Technical Fit 500 rear brake light fault! Help!

[Amrik]

Hi all, newbie here!!

We require some help with our 2008 Fiat 500 1.2 lounge!

Basically the left hand side brake light doesn’t work at all but the high level and rhs work as normal, I’ve tried the following thus far!

* Replaced the lamps several times with no joy
* Checked and replaced fuses with no joy
* Tailgate wiring repaired with a repair kit (eBay) - no joy!
* I’ve replaced the lamp carrier - still have issues
* I’ve replaced the brake light switch under the dash - still no joy!

Has anyone else experienced this issue?

Could this be a faulty ground cable?


Any help would be appreciated.

 

[John202020]

Inside the boot check the Green wire with white stripe has a voltage on it(brake pedal pressed) you could cut it or simply push a knife blade into it with a meter connected assuming you know what you're doing, or just the bulb with the other side properly earthed to see if works? Also test if same voltage is on the same pin at the cluster plug, whilst unplugged.
If you get a feed there the fault is either from that point to the cluster or the cluster? Have you tried removing the cluster and testing it or powering it up to see if it performs as expected?
Also try this turn on ignition, don't start, left hand indicator on, lights on, select reverse, does the cluster perform as expected, any other strange lights on, on the other side?

 

[steveras57]

Hi
As john says its the green/white wire that feeds the brake light on that side you are checking to see if you are getting a full 12v feed to the bulb holder! if you can you need to check the feed to the connector for the left side bulb holder unit first ! .
If you are getting a lower voltage or none at all!- then that green /white wire may have an issue! ie- pinched or broken! the next steps are fiddly and awkward and involve tracing the wire back to the body computer( the internal fuse box which is housed behind the glove box shelf) which has to come out and may be hidden further under a metal "shield".

But before you get to that the plan should be to trace the wire back as far as possible!- as i recall it runs behind the rear inside quarter trim panel then down and along the passenger side sill!- the loom is accessible underneath the plastic sill cover in the passenger door held in by two screws( one at the front one at the back!) and the trim clips!( prise the trim upwards carefully with a long screwdriver once the screws are removed!) if you expose the loom and find the green /white wire!( you will have to remove a fair bit of the insulating tape sheath! )( make sure you get the right one!) you can repeat Johns test by carefully exposing the wire core by carefully striping back a small section of the insulation with a sharp craft knife or something similar(be careful not to cut right through the wire!).

Using your multi-meter set to 20v use the red probe to touch the green /white wire whilst having the black probe wire on a known good earth to the car body! the ignition should be on but car NOT running! and the brake pedal pressed either by another person! or you can use a length of wood jammed between the brake pedal and the drivers seat!.
If you get 12 volt reading here then the signal IS being sent from the body computer! and therefor you have a wiring issue from this point backwards towards the rear light unit and further investigations at other points in the loom working BACKWARDS will be needed until you find the problem area of the wiring which you can then repair!.

If however you are NOT getting 12volts at this point then the problem is further FORWARDS IE- in the wiring up to the body computer and /or connectors/ the relevant connector itself!/or more worrying the body computer itself! which requires an electrical expert! .
If this is the case it might be worth getting a professionals opinion! as it might be possible to to run a bypass wire which might be simplier and cheaper than trying to repair the body computer!.
If you locate the problem and mend/replace any of the wiring dont forget to insure the repaired section( and any other areas you have exposed for"testing" are re-insulated to prevent shorts and /or further problems!.

Its a testing process that can be fiddly and frustrating! be sure to plan ahead as to what to are going to try! and insure you give yourself enough time to work without undue pressure and haste! finally you may reach a point when you have to recognise that you cant solve the issue yourself and need to involve a professionals services! dont take it personally as a failure! sometimes this happens and the idea is to solve the problem not make things worse!
(dont ask how i know!!). hope this helps!.
please post any further problems ! or results so we know whats happening!

 

[Amrik]

Thanks

Hi
As john says its the green/white wire that feeds the brake light on that side you are checking to see if you are getting a full 12v feed to the bulb holder! if you can you need to check the feed to the connector for the left side bulb holder unit first ! .
If you are getting a lower voltage or none at all!- then that green /white wire may have an issue! ie- pinched or broken! the next steps are fiddly and awkward and involve tracing the wire back to the body computer( the internal fuse box which is housed behind the glove box shelf) which has to come out and may be hidden further under a metal "shield".

