Hi,

Focus mk3 2015 1.0 ecoboost estate, right breaklight is lit all the time. I've swapped the P21/5w bulb, which did not help. Any advice on the rear light cable connector pinout, to validate if the voltage is high for incoming signal or the light structure has problems.

Hello Miika from Nottinghamshire 😀

You have a faulty earth on the rear light cluster. It is either missing or is corroded and has a high resistance. Check the light cluster on the other side to the one with the light on for a poor earth.

14 hours ago, mmt75 said:

right brakelight is lit all the time

Do you mean it's lit when all other lights are off?

 

7 hours ago, mjt said:

Do you mean it's lit when all other lights are off?

 

When you turn on the lights, right breaklight is on, where as left light cluster has normal rear light on.

21 hours ago, unofix said:

You have a faulty earth on the rear light cluster. It is either missing or is corroded and has a high resistance. Check the light cluster on the other side to the one with the light on for a poor earth.

Thanks for the tip! This was on the todo list. If it's a grounding issue, should this affect othe lights on the light cluster, signal works ok?

2 hours ago, mmt75 said:

If it's a grounding issue, should this affect othe lights on the light cluster, signal works ok?

Check the earth on the opposite side to the one with the brake light on.

The attached diagram shows an earth broken on one light cluster and what happens when this is fed from the lighting and braking circuits. Both with lights off, brakes on and with lights on, brakes off the current actually flows through all lamps, albeit one path has three lamps in series resulting in reduced brightness**. With lights and brakes both on the lamps on the side with the broken earth won't illuminate because the same voltage is applied to both which will cancel.

The point is, though, that in none of these scenarios does a single brake light illuminate.

** Edit: I should point out the above assumes these are incandescent lamps. If any are LEDs the brightness distribution in the series chains will almost certainly be different. I don't have any knowledge of the characteristics of LEDs used in vehicle lighting so can't predict what the effects will be in such cases.

Its probably the left hand brake/tail lamp filaments touching, replace the left hand lamp

Or the bulb is in the wrong way round or wrong bulb.

Put the LH bulb in the RH side and see if the problem stays or disappears 

On 11/24/2024 at 10:30 PM, unofix said:

You have a faulty earth on the rear light cluster. It is either missing or is corroded and has a high resistance. Check the light cluster on the other side to the one with the light on for a poor earth.

Thanks for the tip! This was on the todo list. If it's a grounding issue, should this affect othe lights on the light cluster, signal works ok?

I removed both tail light clusters and the wiring with the bulb sockets and swapped them between left and right and the problem moved from right to left. The wiring socket has an if of 114521. Any tips for keywords to find a replacement part?

On 11/27/2024 at 10:11 AM, DaveT70 said:

Or the bulb is in the wrong way round or wrong bulb.

Put the LH bulb in the RH side and see if the problem stays or disappears 

Hi DaveT70,

spot on - the bulb was inserted in the wrong way for new and old bulb....

No more continuous break light, thank you all that gave tips to resolve the issue.