Help - battery or alternator problem

[wwest]

Not to rain on anyone's parade...

Ah, hell, why not....

Insofar as overall magnetic, electro-magnetic design/structure is concerned a generator is just as efficient, or can be easily made just as efficient, as an alternator, and at the same speed. The sole difference is the method of rectifying the AC voltage. Yes, internally the generator, like the alternator produces AC voltage. Whereas the alternator uses solid state diodes for rectification the generator uses the brush/commutator method, mechanical method.

The only other difference is that the METAL in the generator will typically retain some small level of magnetism in the "field" over time so a push-started car will be able to recharge a TOTALLY DEAD battery.

Useless feature today.

[deanslist.us]

There are a lot of knowledgeable people in this thread. :eek: Anyway, thanks to the alternator going bad in time for the holidays, shipping is going to take a week...thought I would check back in and see how the debate was going. Which, I have my own opinions about, but all of you are making good points and arguments. Either way, my local wrench tells me that it is possible to kill the alternator if you try to charge a dead battery (that doesn't want to charge) as it will work doubly as hard to reach the require voltage and eventually burn its own diodes out. I know that on my previous 76 and 84 911's it was a no no to disconnect the battery while hte car was running as that would also kill the alternator/generator (whichever it had I don't know).

Edited December 22, 2007 by deanslist.us

[wwest]

There are a lot of knowledgeable people in this thread. :eek: Anyway, thanks to the alternator going bad in time for the holidays, shipping is going to take a week...thought I would check back in and see how the debate was going. Which, I have my own opinions about, but all of you are making good points and arguments. Either way, my local wrench tells me that it is possible to kill the alternator if you try to charge a dead battery (that doesn't want to charge) as it will work doubly as hard to reach the require voltage and eventually burn its own diodes out. I know that on my previous 76 and 84 911's it was a no no to disconnect the battery while hte car was running as that would also kill the alternator/generator (whichever it had I don't know).

"...no no to disconnect the battery..."

Yes, YES, in SPADES.

Never, NEVER, have the battery disconnected with the engine running.

The battery is an integral part of the alternator/regulator voltage/current output regulating functionality.

Disconnect the battery with the engine running and the alternator voltage output will likely go into "orbit", excessive level and possibly blow EVERY solid state device in the car.

I don't know the physics of this but the regulator works using a PWM method. The following happens at the sub-millisecond level. The cycle starts by turning the alternator output FULL ON, full battery voltage applied to the rotor's magnetic circuit. Then as soon as the alternator output voltage reaches the "setpoint", ~13.6 volts, the regulator switches the rotor current full off. Now the regulator waits for the circuit voltage to decline to some nominal low level and then begins the cycle again.

A poorly charged battery will represent a high resistance to the charging current so the alternator voltage will rise to the setpoint with microseconds of full rotor voltage being applied. Since it is poorly charged it cannot sustain a high voltage level once the rotor voltage is switch off so the PWM duty cycle will be short.

Over the long term the battery will become fully charged, the battery internal resistance itself being used to regulate the charge RATE until the regulator high voltage limit is reached. That voltage regulator setpoint is to some extent a function of ambient temperature, the colder it is the higher the voltage will approach the upper limit.

[rlim]

Well, it's that time to look into replacing the alternator which seems to have a mind of it's own. Works fine for a while hovering around 14 volts and then the next moment .. back down to below 12 volts with ABS lights on. When I accelerated , it pops back up at 14 volts. Question ... is there a difference for alternators betrween manual and automatic transmission? I'm looking at getting one from Autozone and they only have one model and they couldn't tell if they're compatible for both trannies. The only difference I see between the auto vs manual are the pulleys. Most of the sites selling alternators also offer a single model ...

The one with the black pulley (right) is for the manual transmission vs the one with the exposed nut (left) which is for the tiptronic... which is similar to the one I have on my car. I pulled the pics off ebay..

Edited February 11, 2008 by rlim

[RFM]

The one with the covered pulley have a free wheel, the other one have a fixed pulley. If the original have a fixed pulley, replace with the same type.