But before you get to that the plan should be to trace the wire back as far as possible!- as i recall it runs behind the rear inside quarter trim panel then down and along the passenger side sill!- the loom is accessible underneath the plastic sill cover in the passenger door held in by two screws( one at the front one at the back!) and the trim clips!( prise the trim upwards carefully with a long screwdriver once the screws are removed!) if you expose the loom and find the green /white wire!( you will have to remove a fair bit of the insulating tape sheath! )( make sure you get the right one!) you can repeat Johns test by carefully exposing the wire core by carefully striping back a small section of the insulation with a sharp craft knife or something similar(be careful not to cut right through the wire!).

Using your multi-meter set to 20v use the red probe to touch the green /white wire whilst having the black probe wire on a known good earth to the car body! the ignition should be on but car NOT running! and the brake pedal pressed either by another person! or you can use a length of wood jammed between the brake pedal and the drivers seat!.
If you get 12 volt reading here then the signal IS being sent from the body computer! and therefor you have a wiring issue from this point backwards towards the rear light unit and further investigations at other points in the loom working BACKWARDS will be needed until you find the problem area of the wiring which you can then repair!.

If however you are NOT getting 12volts at this point then the problem is further FORWARDS IE- in the wiring up to the body computer and /or connectors/ the relevant connector itself!/or more worrying the body computer itself! which requires an electrical expert! .
If this is the case it might be worth getting a professionals opinion! as it might be possible to to run a bypass wire which might be simplier and cheaper than trying to repair the body computer!.
If you locate the problem and mend/replace any of the wiring dont forget to insure the repaired section( and any other areas you have exposed for"testing" are re-insulated to prevent shorts and /or further problems!.

Its a testing process that can be fiddly and frustrating! be sure to plan ahead as to what to are going to try! and insure you give yourself enough time to work without undue pressure and haste! finally you may reach a point when you have to recognise that you cant solve the issue yourself and need to involve a professionals services! dont take it personally as a failure! sometimes this happens and the idea is to solve the problem not make things worse!
(dont ask how i know!!). hope this helps!.
please post any further problems ! or results so we know whats happening!

I’ill try this later today. Thanks for taking the time to respond.

 

[Amrik]

Hi
As john says its the green/white wire that feeds the brake light on that side you are checking to see if you are getting a full 12v feed to the bulb holder! if you can you need to check the feed to the connector for the left side bulb holder unit first ! .
If you are getting a lower voltage or none at all!- then that green /white wire may have an issue! ie- pinched or broken! the next steps are fiddly and awkward and involve tracing the wire back to the body computer( the internal fuse box which is housed behind the glove box shelf) which has to come out and may be hidden further under a metal "shield".

But before you get to that the plan should be to trace the wire back as far as possible!- as i recall it runs behind the rear inside quarter trim panel then down and along the passenger side sill!- the loom is accessible underneath the plastic sill cover in the passenger door held in by two screws( one at the front one at the back!) and the trim clips!( prise the trim upwards carefully with a long screwdriver once the screws are removed!) if you expose the loom and find the green /white wire!( you will have to remove a fair bit of the insulating tape sheath! )( make sure you get the right one!) you can repeat Johns test by carefully exposing the wire core by carefully striping back a small section of the insulation with a sharp craft knife or something similar(be careful not to cut right through the wire!).

Using your multi-meter set to 20v use the red probe to touch the green /white wire whilst having the black probe wire on a known good earth to the car body! the ignition should be on but car NOT running! and the brake pedal pressed either by another person! or you can use a length of wood jammed between the brake pedal and the drivers seat!.
If you get 12 volt reading here then the signal IS being sent from the body computer! and therefor you have a wiring issue from this point backwards towards the rear light unit and further investigations at other points in the loom working BACKWARDS will be needed until you find the problem area of the wiring which you can then repair!.

If however you are NOT getting 12volts at this point then the problem is further FORWARDS IE- in the wiring up to the body computer and /or connectors/ the relevant connector itself!/or more worrying the body computer itself! which requires an electrical expert! .
If this is the case it might be worth getting a professionals opinion! as it might be possible to to run a bypass wire which might be simplier and cheaper than trying to repair the body computer!.
If you locate the problem and mend/replace any of the wiring dont forget to insure the repaired section( and any other areas you have exposed for"testing" are re-insulated to prevent shorts and /or further problems!.

