Trouble adding led indicator to circuit
 

I have electronic locking differential in my truck and attempted to add an led indicator light but it does not illuminate when the differential coil is powered up.
I do not want to use one of the modern automotive switches that have a built in led indicator. I am using a heavy duty commercial toggle switch that I mounted in the instrument bezel choke hole. I wired the led in parallel with the locker coil. The led will illuminate when the locker is not plugged in but will not come on with the locker connected.

I do not understand why the led doesn't work. How is my circuit any different than those cheap led switches they sell at the auto parts stores? Do I need to add a diode or decrease the resistor size. Could it be the coil has so little resistance the voltage going to the led drops?

Thanks for looking, here is a diagram of the circuit.

Re: Trouble adding led indicator to circuit
 

That's mysterious. What are the specs of the LED (rated Voltage and current draw), and how many Ohms is the resistor? Have you tried it with the engine running so the voltage is higher?

Re: Trouble adding led indicator to circuit
 

Doesn't matter if the engine is running or not. It is a typical 5mm, It came pre-wired with the resistor to work with a 12-14 volt supply. I'm not sure of the resistor rating, its under shrink wrap. The LED itself requires 2.8-3.0V and draws 20mA. It works fine when tested on the battery.

Re: Trouble adding led indicator to circuit
 

Have you measured the voltage at the switch where the wire to the resistor/LED is connected when the switch is on and powering the locker?

Re: Trouble adding led indicator to circuit
 

Not a guru by any sense but know my way around a circuit. I do know led's cannot run parallel to each other in a circuit. Look at an led strip, it puts the led's in a series/parallel orientation for resistance and voltage. The simple solution to me is use a small relay. Wire the coil in parallel with locker and put the led on the normally open contacts. Btw Simi resident as well. Maybe run into you sometime.

Re: Trouble adding led indicator to circuit
 

Hey Richard, I was thinking about adding a relay. I still need to check the voltage drop and see if I can solve the issue by reducing the resistors value. I'm just finishing a shortbed conversion and attempting my first paint job so it will be a while before my truck is cruising the town. It won't be hard to spot, I will be the only guy around with a 8 lug SWB. I do plan on going to a couple of the local Cruise Nights once I'm rolling. What color is your truck?

Re: Trouble adding led indicator to circuit
 

I like to figure out why and how things work myself. Found a few articles online mentioning the led in parallel with a motor. I can guarantee you are not the only one in Simi with a SWB 8 lug. My truck is two tone blue/white. Check out my build thread if you are inclined to do so. Hope to be hitting the local meets next year.