99 2.5 Alternator Custom Wiring

[YungICY]

Okay, I have a bit of a head scratcher and I need someone that knows the ins/out of this system to confirm a couple things for me.

Firstly, let’s start with the basics. Truck is an 85 with a 2.5l out of a 99. I did have this running and functional for the last few years but I’ve finally decided to update/clean up the initial wiring I did and I don’t recall how I had this component wired.
The truck is running a factory engine harness through the factory C124 plug mounted behind the PS pump and from that plug I have leaned out the harness to make it a somewhat stand alone harness for my application. That harness goes to my dash where I have an all the switches, fuses, relays etc.

Now for the specifics. Pin 41 of the C124 plug is for the alternator regulator and gauge cluster battery light. My wiring book shows the circuit as: hot in run/start—>fuse panel 7.5 amp—> dash bulb with a 470 ohm resistor in parallel—> alt regulator “I” terminal.
When I had it wired before I did not have a battery light hooked up but on my new panel I put one in and would prefer it to work.

The dash light I have in now is an LED. Will this system work with an LED or does it need to be an incandescent bulb? If the LED will work can someone explain what if any changes need to be made from the original design in my wiring book? If I was to not use a battery light, would that “I” terminal wire just be hooked up to keyed 12v power in start/run and ultimately just bypass the dash bulb? From my understanding this wire feeds 12v to the regulator to excite the coils to get the alternator producing power correct?

I can provide pics and any clarification you guys may need. Thank you.

 

[franklin2]

If you did use an LED, it would be critical that is was oriented in the circuit the correct way. Even then, you would have to try it to see if the LED passed enough current to the regulator.

When you first turn the key on, power passes through the dash bulb to the regulator to bring the regulator/alternator online. At this time, with the engine not turning, there is 12v on the keyswitch side of the bulb, and a very low voltage on the regulator side, so the bulb illuminates.

When the engine is started and the alternator starts generating, 12 volts is applied to the excite wire from the regulator. This puts 12v on the regulator side, and it will have 12v on the keyswitch side, so the dash bulb will go out since it has 12v on both sides of the bulb. Throw a belt or have a alternator failure, the 12v from the excite wire will drop away and the light will come on in the dash since the keyswitch is still on.

The 470ohm resistor is parallel across the bulb in case the bulb burns out. If the bulb burned out without the resistor, the charging system would quit charging. So the resistor is a parallel path in case of failure.

If you did not want to use the bulb, I would think you would at least want to use the resistor to simulate the Ford wiring. The light is a good thing though, I think it's even better than a gauge. A light is much more noticeable.

I still do not know how a LED would act in this circuit. You will find if the idle goes really low, and the alternator output is marginal, the dash light will begin to glow dimly. Not sure a LED would do this.

 

[YungICY]

If you did use an LED, it would be critical that is was oriented in the circuit the correct way. Even then, you would have to try it to see if the LED passed enough current to the regulator.

When you first turn the key on, power passes through the dash bulb to the regulator to bring the regulator/alternator online. At this time, with the engine not turning, there is 12v on the keyswitch side of the bulb, and a very low voltage on the regulator side, so the bulb illuminates.

When the engine is started and the alternator starts generating, 12 volts is applied to the excite wire from the regulator. This puts 12v on the regulator side, and it will have 12v on the keyswitch side, so the dash bulb will go out since it has 12v on both sides of the bulb. Throw a belt or have a alternator failure, the 12v from the excite wire will drop away and the light will come on in the dash since the keyswitch is still on.

The 470ohm resistor is parallel across the bulb in case the bulb burns out. If the bulb burned out without the resistor, the charging system would quit charging. So the resistor is a parallel path in case of failure.

If you did not want to use the bulb, I would think you would at least want to use the resistor to simulate the Ford wiring. The light is a good thing though, I think it's even better than a gauge. A light is much more noticeable.

I still do not know how a LED would act in this circuit. You will find if the idle goes really low, and the alternator output is marginal, the dash light will begin to glow dimly. Not sure a LED would do this.

Okay so I was on the right path of thinking. Thanks for clearing up the use of the resistor for me, I was unclear of the purpose of it but that makes perfect sense.

I think I’ll just wire it up minus the resistor for now and see if it works as it should. I’m hoping the LED being a diode doesn’t affect the functionality of it. If it does I’ll do some reading on a resistor to put in there as it may require something other than the original 470 ohm.

Thank you for throwing some knowledge at me.

 

[YungICY]

Update:
I got my dash panel in with the led battery light wired up. I fired up the truck and let it run for a minute and the battery light never turned off.

Couple of things to note: I do have a volt meter that is built in to the usb port I put in. That meter was reading 14.7-14.8 volts. I need to verify what reading I’m getting directly on the battery. I do know that with my old dash panel I had a similar volt meter and it also read high around 14.6-14.7. I should also note that my old battery is toast and the regulator may be cranking up the voltage to charge it.

In the next couple of days I’ll go out and verify voltage at a few places and see if everything is getting the voltage it wants/needs. In the meantime I’m going to do some reading and see if I can’t find a way to get the light to turn off when the alt is charging. It may be as simple as the 470 ohm resistor in parallel with the bulb. I’ll report back in the next few days.