Do I Need A New Alternator?

[big911fan]

I've read the posts on alternators and still have a few questions. Here's my situation:

Battery= 12.68V
Engine on, no load= 13.8V
Engine on, w/load = 13.5 to 13.8V (depends on how big the load is)
Alternator dash light on with engine running (faint); goes out when engine off

Do I need a new alternator? If so should I buy new, rebuilt, or have someone rebuild the one I have?

The car is an '89 Carrera with 36K miles (original).

Thanks.

[Flieger]

The voltage sounds like your regulator is working right. Is the alternator light on only at idle and goes out when you rev it? If so, that is normal.

[big911fan]

The alternator dash light is faint but stay on at any RPM. Goes off when engine is turned off.

To me it seems like a marginal alternator because voltages are around 13.8V but don't increase with load. It appears to be enough to keep the battery charged though under any load since it's charging above battery static voltage level of 12.68V.

Has me puzzled. I don't want to buy a new alternator if this is acting normally. I wonder if anyone else has tested their system for comparison.

[86 911 Targa]

Originally Posted by big911fan

The alternator dash light is faint but stay on at any RPM. Goes off when engine is turned off.

To me it seems like a marginal alternator because voltages are around 13.8V but don't increase with load. It appears to be enough to keep the battery charged though under any load since it's charging above battery static voltage level of 12.68V.

Has me puzzled. I don't want to buy a new alternator if this is acting normally. I wonder if anyone else has tested their system for comparison.

13.5-14.5V are the nominal voltages for your alternator.

Anything above 14.5 will damage your battery.

How old is your battery?

Get your system checked.

Good luck,

Gerry

[ischmitz]

This is caused by one of the diodes being short. It makes the output having a lot of ripple and that is what keeps the alternator light glowing. Not good for the battery in the long run. And if the battery dies this can cause even more issues with the electronics.

Take it out and have it rebuilt is my advise.

Ingo

[big911fan]

I suspected a diode was bad or going bad. The battery was installed 4/08 and seems strong at 12.68V. I keep it on a Battery Tender all the time.

The system is obviously charging but perhaps not normally as you indicated. It doesn't seem to be at the point that it drains the battery. It just can't keep up with max current demand as strongly as it should. The glowing alternator light worried me. With no load it charges at 13.8V.

[porschenut]

If your alternator light is on while running, it means your alternator is not putting out enough voltage. Normal functioning is that the light comes on with they key in the ON position, and goes out as soon as the engine starts and the balancing voltage from the alternator causes the light to go off completely. Rebuild or replace, definitely.

[ischmitz]

Correct, and the glow is really an on-off-off-off-off-off-on-off........ pattern with engine RPM frequency but the light bulb in the dash (and your eye) can not follow it fast enough. That's why you see a faint glow. It's called avereging.

Hook up an oscilloscope across the indictor light and you will see.

And this is caused by one diode out of six being bad. Ask me how I know.
Ingo

[big911fan]

Started the car up this a.m. and checked the readings. Battery voltage was 12.6V before startup. The charging rate was 14.3V with engine running and no load.

That's higher than yesterday's readings at shutdown. Seems like the alternator is keeping up with the demand when the battery needs it. The alternator light still glows faintly continually while the engine is running. Even when charing at 14.3V.

My question is, if only a single diode is bad and keeps the dash light on, is the alternator still capable of keeping up with a full demand load? If the answer is no, perhaps something else is going on.

Also, are diode sets obtainable and replaceable by the average guy or do I need to find a specialty shop.

[ischmitz]

The diodes are rectifiers. They convert the AC current into a DC current. As I said in my previous post the bad diode WILL cause issues. It will confuse the regulator and over time break the battery.

You made the comment that you keep the battery on a battery minder over night. This and the fact that your alternator now puts out a DC and AC voltage lead to wrong readings with a simple multimeter. The AC confuses the DC voltmeter. You need an oscilloscope. A correct way to test the battery is to switch on the headlights without the engine running for maybe 10 minutes. This will remove surface charges. The with the engine running and the system under load check for a clean DC and voltage level.

Depending on the type of alternator and your tools you might be able to do the replacement by yourself. However, there still is the issue of sourcing the part. These diodes come premounted on a heatsink and are not readily available at your friendly autoparts store.

Ingo

[wwest]

Originally Posted by big911fan

Started the car up this a.m. and checked the readings. Battery voltage was 12.6V before startup. The charging rate was 14.3V with engine running and no load.

That's higher than yesterday's readings at shutdown. Seems like the alternator is keeping up with the demand when the battery needs it. The alternator light still glows faintly continually while the engine is running. Even when charing at 14.3V.

My question is, if only a single diode is bad and keeps the dash light on, is the alternator still capable of keeping up with a full demand load? If the answer is no, perhaps something else is going on.

Also, are diode sets obtainable and replaceable by the average guy or do I need to find a specialty shop.

The regulator adjusts the charging rate as a function of OAT, the higher the surrounding temperature is the lower the charge level of the battery will be.

The charge rate "glow" is normal for older 911's, '78ish.

Everything you have said, tested, seems like a normal system to me.

Trickle charger will keep the battery topped off so alternator never needs to go to high charge rate.

[wwest]

Originally Posted by ischmitz

The diodes are rectifiers. They convert the AC current into a DC current. As I said in my previous post the bad diode WILL cause issues. It will confuse the regulator and over time break the battery.

