rear LED bulbs and electrics

An archive of any Porsche 911 technical threads

Moderators: hot66, impmad2000, Barry, Viv_Surby, Derek, Mike Usiskin

Post Reply
User avatar
hot66
Moderator
Posts: 18330
Joined: Tue Nov 11, 2003 4:17 pm
Location: North Yorkshire

rear LED bulbs and electrics

Post by hot66 »

thought Id have a dedicated thread as this was lost in a different one
hot66 wrote: Sat Apr 22, 2023 3:36 pm
hot66 wrote:After fellow ddkers said they couldn’t see my rear lights on my 964 on a Belgium motorway in the wet , I fitted rear led bulbs in prep of euro trip this year

Image
So … having had a few runs out with these bulbs I’m now of the opinion they are too bright as tail lights ( theyre brake light bright ) and the difference between tail light and brake light isn’t big enough .

I’m going to keep them in for a while longer and do some trips with others following me and see what they say.

Anyone know of led bulbs that are maybe less bright as a tail light but much brighter as a brake ?
update on this , was observed by a mate when following me they couldnt tell when the brakes were applied if I had the rear lights on .... I've been in contact with the bulb suppliers and they suggest the following. Does this sound right ? :

Hi James,

These actually have a good differential between stop and tail, however you may be suffering from current bleed from the tail light circuit that is lighting the LED at full brightness when you have the sidelights on.

If you are seeing very little difference between stop and tail when they are fitted to your vehicle, you may have a wiring issue that may be causing this problem. You will need a multimeter to ascertain this.

Would you remove the lamp from the fitting and:

1. With the tail lights switched on, measure for current at the stop light contact in the lamp holder.
2. With the tail lights off, measure for current at the stop light contact in the lamp holder.
3. With the brakes activated, measure for current at the stop light contact in the lamp holder.

1. and 2. should be zero. 3. should be around 12 volts.

If 1. has a reading, you are losing current to the brake light contact when the sidelights are on which is enough to light the LED at full brightness (hence not seeing any difference when you brake). This can be caused by a poor earth to the light unit or rarely a faulty brake light switch allowing current to pass through it when the brakes are not active.

LEDs will light with a lot less current than an incandescent lamp so this issue will not show with incandescents fitted. They can also be tested direct to a 12v battery off of the vehicle so that you can see the difference between the tail light and brake light illumination.

Do let us know if you need more assistance.
James

1973 911 2.4S
1993 964 C2
2010 987 Spyder
1973 MGB Roadster

Its not how fast you go, but how you go fast ;)
markm
Put a fork in me, I'm done!
Posts: 1878
Joined: Thu Aug 23, 2007 2:55 am

Re: rear LED bulbs and electrics

Post by markm »

That sounds very similar to what happened with mine when I put them in the back of the 993RS.

When I fitted one, the brake light worked fine. When I fitted both they didn't work at all and the chappie explained it very similar to your 1,2,3. One thing I did notice also was that one LED was glowing slightly when nothing was on when both were fitted.

Ended up just taking them out and going back to the originals
72 2.4s
96 993 RS
Series 1 DDK membership card
User avatar
hot66
Moderator
Posts: 18330
Joined: Tue Nov 11, 2003 4:17 pm
Location: North Yorkshire

Re: rear LED bulbs and electrics

Post by hot66 »

Update …… ironically considering their brightness I started to feel a bit nervous when driving at night with these as there just wasn’t enough difference between lights and brakes.

I’ve now swapped back to original bulbs too. I did measure voltage as suggested and yes there was a very small amount of voltage present in test 1
James

1973 911 2.4S
1993 964 C2
2010 987 Spyder
1973 MGB Roadster

Its not how fast you go, but how you go fast ;)
Post Reply