[KevinMac]

Well, it's that time to look into replacing the alternator which seems to have a mind of it's own. Works fine for a while hovering around 14 volts and then the next moment .. back down to below 12 volts with ABS lights on. When I accelerated , it pops back up at 14 volts. Question ... is there a difference for alternators betrween manual and automatic transmission? I'm looking at getting one from Autozone and they only have one model and they couldn't tell if they're compatible for both trannies. The only difference I see between the auto vs manual are the pulleys. Most of the sites selling alternators also offer a single model ...

The one with the black pulley (right) is for the manual transmission vs the one with the exposed nut (left) which is for the tiptronic... which is similar to the one I have on my car. I pulled the pics off ebay..

I steer away from 3rd party rebuilds, been there and burned too many times. My feeling is these rebuild houses use cheap components such as regulators etc.. They don't last!

[rlim]

Well, it's that time to look into replacing the alternator which seems to have a mind of it's own. Works fine for a while hovering around 14 volts and then the next moment .. back down to below 12 volts with ABS lights on. When I accelerated , it pops back up at 14 volts. Question ... is there a difference for alternators betrween manual and automatic transmission? I'm looking at getting one from Autozone and they only have one model and they couldn't tell if they're compatible for both trannies. The only difference I see between the auto vs manual are the pulleys. Most of the sites selling alternators also offer a single model ...

The one with the black pulley (right) is for the manual transmission vs the one with the exposed nut (left) which is for the tiptronic... which is similar to the one I have on my car. I pulled the pics off ebay..

I steer away from 3rd party rebuilds, been there and burned too many times. My feeling is these rebuild houses use cheap components such as regulators etc.. They don't last!

-----------------------------------------

I figured if I'm to purchase from Autozone, they offer lifetime warranty on the alternator. I can alway return it for warranty replacement. At $225.49 and $90 core, its reasonably priced . Their alternator does not come with a pulley which I assume I can easily transfer from my alternator.

[KevinMac]

Well, it's that time to look into replacing the alternator which seems to have a mind of it's own. Works fine for a while hovering around 14 volts and then the next moment .. back down to below 12 volts with ABS lights on. When I accelerated , it pops back up at 14 volts. Question ... is there a difference for alternators betrween manual and automatic transmission? I'm looking at getting one from Autozone and they only have one model and they couldn't tell if they're compatible for both trannies. The only difference I see between the auto vs manual are the pulleys. Most of the sites selling alternators also offer a single model ...

The one with the black pulley (right) is for the manual transmission vs the one with the exposed nut (left) which is for the tiptronic... which is similar to the one I have on my car. I pulled the pics off ebay..

I steer away from 3rd party rebuilds, been there and burned too many times. My feeling is these rebuild houses use cheap components such as regulators etc.. They don't last!

-----------------------------------------

I figured if I'm to purchase from Autozone, they offer lifetime warranty on the alternator. I can alway return it for warranty replacement. At $225.49 and $90 core, its reasonably priced . Their alternator does not come with a pulley which I assume I can easily transfer from my alternator.

I guess the question is how many times do you want to continually replace it!. You may luck out, but in my past expierence, they just don't last.

[vette67]

Well, it's that time to look into replacing the alternator which seems to have a mind of it's own. Works fine for a while hovering around 14 volts and then the next moment .. back down to below 12 volts with ABS lights on. When I accelerated , it pops back up at 14 volts. Question ... is there a difference for alternators betrween manual and automatic transmission? I'm looking at getting one from Autozone and they only have one model and they couldn't tell if they're compatible for both trannies. The only difference I see between the auto vs manual are the pulleys. Most of the sites selling alternators also offer a single model ...

The one with the black pulley (right) is for the manual transmission vs the one with the exposed nut (left) which is for the tiptronic... which is similar to the one I have on my car. I pulled the pics off ebay..

I steer away from 3rd party rebuilds, been there and burned too many times. My feeling is these rebuild houses use cheap components such as regulators etc.. They don't last!

-----------------------------------------

I figured if I'm to purchase from Autozone, they offer lifetime warranty on the alternator. I can alway return it for warranty replacement. At $225.49 and $90 core, its reasonably priced . Their alternator does not come with a pulley which I assume I can easily transfer from my alternator.

Glad you are doing the work yourself. I spent $1200 on getting my alternator replaced and later discovered (after reading the FAQ you referred to) how easy it is to remove the darned thing. Ive had it off twice recently in order to get to some vacuum leaks and it is a no-brainer!

Edited February 13, 2008 by vette67