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Re: Alternator question (long - sorry!) Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 3 Dec 2001 14:39:09
In Reply to: Alternator question (long - sorry!), Rod M, Mon, 3 Dec 2001 12:28:38 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Some alternator theory first -
An alternator is like a generator, except for two things - it produces alternating current instead of DC, and it uses electromagnets instead of permanent magnets. There is a diode pack in the alternator to turn the alternating current into direct current.
In order for the alternator to produce current, the electromagnets (called field coils) need to be energized. This is how it works:
Say the car is off. When you turn the key to ON, but before the engine starts, current flows from the battery, through the BAT light in the dash, through that skinny wire to the alternator, and into the field coils. This creats a magnetic field in the alternator. Since current is flowing through the BAT light, it glows.
You start the car, and the alternator starts to spin. As the alternator spins, it start to creat current. When the alternator is spinning fast enough (around idle speed), the alternator is producing enough current that the Voltage Regulator takes over the job of powering the field coils. Basically, it sticks 12 volts onto the field coils where that wire from the BAT light comes in. Since the voltage regulator is powering the field coils, no current is flowing through the wire from the BAT light, so the light goes out.
The voltage regulator regulates the voltage (whew!) by varying the amount of current through the field coils. When the engine is at high RPM, it puts less current into the field coils (makes the electromagnet less magnetic) so the alternator output doesn't go too high and fry all your electronics.
So, what does that all mean? It means that you NEED that thin wire to the alternator to have 12 volts on it to get the alternator going. But there are some critical things to know -
That wire can only have power when the car is ON. If you tied it to some unswitched power, it would drain the battery.
YOU MUST HAVE THE BATTERY LIGHT BETWEEN THE BATTERY AND THE ALTERNATOR. Ahem. Sorry for shouting. The alternator isn't designed to get full battery voltage on the field coils. The BAT light is a special wattage, and it acts as a resistance to drop the current into the field coils. Too much current is a very bad thing, so don't wire it to whatever handy 12 volts comes around. Also, always replace the BAT light with the exact one specified in the owner's manual.
Yes, a bad connection in the thin wire to the alternator will screw things up, but I don't think you've got to trace it all the way back to the instrument panel. Just make sure where you spliced in the new wire is good - I would solder it if possible. Places like Radio Shack sell solder tape that you can wrap around a joint and heat with a match. Also - you MUST insulate that wire. If that wire touches ground, two things will happen - (1) the BAT light will come on, because one side is grounded, and (2) the alternator won't get the current to turn on the field coils, so it won't produce current.
Make sure that wire isn't shorting. Make sure there are no exposed connections, and that it isn't rubbing anywhere.
I agree with Aaron - replace the voltage regulator. Worn brushes are worn brushes, and the regulator itself may be flaky.
One thing to know - sometimes you'll start the car and the light will be on and the voltage low. Blip the throttle to about 2K RPM for a second or two. See if the light goes out and the voltage jumps up. Sometimes the current produced at idle isn't enough to 'kick over' the voltage regulator, and it needs more speed to get things going. This isn't great, but it's a way to get things going before you replace the voltage regulator.
Good luck!
posted by 140.157....
, Mon, 3 Dec 2001 14:39:09 <-- Viewing This Message
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