Probably somewhere between the obs coming out, and 94.
Because far as I knew, they (obs) all had it.
Ford has put a resistor in parallel ever since they switched from the amp meter to the charge light, the placement of the resistor has moved over time from the early 87 being in the bulb socket to later ~89 being on the cluster circuit board.
Many say the bulb acts as a fuse. But on the other hand, that's supposedly what the resistor is for.
Is ifrythings still a member here? I was just reading some of his post on this exact thing on another forum.
The bulb is not a fuse, it's a current limiter so you don't blow out the lamp driver circuit.
In the red box in the photo below is a typical lamp driver circuit use by ford, as you can see when the ignition switch is on, current flows through the bulb (and resistor) into the "I" terminal of the regulator, then through R7 (10 ohm resistor) and into the transistor. Now if you put 12v directly to the "I" terminal the only thing limiting current to the transistor is the 10 ohm R7 resistor which will let 1.2 amps of current through, the transistor is rated at max 1/2 amp, guess what releases the magical smoke?
Depending on how the regulator was design this failure may cause the light to be on or off forever and the alt charges the system or no charging with or with out the light being on.
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I have seen poor connections between the bulb socket and instrument cluster which would cause problems with the earlier design of the resistor being part of the bulb socket which is probably why ford moved it to the back of the cluster circuit board.
Also the exciter circuit must be switched with the ignition, if tide to a constant power, the regulator will keep the field coil in the alternator on (~3-5 amp draw) and kill your batteries.