Its a testing process that can be fiddly and frustrating! be sure to plan ahead as to what to are going to try! and insure you give yourself enough time to work without undue pressure and haste! finally you may reach a point when you have to recognise that you cant solve the issue yourself and need to involve a professionals services! dont take it personally as a failure! sometimes this happens and the idea is to solve the problem not make things worse!
(dont ask how i know!!). hope this helps!.
please post any further problems ! or results so we know whats happening!

I’ill try this later today. Thanks for taking the time to respond

 

[Amrik]

I’m still to fully test the feed to the rear LHS brake light.

But I have tackled the ground cable as it arrived today - as you will see in the pictures it’s been a ground cable for too long!

A relatively straightforward job once the rusty bolts had been doused in WD40 & my knuckle are all in tact!

 

[steveras57]

A good thing to get done for sure!
come-on get yer voltmeter out we are all waiting for the results of what you find! orhopefully not.

 

[Amrik]

A good thing to get done for sure!
come-on get yer voltmeter out we are all waiting for the results of what you find! orhopefully not.

Just ran a quick test (light tester and volt meter) on the cable before it passes through the grommet to the light cluster and I’ve got no light and no reading at all.

I need to take out the sill cover, plastic cover over the rear quarter inside and possibly the rear bench seat? To test the cable further into the car and towards the bcm!

A joyous job for Friday afternoon!

 

[steveras57]

Yep you have your work cut out there mate!
dont forget when taking off the rear quarter internal panel that the seat belt anchor bolt should be undone with a "rattle gun"- if you try to use a socket and manual ratchet its well known for the bolt to sheer-off! flush making it extremly difficult to remove the remains! take care!.

 

[steveras57]

To be honest id be tempted to test it under the sill plate first to see if you are getting any voltage from the bcm rather than struggle with the panel and seat first! if you have power there then you know the problem is further back towards the light cluster! if not then its forwards towards the bcm!.

 

[Amrik]

To be honest id be tempted to test it under the sill plate first to see if you are getting any voltage from the bcm rather than struggle with the panel and seat first! if you have power there then you know the problem is further back towards the light cluster! if not then its forwards towards the bcm!

 

[Amrik]

That was my plan test at the sill first then work from there. I’m on the road tomorrow so a few days before my next update. Fingers crossed it’s an easy fix!

 

[steveras57]

I sure hope so too!!! fingers crossed for a broken or corroded wire or the like.
I can say from personal experience that if its related to the bcm then your in for a world of pain frustration and expense!

 

[John202020]

DON'T remove the seat belt bolt to remove the trim, cut the trim if it's really necessary to remove at the least noticeable point, or you'll end up with a much bigger problem, our resident expert Typecastboy suggests.
If the number plate lights work then there's power all the way up to the flexi, so the issue is where/after it splits to the two rear lights

 

[Amrik]

I sure hope so too!!! fingers crossed for a broken or corroded wire or the like.
I can say from personal experience that if its related to the bcm then your in for a world of pain frustration and expense!

I have a BCM from eBay for £80 and im
shopping around for quotes to clone, I’ve been quoted between £150 -£350! Hopefully I don’t need the bcm cloning and it’s a wiring issue.

 

[Amrik]

DON'T remove the seat belt bolt to remove the trim, cut the trim if it's really necessary to remove at the least noticeable point, or you'll end up with a much bigger problem, our resident expert Typecastboy suggests.
If the number plate lights work then there's power all the way up to the flexi, so the issue is where/after it splits to the two rear lights

Where do the wires split?

 

[John202020]

Looks like they are individually fed looking at the wiring diagram each light feed going back to the fuse box in the passenger compartment
Left hand light supply wire yellow red stripe pin 20 at the fuse box
Right hand supply yellow black stripe pin 5 at the fuse box

Number plate feed yellow green stripe pin 15 at the fuse box
So it maybe that the issue is more at the front there is a common earth point far rear nearside

 

[p0689109]

I had a problem on my 2009 rear left reverse light. After checking the circuit I eventualy traced it to the light unit itself. The plug part of the unit was the culprit. I got a second hand one and it worked. So check that in case its the culprit.

 

[Amrik]

Transpires that a cable was damaged in the sill. I’ve jointed the cable and all appears to be working now.

 

[steveras57]

hope your thanking the gods for a nice easy fix!
Do you have any idea how a wire got damaged under the sill plate?