LED Light Bar occasional flickering when engine running

Question

I am in the process of installing a KC Hillites Chase Bar on my Volvo XC90. I am wiring up the functions on the light bar controllers to my existing rear lights, such that when power is sent to a bulb on one of my rear lights (indicator, brake, reverse light etc.) it will activate one of the functions on the light bar.

I have simply cut and spliced in a wire into the positive power wire for each bulb, and run this to one of the inputs on the controller (below).

I thought this would work just fine, however, when I have the engine running the lights on the chase bar flicker (turn off for a moment and then back on, every few seconds), the timing of the flickering is not consistent. I have ruled out any loose connections (and the flickering specifically seems to only happen when the engine is running). Note, if I connect the controller pin straight to 12v, it stays on consistently. So the issue seems to be in the bulb splice setup.

I don't have an oscilloscope, but I connected a multimeter to some of the bulb/splice outputs and watched the voltage and it was sometimes inconsistent (voltage would jump from 12.9v to 13.1v every second), but still hovering around 13v. I was wondering if this inconsistency could be triggering some kind of rising/falling edge logic on the controller? But I was only seeing this inconsistency on one wire, the rest seemed stable as far as my multimeter can sample.

What may be causing this? Is there some kind of electrical component I need to install in the splice to ensure the voltage stays consistent or something for my light bar controls?

If it's relevant, I do have two batteries in the vehicle and the chase bar is powered from the auxiliary battery, but of course, I have common grounds between the batteries.

Update from further testing: If I have the bulb splice drive a relay, and use the relay to control the light controller, everything works as expected. There is no flickering and the relay stays correctly energised. This has solved my immediate problem, but I do still want to understand what the cause of this issue is so I can determine if this relay set up is the best way to resolve it.

Answer 1

The relay setup is probably the best way to go and it was something I was going to suggest even before I got to your update. I don't think you can go wrong there. That way your power source is only powering the relay which takes relatively low amount of power to energize and keep it that way, whereas the lights put a certain amount of drain on circuit which may have been too much for it. I don't know how the control box works exactly, but if it is a solid state type solution, if it gets below a certain power threshold, it might stop power going to the lights until the power was back up to where it wanted it to be. This is sort of a SWAG on my part, but seems reasonable considering. No doubt the relay is the best method to power something like what you're trying to drive.