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Voltage regulator
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 22 Jan 2003 15:19:58 Share Post by Email
In Reply to: Electrical Oddity - voltage increases with engine speed, Simon P. Coffin, Tue, 21 Jan 2003 19:16:39
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The voltage regulator is responsible for keeping voltage stable as engine speed changes. It does this by controlling the current in the alternator's field coils. So I would expect you have a faulty voltage regualtor.
The VR is the diamond shaped device bolted to the back of the alternator. Very easy to replace on the 900. Just disconnect the negative lead from the battery first. A new VR is about $35-$50, depending on where you find it.

To be sure, stick a voltmeter on the battery and start the car. With most electrical loads off (headlights, defroster off, fan on low), you should see around 13-13.5 volts at idle. At full RPM it should be no more than 14.0 volts. A good VR will hold the voltage very stable.

It isn't the battery. The battery is there to start the car. Once the engine is running, the alternator provides all the electrical power the car needs, and recharges the battery to replenish what was lost from starting the car. Normal battery voltage is around 12.6 volts. When the engine is running, the alternator is putting out voltage in the 13 range. Since this is higher than 12.6, it's pushing current INTO the battery. The battery will help with some very transient high loads, but those are on the order of tenths of seconds.

posted by 192.249....

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