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The battery warning light comes on if either the alternator fails or battery voltage is significantly below normal.
See typical wiring diagram below:
If the alternator fails, current flows thru warning lamp "A" in the left to right direction; if the battery voltage drops significantly while the engine is running, current flows from right to left thru warning lamp "A." In either case the battery warning lamp glows. (Unless a diode placed in series with lamp "A" which sometimes occurs.)
Transistors T1, T2, & T3 plus associated diodes and resistors form a current regulator circuit that regulates field current to keep alternator voltage constant (more or less) as alternator rotational speed varies with engine speed.
Most charging systems like this have a bypass resistor so that if the alternator light burns out, that does not in itself prevent the alternator from exciting, but some older rigs don't have that. If the alternator warning light does not come on when you turn the key to "on" without starting the car, it's possible that the system won't charge and all it has to have is to replace that bulb.
Yes, most of the newer cars have a bridge resistor. They probably use an LED as well. They could use a bridge rectifier to turn on the LED regardless of the direction of current flow. But there are LEDs that glow red or green (or other colors) depending upon direction of current flow (consider them as parallel but oppositely directed diodes in one bulb body) so it would be possible to make the charging warning light glow red or green depending upon whether the alternator or the battery were failing. Just a thought.
Really? You should tell this to all the high rpm drag racers that don't run alternators. Or maybe it was sheer luck I was able to once drive 80 miles on the interstate trying to make it home and the alternator wasn't charging.
Deez - You and I both know there is a major difference between race ignition systems and those found on commuter cars. Not to mention most racers use batteries such as Optimas (which are designed for such abuse) and also charge them between passes. And yes, you were lucky, very lucky, to make it 80 miles running on the battery alone... Obviously you weren't running a huge draw on your vehicle's electrical system, or a power-hungry MSD box.
Quote:
Originally Posted by Tourian
I actually had an alternator go out on me while I was driving. I watched my voltmeter quickly drop and one by one warning lights start to come on as the voltage dropped too low to keep my SRS system going, etc until the car died.
The battery provides a buffer between the alt because at idle, your alt isn't spinning as fast and doesn't provide the amps and voltage necessary to run your vehicle and all its systems - especially if you have you a/c, headlights or stereo on.
So stating all cars can run without one or the other is way too broad a stroke. It is much safer to say a typical car (not a stripped out drag box) has to have both in good condition to run properly and its just a guess as to which one is bad - both need to be tested on the OP's vehicle.
I guess I should have been more specific in my post. MOST vehicles will run off the energy provided by the alternator... granted that you're not running every accessory in the vehicle, or those that require large amounts of voltage.
Drover, are be confused now?
Simple test: if engine keeps running after positive battery cable is disconnected then battery is bad, if engine does not run after battery is disconnected then alternator is BAD, however battery could be in good shape.
If battery does not start engine, it is usualy low in voltage, if battery has been jumpped over and over again, sign that something is wrong, next time do not wait to long that way you will be able to salvage one or the other instead of replacing both as you might need to do now.
Good luck
Drover, are be confused now?
Simple test: if engine keeps running after positive battery cable is disconnected then battery is bad, if engine does not run after battery is disconnected then alternator is BAD, however battery could be in good shape.
If battery does not start engine, it is usualy low in voltage, if battery has been jumpped over and over again, sign that something is wrong, next time do not wait to long that way you will be able to salvage one or the other instead of replacing both as you might need to do now.
Good luck
Ain't taking no battery cable off nothin' while the car is running (which it won't with or without the positive battery cable connected, and I'm guessing the ultimate problem is the alternator). Saab electricals aren't cheap and I'm not messing with voltage spikes and no battery to quell them.
Ain't taking no battery cable off nothin' while the car is running (which it won't with or without the positive battery cable connected, and I'm guessing the ultimate problem is the alternator). Saab electricals aren't cheap and I'm not messing with voltage spikes and no battery to quell them.
Right answer. Taking the battery cables off the car as it sits *might* be the same as doing the old hillbilly *generator* test, which can blow out the diodes in your alternator.
If you have not yet tried this, try taking the existing battery out of the car and put in another 12V car battery - does not have to be new, or even a perfect fit, just drop one in from your other car, it just has to hook up to the cables and more or less fit in the battery tray. See if it will start/run with the known good battery.
Alternative test would be to charge the existing battery, see if it can be charged.
When I had a roughly similar situation with my 68 GTO, the car would not start even when jumped with the existing battery, but would start and run fine with a borrowed battery. Alternator was fine.
I have the same problem for 2000 towncar my car is dead,in order to start the car I have to jump cables to it once I take one of them off it turns off
I just bought my battery new last year
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