Rear lights (inside) LED conversion

[maccmowers]

Hi all,

I've got LEDs in the back of my MK7 to replace the factory fitted candles in jam jars. I've had them in for a year now and they have been flickering, sometimes working/sometimes not (individual lights - there are about 30 LEDs) so I'm trying to sort out why. I think its because the lights dim as they shut off and the dimming action is playing havock with the LEDs (maybe?). I also have a normal 12v (non LED) wired to the same circuit because without it the LEDs stay on very, very dimly when the van is locked. I assume it needs a resistor to shut down the micro amps of current in the system.

Today I have fitted 60 new LEDs and ran the whole circuit through a relay so that a battery live powers the LEDs and the van wiring powers the relay, but they still shut down in a dimming fashion and flicker as the voltage drops on power up/shutdown.

Any ideas how to make them come on and off without the slow rise and fall of voltage? I don't want to put it on a manual switch because I'm lazy and want it to work on the key!

[the dutch guy]

check if there is a capacitor somewere in the unit. it is there to give the lights more bulb-like effect instead of the annoying instant-sun effect leds can have. just look at audi's led rear lights. it almost seizure inducing and dangerously blindingly bright some rear units have capacitors and resistors to make the computer think there is a real bulb in the unit and it has the proper power consumption. with the mk6 it does not matter as it has not computers to speak of, the mk7 is too smart in this regard.

open the unit and put in a smaller cap if one is present or put a resistor across the input to increase the power consumption and drain it faster shortening the "hold" effect the capacitor creates.

the dim effect is because of the leakage current of the mosfet inside the computer andif you have it bypassed with a relay and power it directly from the battery you see the capacitive charge caused by the wiring. you also see this in led's in your house were the negative side is swiched. if you put the switch on the positive side you dont have that problem.

do not worry, the light being created was also used by the old bulbs but those do not light up with such a insanly low current. leds do. it can run for YEARS before draining the battery. it is just a extremely slight current used for the bulb check, nothing more. (also a slight leakage current of the mosfet depending how its set up in the computer)

[maccmowers]

I think I over complicated my original message so here's a simplified version...

I want the LEDs to come on, and go off rapidly. Not slowly like they are doing.

I tried using a relay to rectify the above issue so the LEDs are powered from battery live and the relay is powered by the original van wiring, but the slow increase in voltage from the van wiring just pulls the relay on slowly so it has the same effect.

Does a relay exist that only activates when voltage reaches 12v?

[the dutch guy]

the relay cannot go on "slowly" as it is a relay. it either moves or it does not. there is no inbetween. as soon as there is enough voltage to overcome the spring tension it will pop the contact into place. depending on the relay that happens around 8~9 volts and disconnects around 6~7.

also: if you have a mk7 you MUST put a diode across the relay to prevent burning out the computers output. turning a relay off causes a massive voltage spike wich can kill the driving mosfet inside the computer.

the slow on and off ecffect is casued by a capacitor in the light unit itself or a driver chip if present.
if the drive signal is done by a relay and you still see a weak light coming from it then you need to use a double pole relay and disconnect both the positive and negative. it lights because of the capacitive action you get from having long wires taped against eachother and you are switching off the wrong polarity.

[maccmowers]

Thanks for the in depth explanation. I have already used a relay with a diode built in and also have a normal brake light bulb in addition to the LED's to soak up the 'capacative action'.

What I don't understand though is your explanation that the dimming effect is done by a capacitor in the light unit. The original lamps are just light bulbs (382 maybe) and as far as I'm aware don't have a capacitor built in, and they light up/go out slowly. Also you said about the relay being on or off (with no in between) but I can feel the relay vibrate as the voltage increases/decreases.

[the dutch guy]

if the relay is audibly bouncing then it probably means the thing driving the relay (the computer) is slowly ramping up voltage causing the relay to flutter. this can be solved with a few extra components.

dont use a bulb to try and absorb this soft-on soft-offstuff. it will never work properly, it will only cause more power consumption.

also if your units are decent quality you will have led drivers in them wich usually "hold" the led on with their reserve capacity. (onyl a few dozen milliseconds, but enough to notice)

[maccmowers]

I know the computer is the thing slowly ramping up the voltage, that's what I'm trying to stop! That's why I added a relay hoping it would work like a light switch.

The bulb isn't there for the soft on soft off. It's there so the LEDs don't stay dimly lit when the doors are locked.

If you know what the extra components are can you please explain?

[the dutch guy]

How do you have everything connected now? Can you make a drawing?

[maccmowers]

It's no Picasso but it does the job...

[the dutch guy]

is that for only the regular rear lights or do you have the same setup for the brake and indicator? also: do you use those drop in replacement leds or do you have a entire new led unit?

[maccmowers]

Brake and indicator? I've changed the loading area lamps from 2 normal bulbs to LEDs (hence title 'Rear lights (inside) LED conversion).

No lamp units, just LEDs in white blocks of 3 strung together.

The original bulbs lit up/dimmed slowly (as designed by Ford), and so do the LEDs. Thats what I'm trying to stop. I want them to light up and die in a millisecond.

[the dutch guy]

right i misunderstood then.


the reason why it still lights is because there is leakage current in the cables and other stuff it is connected to like the rear-light-in-the-front. that is what causes the leds to dim, current is bypassing the relay via another route as you are still using the old wires. putting a bulb in parralel just causes a voltage drop, you are still using (a very small amount) of current.

use a double pole relay and disconnect both the positive and the negative of the lights. you might want to run dedicated wires to the lights to do it proper.

[maccmowers]

I know you're trying to help me, which I appreciate, but you need to read what I'm writing!

I'm not trying to solve the 'staying on dim', that is already sorted. The original lights didn't stay on, it'was just the LEDs. That's why I've left a normal bulb in the circuit. (I now have LEDs AND a normal bulb) Please forget everything we have already talked about. I'm going to start again.

The original transit lights (just 2 x normal bulbs) have a soft on/off feature. The voltage comes from the computer in a slow fashion giving the effect of slowly illuminating and then going out slowly.

I have now replaced those lights with LEDs. The LEDs don't like the slow on/off as they only work at 12V, so they flicker at the lower voltages. That's why I thought of using a relay to power the LEDs from a solid 12v (battery live). The trouble is, the slow voltage (switch wire from BCM) pulls the relay on slowly (vibration).

My question is this... Can I get a relay that only pulls in when it reaches 12V?

[the dutch guy]

yes and no. the problem is that the lights also recieve power (leakage current) from elsewere. this is why you see the leds light dimly, because you have not secured the ground properly. if you also disconnect the ground line properly they will not light dimly anymore.

you can use a simple solid state relay. it will trigger much sooner and with a simple fairly large (470uF or bigger) capacitor on the output you can absorb any bouncing the SCR might do.

[maccmowers]

Thanks I'll try one out and see what happens.