You made the comment that you keep the battery on a battery minder over night. This and the fact that your alternator now puts out a DC and AC voltage lead to wrong readings with a simple multimeter. The AC confuses the DC voltmeter. You need an oscilloscope. A correct way to test the battery is to switch on the headlights without the engine running for maybe 10 minutes. This will remove surface charges. The with the engine running and the system under load check for a clean DC and voltage level.

Depending on the type of alternator and your tools you might be able to do the replacement by yourself. However, there still is the issue of sourcing the part. These diodes come premounted on a heatsink and are not readily available at your friendly autoparts store.

Ingo

Strange....

The converter in my MH charges my battery just fine with PURE full-wave rectified AC(DC "pulses") voltage.

Alternators NEVER supply DC voltage for battery charging, only full-wave rectified AC. The battery itself provides the "smoothing" of the pulsating DC. A shorted diode typically results in the alternator overheating and soon failing but in the meantime functioning fairly normally.

[rick-l]

My 88 has two rows of diode modules each module contains 8 diodes. If you could solder they are easily replaceable if you can source them (and you can't have mine).

Direct current (DC) is the unidirectional flow of electric charge, the varying amplitude does not matter.

[wwest]

Originally Posted by ischmitz

The diodes are rectifiers. They convert the AC current into a DC current. As I said in my previous post the bad diode WILL cause issues. It will confuse the regulator and over time break the battery.

You made the comment that you keep the battery on a battery minder over night. This and the fact that your alternator now puts out a DC and AC voltage lead to wrong readings with a simple multimeter. The AC confuses the DC voltmeter. You need an oscilloscope. A correct way to test the battery is to switch on the headlights without the engine running for maybe 10 minutes. This will remove surface charges. The with the engine running and the system under load check for a clean DC and voltage level.

Depending on the type of alternator and your tools you might be able to do the replacement by yourself. However, there still is the issue of sourcing the part. These diodes come premounted on a heatsink and are not readily available at your friendly autoparts store.

Ingo

"...It will confuse the regulator..."

No, the regulator will simply supply a higher excitation current to the rotor windings in order to keep the battery charged to the normal target level even though 1/3 of the alternator power is being absorbed within the alternator itsself, windings.

"..and over time break the battery.."

How so....??

Back on the farm/ranch I sometimes opened the shorted diode section/phase (cut the shorted diode out of the circuit) leaving the alternator with only two output phases rather than three and continue working. Battery doesn't "care" how many charging pulses per second it receives otherwise a constant engine RPM would be required.

[ischmitz]

Tell that to the DME once the battery "decides" to behave open. I have seen my share of bad shunt diodes and burned circuit boards when I repair these.

I agree that farm equipment can take quite some abuse and all that. However, there are stern warnings to never run these modern cars without battery. And if the ripple amplitude is too large it will wear the battery. Modern batteries do care...

[big911fan]

You guys are way ahead of me on the electronic theory. First, I know to discharge the "surface charge" on the battery before testing with my simple multimeter. My readings were done that way. Second, it appears that the charging voltage drops after driving the car for a while. That sounds like what it should do after the battery recovers from the initial startup load drain. Yesterdays charge rates were done after driving the car in 90 deg. heat for an hour or more. This a.m. it was 70 deg. That makes the comment about OAT perhaps relevant to my situation.

It sounds like the alternator is behaving normally except for the dash light "glow". One of the posts suggested the glow was normal for the older 911's. Mine is an '89 so I don't know if that applies.

Perhaps the only clear answer lies in having a specialized shop test it for me. If the faint glow, barely visible in daylight, is normal, then I'm waisting everybody's time. If not, I do have a bad diode it seems. In either case, I'm learning a lot here. Thanks.

[tyro]

I just finished troubleshooting mine, also had the dim light - however it was intermittent. Checked connections and grounds over and over, no difference. Took the alternator to a rebuild shop and they declared it good. Reinstalled. No change. New regulator installed, no change. New battery, no change. Finally, a rebuilt alternator from Rock Auto seems to have done the trick. What a PITA.

Here's a good link that describes our charging systems:

Bosch Charging systems

If you end up needing a new alternator, I have a Valeo alternator that looks like this one. It was tested by a different shop, new brushes were installed. $120+shipping and it's yours:

[big911fan]

That Bosch Charging article seems to hit the nail on the head. I obviously have the beginning stages of a problem with the dim bulb. My connections and grounds seem good so I'll assume the first step is test/replace the alternator. Sounds like other things could go wrong but that's the most likely.

What bothers me is the number of posts that said they went through multiple alternators before they found a good one. Is there a reliable shop in Northern CA that I can ship mine off to for rebuild or a source for good ones?

[big911fan]

Sorry. Didn't mean to overlook your offer. Just wasn't sure if it fits a 90A that will work on my '89 Carrera. If possible I'd like to get one locally to save time and returns if necessary. Will keep it in mind.

I have a Bosch repair facility in town but I'm not sure what their capabilities are until I check with them.

[AlfonsoR]

Interesting discussion.

Not much to add, but I have an 89 as well and the alternator light does not stay on, so I would say it is not normal.

If I understood corretly, I think what WWest and ISmitch are both saying that the alternator will function adequately as is, but due to the issue of one phase being out, relatively shortly down the road, it will fail. Correct?