for that, so I may need to look into a wiring upgrade. (hmmm...another epic project materializing as we speak )

[Rex Karrs]

I've got a similar issue with my '86 944. I install a new battery and she runs fine, with perhaps a slight but noticeable deterioration in starter cranking time, for about 90 days. Then, one morning, nothing. Not even a click when I turn the key. I remove the battery, and find it open. Complete failure. I replace the battery and that resets the issue - in 90 days, I'll be doing it again, perhaps a little sooner if I don't drive her every day. The fuel gauge and speedo are intermittently erratic as well, but I'm not sure that's a related problem. The power windows are slow, but they work, as does everything else electrical right up until the 90-day battery failure. She's dead in the driveway right now, and I'm tired of changing batteries. Any ideas?

[bearone2]

Originally Posted by wild man

When I stated that the battery showed 12.5V with the engine off, I actually meant that in a "good" way. It probably would read a quarter volt or so higher if it is given a full charge and then left to stabilize for 2 hours. It is a NAPA professional line (The Legend) 75 month battery with CA rating of 815, and a CCA rating of 660. Although at first glance, it does appear to be the "unmaintainable" variety, it really is not. The caps are removed as 1 rectangular piece of plastic covering half the battery. And I do maintain it well, keeping it (and the terminals) clean, and always making sure the electrolyte levels are up to the split rings. Specific gravity testing comes up good on all cells.

I agree that "a high rate discharge test is the best" way to test a battery. And there is no more convenient of a way of conducting such a test, as cranking over the engine. It is not too often that VR's start to fall short on their regulating capabilities, so most likely some turns in the alternator coils are starting to short out. But I still see where there is a slight possibility that it could be the VR. That is because the brushes that feed the VR juice to the alternator's field winding are attached to the the VR, itself. And if they are worn down to the point where they are starting to loose connection with the alternator's commutator rings, I think that could possibly be making it show that symptom's that I am seeing. In summary, the question I was asking in this thread was, what are the chances that is the problem, and not the alternator itself?

no, starting isn't a hi-rate discharge test.

connect the tester to both terminals, hold the switch for 10 secs and see if the batt drops below 9.6.

[bearone2]

Originally Posted by wild man

When I stated that the battery showed 12.5V with the engine off, I actually meant that in a "good" way. It probably would read a quarter volt or so higher if it is given a full charge and then left to stabilize for 2 hours. It is a NAPA professional line (The Legend) 75 month battery with CA rating of 815, and a CCA rating of 660. Although at first glance, it does appear to be the "unmaintainable" variety, it really is not. The caps are removed as 1 rectangular piece of plastic covering half the battery. And I do maintain it well, keeping it (and the terminals) clean, and always making sure the electrolyte levels are up to the split rings. Specific gravity testing comes up good on all cells.

I agree that "a high rate discharge test is the best" way to test a battery. And there is no more convenient of a way of conducting such a test, as cranking over the engine. It is not too often that VR's start to fall short on their regulating capabilities, so most likely some turns in the alternator coils are starting to short out. But I still see where there is a slight possibility that it could be the VR. That is because the brushes that feed the VR juice to the alternator's field winding are attached to the the VR, itself. And if they are worn down to the point where they are starting to loose connection with the alternator's commutator rings, I think that could possibly be making it show that symptom's that I am seeing. In summary, the question I was asking in this thread was, what are the chances that is the problem, and not the alternator itself?

no, starting isn't a hi-rate discharge test.

connect the tester to both terminals, hold the switch for 10 secs and see if the batt drops below 9.6.

see post #4.

[westwind999]

Have you checked the voltages after driving the car for a couple days to see how they compare to a just charged battery? If the battery is drained down to 12.0V do you still get 13.8V when it's running.

I replaced the voltage regulator, the brushes were worn out on mine. It was pretty easy to do and much cheaper than a new alternator.

[FLYGEEZER]

Had the same symptoms with my 125,000 mile 944. First clue was a very dim alt light on with all accessorys running. Old school mechanic sez...one bad diode in alt. He was right on the money. Replaced diode all better now. Good luck!

[Arizona_928]

Don't get a rebuilt ebay crap. I bought one of them and the damn thing is only putting out 12.7V.. Which reminds me. I need another alternator.

[AA_Ezra]

Hey Az_porschekid. did you're alt have an adjustable voltage regulator?