Originally Posted By: Ethan1
Originally Posted By: George7941
Originally Posted By: Ethan1
The warning light does not work the way everyone seems to assume. On an older car like this, the alternator itself powers the warning light when it fails internally. No belt = no power from the alternator = no power to the light.
Wrong! The OP stated that the lights powers up when the ignition is turned on.
On the older cars the alternator powers up one side of the bulb, the other side has ignition power.
How are the two mutually exclusive? Why do you think that a self test is impossible for a bulb that is powered by the alternator? Integrated circuits are weird... it's been a wild ride, these last 50 years.
I will tell you why. Older cars had a very simple circuit for the warning light. One side of the bulb had ignition power, the other side was hooked up to the alternator such that it had roughly 12V when the alternator was charging. Thus when the alternator was charging, the bulb had equal voltages on both sides, hence 0V across it and therefore would go out.
I know of no car where the alternator powers the tell tale on.
Re StevieC's example, lots of older cars would not charge if the tell tale bulb burned out because the current through the bulb when the ignition was turned on and the bulb lit was the current for the initial excitation of the field windings. Some alternators retained enough residual magnetism in its field poles when the engine was not running to self-excite once the alternator started turning and would produce even with a burned out tell tale.
