The Stovebolt Site Forums The Shop Area The Electrical Bay LED Side Effects in original style wiring

This might be relevant to some of you.

LED Turn Signals

I completely rewired the truck and put in dual filament switchback LED bulbs with white "filaments" for the running lights and amber "filaments" for the turn signals.

When I went to test it, I found the the commanded turn signal worked as expected, but the opposite side flashed at half the frequency. Only the front showed that behaviour.

I verified that both sides had the correct voltages on the correct pins at the correct times, so why was this happening?

I did install an electronic flasher unit for the low power LEDs, so that wasn't the issue. Switching to traditional incandescant bulbs made the issue go away, so the base wiring seemed to be correct (though the current draw for my lighting went from 923mA to 4.6A).

The issue turned out to be the switchback functionality, coupled with the lack of pulldown on the turn signal lines. There was a very, very short ground bounce relative to the turn signal line that triggered the switchback to turn off the running light, but did not have the current to power the turn signal.

The solution was a 10k pulldown on the turn signal wires to prevent the switchback logic from triggering.

So you put a load resistor (10K) for the LEDs?
I've heard of that in more newer vehicles when converting from incandescent to LED especially with CANBUS systems, but haven't dealt with switchback LEDs.
Very interesting.

No, a pulldown not a load resistor.

They install the same way (from power to ground) but have different resistances and serve different purposes.

A load resistor is generally < 10 Ohms and is intended to make the circuit draw enough current for the flasher and/or ECU to think there is an incandescent bulb present. They are high power (measured in watts) and need to be open air to avoid burning up.

A pulldown is much higher resistance (10,000 Ohms in this case) and serves to keep the pin from floating when power is not connected. It is about the size of a piece of pencil lead and is hidden inside the loom.

I have a couple of unused load resisters in the shop that are pretty much heat sink, but haven't heard of a pulldown, let alone switchback LED bulbs... but have only been working on just this project for the last 4 years.

I would like to know more about this pulldown resistor stuff, especially when it pertains to vehicles 12v & 6v uses. Is a pulldown resistor different from a "normal" resistor? How do you id one? How does it hook up? I am assuming that it us only used with led bulbs. Thanks!

I will try to get some time to do a write-up with pictures this week.

The somewhat important note is that "load" and "pulldown" are describing what the resistor is being *used for*. Not the *type* of resistor.

Certain uses are more effective with resistors of different specifications, but we need to be careful not to conflate the function with the type.