dr bob

Hardly smart-assed comments. This is kind of discussion that gets folks thinking. I have to say that, --in spite of the supposed eedikation and all--, I spent way too much time analyzing when I ultimately just played dumb and put the new one in. All of the testing and measuring and cleaning was fun. Right...


Anybody need a real pretty S4 alternator? Tests fine in the car and all. Just doesn't keep the battery charged.

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Tmpus~, my used one looks better than the 'rebuilt' you show. Slip rings less worn, brushes still plenty good just looking through the regulator access. Hence the warning to get a known-reliable remanufactured or new unit.

tmpusfugit

Yeah, can't tell you how pleased I was to see the slip rings in this device. I am not certain how that much wear could be caused in an alternator that has only been available for 4-6 years....must have run it in more than one Texas sand storm.... And this was "rebuilt" by a very highly recommended and aclaimed Houston rebuilder.....

borland

dr bob,

It probably just needs one of these $20 regulators.... set point at 14.5V....

http://cgi.ebay.com/ebaymotors/Bosch...Q5fAccessories

tmpusfugit

When an alternator does not keep a battery fully charged over time and use there are several suspects, including a faulty regulator. Other causes can be a too loose fan belt, worn brushes on the slip rings, an open winding in the stator (one segment), or a defective diode. Frequently all will appear fine with the output voltage of the alternator, until such time as it becomes heavily loaded by electrical requirements. Generally the charge voltage will drop well below the intended 14.5 vdc and may drop below 12 vdc.

Unfortunately, some of the rebuilders do not do the best work, they do not test their products well, may not test them under anything more than running it unloaded and verifying an output of 14.x +/- a bit.... I have 2 very recently rebuilt alternators here now, one from fleabay and the other from a well respected local shop, neither of which will generate the 14.5 vdc under load. The 95 amp Bosch will drop to 11 vdc or less with the lights on, after a few minutes. The 130 amp Bosch drops to 12.x +/- under moderate load. I am reading all voltages with an accurate DVM, I have cleaned grounds etc, and have measured the voltages at the battery, at the alternator, and at the main fuse panel.....

What's my point? Buy your alternators from one of the big 4 or 5 suppliers and pay the bit extra they may charge....I am darned certain I would be money ahead had I done so.....and less frustration too.... but we can just consider it OJT?

Landseer

I bought a factory-Bosch rebuild a few weeks ago.
Its supposed to be a complete high quality rebuild / looked and worked great.
Needed it in a rush -- wife and daughter were stranded with all dash warning lights illuminated, a few hours away.
Bought it at Advance Auto for about $150 which I think is higher than our suppliers, but it did have free overnight delivery which is what I needed.
Ordered it one afternoon, picked it up enroute at 9am the next day.

WallyP

Had a friend in the auto electric business a few years ago, and I read some of his professional magazines - pretty interesting.

One thing that I remember was that the industry average for faulty (all causes) rebuilt alternators was 28%.

Landseer

One thing that's in our favor is ease-of-access, relatively speaking, on the 928.

WallyP

The alternator on my wife's Suburban is sitting on top, and removal takes two or three minutes. On her STS, it is buried, and removal is a two-hour job - ask me how I know...

dr bob

The voltage rating on the original alternator/regulator isn't obvious. On the replacement piece it says 14V. I'm not sure that 14.5 will buy anything except shorter bulb and battery life. I'm pretty appy with the 13.8V or so it delivers at the battery and the jumpstart terminal, and the corresponding reading at the dash gauge.

Increasing the voltage demand forces the alternator to work harder trying to keep the battery at the higher terminal voltage. It takes more current to do that, so more mechanical load and more heat result. My memory harkens back to my now-gone Explorer DD, which had a nice healthy 15 volts from the alternator. It's what they were rated for, apparently. Very bright lights but I replaced a lot of bulbs, and three years max out of a battery with regular water replenishment along the way. Otherwise no negative effects. Higher system voltage does mask a lot of the effects of corrosion and wire/connection resistance. It allows car makers to get the same total energy through a smaller wire too. So for a tired car the higher charging voltage might be a plus.

borland

Don't forget that the battery is in the rear, while the regulator is up front. What the voltage regulator sees in voltage will be higher than what the battery sees. So a higher rating may be just right for some 928s.

13.8-14.7 volts is an acceptable range for charging voltages.

dr bob

Just happened to read the Bosch literature that came with the new alternator. By ~80% of rated output current, they expect the terminal voltage to be just north of 12 volts. Not the stated voltage rating on the regulator.

Maleficio

I just replaced my Valeo alt with a rebuilt Bosch. It worked great until I adjusted my idle too low. I dropped the idle to below 750, and the alt rested at 11 volts. I cranked the idle back up to 750, alt puts out just fine, 13.5.

borland

A charging voltage of just north of 12V would be on a fully discharged battery. As the battery get fully charged, the charge voltage will increase as the charge current tapers off. The regulator simply adjusts the current supplied to the rotor.

A regulator with a higher set point will charge faster (higher current) than one set lower, but the charge voltage is dependent on the batteries state of charge.

Maleficio

Makes sense.

I wonder if people's exciter wiring is rotten? I've fixed plenty of rotten wires on my 82 OB so far, I'm thinking I should have a try at replacing that wire. It's very flimsy and soft from age and heat.

dr bob

Re-read post 9 in this thread. The exciter circuit, through the bulb and the dash resistor in parallel, supplies the -initial- excitation needed to light off the alternator on engine start. As soon as the alternator internally generates voltage greater than what's coming from that circuit (minus a couple voltage drops through internal diodes), that exciter circuit and wiring is no longer used. Evidence is that bulb is not lit. Bottom line is that if the alternator is working at all, that exciter wiring is OK. Replacing it may make you feel better, but it's a heck of a lot of work rebuilding a couple harnesses for no change in alternator performance. My too sense.