alternator light resistor size

[lindbergh]

Does anyone know the size of resistor that needs to be placed in parallel with the alt. bulb for the purpose of keeping the alternator charging if the bulb goes out? I was thinking at starting at 100k ohm and using the trial and error method, but would help if someone knew to avoid that method. Was wondering what the car mfrs. use.
Thanks.

[Bitsyncmaster]

My guess would be to start a 1 Kohm. But it should be quick to check just by removing the bulb and testing if it charges.

[David T]

My guess would be to start a 1 Kohm. But it should be quick to check just by removing the bulb and testing if it charges.

I am not sure you can do what you are trying to do. Having a bulb and a resistor in parallel can be done but the resistance will change when the bulb burns out. You may be able to get the circuit to work when the bulb is working but with the bulb burnt the resistance may be too high. The typical failure mode is not a burnt bulb, the regulator goes bad or the brushes get worn. Always watch the charging bulb when starting the motor. It has to light when the key is on before the motor is running. It is your bulb and charging circuit check. If it doesn't light at that time you know you have a problem in the charging circuit.

[mluder]

There is a method that allows you to install a bulb in another wiring location that's easier to access - i.e. under the coil cover in the engine bay. However it requires the splicing of a new bulb socket into the wiring. Quite honestly, I'd just remove the binnacle and instrument cluster and replace the bulb. It will take you all of 15 minutes and is good practice. Be careful of the disconnect and reconnect of the speedo cable - gotta make sure it's seated correctly to function. Also the gold fingers on the two instrument control wires can be fragile.

Take your time and go slow and you'll be fine.