Odd voltage.....

[Tangerine_Dream]

Does anybody else get just over 8v down the Brake light feed wire when the tail lights are switched on, but the brake pedal is not pressed???

goes to normal voltage when brake pedal pressed, and back to just over 8v when brake pedal is released, but to 0 volts when the tail lights are off.

I only found this out because i fitted some led stop/tail bulbs in, and only the stop lights work, so decided to poke about and find out why.

More oddly, when i unplug the 3rd brake light feed from the rear windscreen, the tail led part then start to work as it should!!

Thats where im getting the 8v feed when the lights are on. seems to turn the earths for the tail lights into a very slight positve feed, hence stops the tail light led part from working.

Have traced the wire back to the fusebox. According to haynes it gets fed from the dash cluster, via the brake switch, but the voltage is as it should be at the switch. Faulty dash cluster maybe?

sorry to waffle a bit, but its got me a bit vex'd!!!

Any thoughts greatly appreciated

 

[GTV]

if you put the old bulbs back is it ok? could just be that there is less resisance with the led lamps and you are getting less voltage drop

 

[Tangerine_Dream]

Hi!

Yeah, the old filiment bulbs work fine, but my thought train is that there shouldnt be any voltage in the brake light circuit untill the brake pedal switch is activated, and not when the running/tail etc lighting circuit is powered??

 

[dimond]

are the new buls you got for relacing double filiment bulbs as stop light bulbs are double filiment and if you put a single in only 1 or the other will work

 

[Tangerine_Dream]

hello,

Yeah, they are 380 bulbs stop/tail 5w/21w led replacements

 

[dimond]

if they are 100% replacement for double filiment bulbs im not sure but seems strange it should have 2 contacts on the bottom and 2 pins, it hasnt blown a fuse has it as the stop and tail will be on different circuits

 

[Tangerine_Dream]

Yeap, they are 100% the right bulbs. all fuses are intact etc.

as i said, the 3rd brake light feed is picking up just over 8v when the running lights are switched on, but im pretty sure it should be remaining 0v till the brake switch is activated.

and as a result of that voltage, its turning the earth of each of the light clusters ever so slightly positve, literally 0.3v.

Switch the lights off, or unplug the 3rd brake light feed with the lights still on, the light clusters earth returns to 0v, and the tail-light part of the led bulb as well as the brake light part will both work fine!!



I know the easiest thing is just to ditch the led bulbs and replace the old filiment ones, but im thinking that it could be an early warning to something more sinister, so ideally want to sort it.

 

[dimond]

its ok to have 0.3v on an earth max but it sound like you got an open or short circuit if you are electricly minded id be tempted to check the whole circuit from brake light switch to the consumers

 

[Tangerine_Dream]

I have tested the wire, right back from the lights to the fusebox.

With lights on...
...when i unplug the connector block from the fusebox, the wire goes dead. reconnect and it picks up the 8v again.

BUT, the 8v is not present at the brake light switch, on the switched side.

so it is acquiring it after the switch, either via the abs unit, or the dash cluster.

From there it goes to the fusebox, and then to the lights.

with lights off...
...no voltage except on the live side of the brake switch, which is the normal 13.8-14.4 v.

 

[GTV]

sounds like tbh its just because there is no/little load on there its the same as if you have a light fail system, what you need is load resistors to put a dummy load on.

 

[CraigW]

I dont know if this will help but this is a problem I had.

My rear light (passenger side) was going weird, sometimes when you had the lights on the light would be on, then sometimes it would be off. The same thing happened with reverse light, sometimes on when in reverse and sometimes not. And my indicator did similar things.

Then it started that when I pressed the brake pedal the bulb wasnt lighting up on the rear but on my clocks the left arrow for the indicator started lighting up slightly on the dash. And if you were indicating and braking at the same time the indicator would stop flashing both inside and outside the car.

I unplugged the wiring on the rear light, removed the bulb holder and the bulbs out of it and cleaned up all the connections with a wire brush and it has fixed the problem so far.

Maybe worth a try.

 

[truCido]

Sounds like the symptoms of a wrongly fitted bulb (rammed in the wrong way), the wrong bulb or a funny earth

 

[Fl@pper]

if it works fine with old bulbs and goes to crap with led bulbs then do the maths or at least refit it all back as you started and fit one at a time until you work it out

why you want led bulbs in close lens lamps ? usually show dimmer than a standard bulb from what i have seen

 

[Tangerine_Dream]

"Sounds like the symptoms of a wrongly fitted bulb (rammed in the wrong way), the wrong bulb or a funny earth"

Nope, nope and nope lol. all fine.

and as for why i want them. less power draw, and quicker reacting lights. And the are just as, if not a bit brighter than the normal filament ones.

The reason i want to sort it is because there is an oddly sized voltage being put down a 'suposedly' un-related circuit, by the lighting circuit. And i assume shouldnt be doing this, and could be a symptom of somethng worse to come. Thats why i was asking if anyone else had the same voltage, or any thoughts .

Thanks for all your suggestions so far though, itsn nice to hear what others are thinking

Rob.

 

[Fl@pper]

gis a pic of them then