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Not uncommon Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 7 Jan 2003 10:32:58
In Reply to: Alternator?, Carl, Mon, 6 Jan 2003 23:39:52 Members do not see ads below this line. - Help Keep This Site Online - Signup |
If you've just put in a new (or rebuilt) alternator with a fresh voltage regulator, you're probably just seeing the brushes wear in. If the problem doesn't go away after a few weeks, you may want to replace the VR.
However, this isn't necessarily a problem. It has to do with how the alternator works.
An alternator has electromagnets to create a magnetic field - they're called the 'field coils'. The faster an alternator or generator goes, the higher the output voltage. Since you want to control the output of the alternator (you don't want 18 volts at 4000 rpm!), the voltage regulator adjusts the current into the field coils - at higher speed, the VR puts less current into the field coils, so the alternator output stays regulated.
But an alternator can't produce current until the field coils are engergized, and the field coils are engergized by the alternator. Chicken and egg. Well, not quite.
When you turn the car to ON, current flows from the battery, through the key, THROUGH THE ALT LIGHT in the dash, to the field coils. This current energizes the field coils, so the alternator can produce current when the engine starts spinning. Since current is flowing through the light in the dash, it lights up.
Once the alternator gets spinning, it produces current on it's own, and the voltage regulator uses this voltage to drive the field coils. Since there is now around 12 volts on both sides of the light bulb, the light goes out.
However, there isn't a lot of current through that bulb, so the field coils don't get a whopping lot of current. When you start the car, if the engine isn't turning fast enough, the Voltage regulator can't supply the field coils with enough current, so the light stays on. If you blip the throttle, the alternator speeds up just enough that the voltage regulator can take over, crank up the current to the field coils, and get enough output from the alternator to take over - the light goes out.
As long as the light is staying out after that first blip, then you've got no problems. It wouldn't hurt to stick a voltmeter onto the battery - with the light out, you should be seeing 13.0 volts or more on the battery at idle.
I've had cars where the VR would go through the 'I need a throttle blip to get going' for a few weeks, then be fine. An annoyance, but not a big problem, as long as the alternator is putting out the proper voltage when driving.
posted by 192.249....
, Tue, 7 Jan 2003 10:32:58 <-- Viewing This Message
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