Alternator life expectancy ?
12-01-2005, 12:28 PM
#16
"Isn't the alternator output constant with excess power basically running out to ground?" - epj993 -
The automotive alternator is a constant voltage source and as such has no current
limiting function like the old generators used prior to alternators. This means that
the alternator will be damaged by excessive loads, e.g. a bad battery cell.
The Bosch 964/993 alternator (same for both) is probably one of the most reliable
911 type alternator ('65 - '98) except for the 911SC ('78 - '81) which can exceed
150K miles. The most common problems for the 964/993 alternators are worn slip
rings and regulator brushes which reduce its life to about 75K miles. Excessive
belt tension causes bearing failures or broken alternator shafts, i.e. the result of
the extra long alternator shaft on the 964/993 alternator. The next common problem
is reverse battery jump starts which occurs more on the 964/993 than the early
911s.
The automotive alternator is a constant voltage source and as such has no current
limiting function like the old generators used prior to alternators. This means that
the alternator will be damaged by excessive loads, e.g. a bad battery cell.
The Bosch 964/993 alternator (same for both) is probably one of the most reliable
911 type alternator ('65 - '98) except for the 911SC ('78 - '81) which can exceed
150K miles. The most common problems for the 964/993 alternators are worn slip
rings and regulator brushes which reduce its life to about 75K miles. Excessive
belt tension causes bearing failures or broken alternator shafts, i.e. the result of
the extra long alternator shaft on the 964/993 alternator. The next common problem
is reverse battery jump starts which occurs more on the 964/993 than the early
911s.
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12-01-2005, 01:26 PM
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Alternator lifespan
My original alternator went out at approximately 67,000 miles on my 993 tt. The rebuilt replacement had to be replaced twice (under warranty). My 1992 BMW 525i had an alternator failure at 151,000 miles. Like others have said, it is difficult to predict the lifespan.
12-01-2005, 07:04 PM
#20
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Originally Posted by kurtv
The rebuilt replacement had to be replaced twice (under warranty).
12-02-2005, 02:42 AM
#22
"so what role does the voltage regulator play? Does it limit/maintain voltage on the output side of the alternator?"
limit - typical motorcycle battery regulation
maintain - automotive alternator regulation
Regulator Function:
1. If battery voltage is less than preset regulator voltage by some "delta" (Vout- Vd),
then turn "on" the alternator to full output by applying the battery voltage to
the field winding.
2. If battery voltage is greater than the preset regulator voltage by some "delta" (Vout+Vd),
then turn the alternator "off".
The battery regulation or "ripple" voltage is then twice the "delta" or 2Vd (<0.50 volts).
limit - typical motorcycle battery regulation
maintain - automotive alternator regulation
Regulator Function:
1. If battery voltage is less than preset regulator voltage by some "delta" (Vout- Vd),
then turn "on" the alternator to full output by applying the battery voltage to
the field winding.
2. If battery voltage is greater than the preset regulator voltage by some "delta" (Vout+Vd),
then turn the alternator "off".
The battery regulation or "ripple" voltage is then twice the "delta" or 2Vd (<0.50 volts).
12-04-2005, 02:10 AM
#23
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Mine died on the way home from Champion Porsche in Pompano Florida right after they did the engine wiring recall.
My car has 34K miles. They not only tried to hit me for a $1200 fan claiming that the bearing wasn't straight but they wanted to charge me $800 for the rebuilt alternator and $600 to install it!!!!
I wonder how many people who don't know better get ripped off daily by them.
Chris
My car has 34K miles. They not only tried to hit me for a $1200 fan claiming that the bearing wasn't straight but they wanted to charge me $800 for the rebuilt alternator and $600 to install it!!!!
I wonder how many people who don't know better get ripped off daily by them.
Chris
12-04-2005, 02:14 AM
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Mine went at about 108K. It's a '97. It may be that time is a bigger factor than miles given that some (like mine) are daily - as much as possible - drivers and others sit waiting for sunny weekends and dry roads. 
12-18-2005, 12:15 AM
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Well mine just stranded me at 47k miles. Its being towed back to the house now. I wish these cars had voltmeters to let me know if the alternator is about to go. No warning. Just started right up after dinner out and after about five minutes some of the warning lights came on. I pulled over to a gas station and shut off the engine . Would not start right back up. Waited a few minutes and pushed the car to a hill and started it, got out on the highway and then all the warning lights started to come on, then the lights dimmed and I knew I was losing electrical power. Got off at the next exit and the car died.
I just had the belts replaced so its possible they weren't tensioned properly. When this happened on my 87 Carrerra the alternator light would come on sporadically. Not so with the 993. Looks like a similar diy to replace it. I also was quoted about 1200 to replace it on the 87 which had 91k miles when the alternator went.
I just had the belts replaced so its possible they weren't tensioned properly. When this happened on my 87 Carrerra the alternator light would come on sporadically. Not so with the 993. Looks like a similar diy to replace it. I also was quoted about 1200 to replace it on the 87 which had 91k miles when the alternator went.
