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  #1  
Old 09-14-2001, 07:35 AM
Joe-1
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Alternator output voltage at the battery

I would like to know about the alternater output voltage on my 1986 190 16 valve. I used my volt meter on the battery with the engine running and got a battery voltage of 13.72 volts. I noticed that most european cars never hit 14 volts. My toyota gets to 14.5 when it is running. Is this normal for all euro cars? My old VW only got to 13.8 as well. Is this normal for Bosch alternators? Thanks, Joe
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Old 09-14-2001, 08:02 AM
LarryBible
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Most alternators put out around 13.5 volts when everything is doing okay. I would be a little concerned about 14.5 volts, but it would probably be okay, although it would probably shorten battery life a little.

Your system is in good shape.

Have a great day,
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Old 09-14-2001, 11:27 AM
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It generally takes about 14.2V to charge a conventional lead-acid battery fully, but modern voltage regulators can get pretty sophisicated. Most 12V accessories don't last as long with a high voltage.

A test of the charging system is to monitor the voltage across the battery at idle, then turn on the lights (high beam & fog). The voltage should stay about the same. If it drops by 1/2 Volt or less, increase the rpms a bit and see if it goes back up. If you have any suspicions after these tests, have the charging system performance tested.

Steve
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Old 09-16-2001, 10:50 PM
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The one MB I've owned puts out about 13.25-13.30 at idle with engine fully warmed up. It reads higher after initial startup when cold. It read a bit higher a few yrs. ago. I suspect a set of brushes might kick it back up. One of these days I'll deal with that.

All of the Asian cars I've owned read much higher than my MB. Readings in the high 13's to low 14's are not uncommon in Honda and Toyota vehicles and these are cars with MANY miles on them.
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Old 09-17-2001, 09:17 PM
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I have noticed this as well. As long as you are above 13, you are OK. Mercedes uses alternators that are at almost 90% duty when all accessories are on...This did present a problem with the early 124's. I have converted several over to the later Diesel alternator. It puts out about 35 more amps.
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Old 09-18-2001, 11:40 AM
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"The one MB I've owned puts out about 13.25-13.30 at idle with engine fully warmed up. It reads higher after initial startup when cold. It read a bit higher a few yrs. ago. I suspect a set of brushes might kick it back up. One of these days I'll deal with that."

I doubt if that is the culprit. Unlike with DC generators, the brushes in an alternator carry very little current. Also, they are actually slip-ring contacts, not commutator brushes, so there is no interruption of contact to cause sparking. They allow the regulator to adjust a magnetic field in the rotating iron structure - a spinning electromagnet - which then induces an AC field in the high-current fixed windings.

Although I have not experienced brush failure in an alternator, I would expect it to cause an erratic, then complete failure of output within a short period of time. I HAVE found many instances of diode failure, disabling one or more of the three high-current fixed windings, and this causes a reduced current capability under load, with resulting reduced voltage. Most other faults cause complete failure.

Steve
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