auto-electrical help- power always going to taillight

[sammich]

This is an ongoing auto-electrical problem that i've had with my 2004 alfa romeo 156.

There has been constant power being delivered to the right hand rear taillight (even when my car was off). when the car was off you could see a very dim light coming from one of the taillight globes indicating that some current was getting through to it.
as a result, it caused the battery to go flat if i didn't run the car for days.

so i've had to disconnect one of the taillight globe as a temp fix (indicator one is still connected)

anyone have some ideas what the root cause might be and what a solution might be?
(i remember someone telling me i would have to replace the entire tacho ($300+ job) but i don't see how the tacho is related to my problem)

thanks

 

[SirKit Breaker]

You have all the classic symptoms of a bad earth. The fact that the car is Italian further reinforces my thinking. Even thought he lights are switched off their are systems still powered up within the vehicle, to maintain memory like the radio, engine management control unit and probably a good few others as well. Although these systems use very little power, if you have a bad earth problem then the current will find a way out, usually the easiest path, in your case the tail light. This isnt going to be easy, but unless you want to go to an auto electrician then you are going to have to go all over the car and check for corrosion anywhere where the wiring is connected to the chassis. The best place to start is in and around the engine bay, check battery terminals, check battery earth lead connections, check engine earth strap whilst you are there. If anything looks dodgy undo it clean it and refit, then keep trying the light till you clean the one that turns the light off.

Good luck............Howard

 

[circles]

No idea what the cause is as modern cars to me are a mystery. However, if it turns out that the £300 is the only option then here is a fix for you.
Disconnect the cable going to the light terminate it and stick it out the way so it is no longer in use. Then run another cable in parrallel from the other tail light to the one which is the problem......Hey presto two lights on and two lights off...... Again my motor knowledge is limited you may have to recalculate fuse values.
Hope this may help if the £300 job raises it's ugly head..

 

[circles]

Thinking on it a bit more......If it is actually on when it is supposed to be off but very dim then cars are usually 12 volts or no volts right? So therefore you are getting a leakage current to that lamp. so it could be some bad corrosion, leaking current through the corrosion, from a positive terminal, to a this lamps cable terminal somewhere. Something which is wet or damp and passing current albeit very low.

 

[baldsparkies]

If its anything like the jag I once owned it could be controled by an electronic lighting management unit. if thats the case dont go cross connecting directly with your other tail light, it may mess the electronics in the module up (could be expensive)
My cheap option was a 12v relay taking power from a seperate power source. and then supply the relay coil from the other tailight which is a low current feed and fit a diode inline with that feed to prevent the module seeing the added coil resistence. it worked for me and still did when I sold the car.

 

[circles]

Well I don't know anything about car lighting control thingy wotsits but as far as the diode is concerned then not sure about what the light management thingy sees when looking at the resistance of a coil. We are talking about dc here so there is no inductance to speak of. However, a diode would needs to be connected across the relay coil in parallel, as the back emf from the coil when the field collapses would produce spikes well capable of destroying semiconductors back at the old management thingy...A 1n4148 would be a classic one for this application.

 

[sammich]

thanks for all the replies!

i did a bit more googling and found someone with the same problem:
Alfa Romeo forum: Rev Counter/Battery drain - remove fuse ?

turns out the rev counter is faulty and needs to be replaced! apparently a common problem on the 156.