LED 5/21W rear lights problem

BackintheFold · 27-09-19, 01:10 PM

Has anyone replaced the 5/21 w light bulbs in their car with LEDs, found a problem and fixed it?
I tried fitting 5/21W LED replacement bulbs but they don't work properly.
I'd be interested if anyone had already encountered a similar issue and resolved it.

Thanks

SwallownAmazon · 27-09-19, 09:01 PM

I think you need to fit a resistor in parallel across both circuits, I did that with my B3 and they seem to work OK. If you don't, the bulb failure warning light comes on.
I tried them in my BMW 740 and they didn't work even with the resistor; the bulb failure system takes over and all sorts of lights come on.

Cheap LED bulbs are pretty useless; this lot are very helpful:

https://www.classiccarleds.co.uk/

BackintheFold · 28-09-19, 07:37 AM

Thanks. My issue was using cheap LEDs with just a resistor to differentiate between side and brake lights, when I braked, they’d go out and get hot.
I figured this was due to the wiring in the car resulting in some reverse voltage and confusing the led bulb

I considered putting a diode in the wiring to prevent back voltage, but w friend gave me two spare LEDs which he thought were CANBUS compliant, and were certainly more expensive than mine.
I put those in and they worked perfectly. Now I’m sourcing another two of the same to give me a full complement of four. I may use red ones in addition to the two white ones I now have in.

SwallownAmazon · 28-09-19, 12:41 PM

Quote:

Originally Posted by BackintheFold

when I braked, they’d go out and get hot.

That's usually indicative of a bad earth.

I trust you have read the bit about using white LED's with coloured lenses in the classiccarsled web site.
One of the reasons for using LED bulbs is to reduce current consumption but the built-in resistors to make them canbus compliant negates that and can occasionally actually increase the current used.
My Volvo and pre-war Morris both have dynamos and no canbus (!) so LED's are ideal and the warm white is ideal for brighter sidelights.

BackintheFold · 28-09-19, 01:34 PM

Thanks-yes I checked the current draw for each and calculated wattage.

Type. ...........side light....brake light....resistance between two positive terminals
Original bulbs-5.2w and 21.6w. 4.8 ohms
Cheap led bulb-0.5w and 1.4w. 80 ohms
CANBUS led bulb-2.4w and 4.8w. infinity ohms.

So I ‘save’ 6.5 amps when braking with the sidelights on. Pretty good I think.

Regarding colour, yes, you are absolutley right, The white LEDs produce a pinker result than the standard bulbs, but that’s what he gave me to test so I was happy with that . I have ordered red LEDs for final use.

The takeaway from this is, I think, that for an ur electrical system you need to use LEDs with infinite resistance between the two positive terminals on the LED bulb. Whether you end up with a saving of Amps will depend on how the LED bulbs work.

EDIT: Also, there is a definite difference in light output form the cheap LEDs and the CANBUS ones, so the difference in current draw is not solely due to the clever CANBUS compliance.

28-09-19, 01:34 PM	#5
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,171	Thanks-yes I checked the current draw for each and calculated wattage. Type. ...........side light....brake light....resistance between two positive terminals Original bulbs-5.2w and 21.6w. 4.8 ohms Cheap led bulb-0.5w and 1.4w. 80 ohms CANBUS led bulb-2.4w and 4.8w. infinity ohms. So I ‘save’ 6.5 amps when braking with the sidelights on. Pretty good I think. Regarding colour, yes, you are absolutley right, The white LEDs produce a pinker result than the standard bulbs, but that’s what he gave me to test so I was happy with that . I have ordered red LEDs for final use. The takeaway from this is, I think, that for an ur electrical system you need to use LEDs with infinite resistance between the two positive terminals on the LED bulb. Whether you end up with a saving of Amps will depend on how the LED bulbs work. EDIT: Also, there is a definite difference in light output form the cheap LEDs and the CANBUS ones, so the difference in current draw is not solely due to the clever CANBUS compliance. Last edited by BackintheFold; 30-09-19 at 08:37 AM.

27-09-19, 09:01 PM	#2
SwallownAmazon Grown up member Classic Audi Club Member Join Date: Jun 2017 Location: Tonbridge, Kent Posts: 511	I think you need to fit a resistor in parallel across both circuits, I did that with my B3 and they seem to work OK. If you don't, the bulb failure warning light comes on. I tried them in my BMW 740 and they didn't work even with the resistor; the bulb failure system takes over and all sorts of lights come on. Cheap LED bulbs are pretty useless; this lot are very helpful: https://www.classiccarleds.co.uk/

28-09-19, 07:37 AM	#3
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,171	Thanks. My issue was using cheap LEDs with just a resistor to differentiate between side and brake lights, when I braked, they’d go out and get hot. I figured this was due to the wiring in the car resulting in some reverse voltage and confusing the led bulb I considered putting a diode in the wiring to prevent back voltage, but w friend gave me two spare LEDs which he thought were CANBUS compliant, and were certainly more expensive than mine. I put those in and they worked perfectly. Now I’m sourcing another two of the same to give me a full complement of four. I may use red ones in addition to the two white ones I now have in.