Brake light issue - advice needed

dcouponzzzz

Hi All,

So my MK1 2004 Focus (5 door LX 1.8 petrol) has a brake light problem. When the headlights are switched on, everything looks fine, then when I apply the brake, the right light goes out completely.

I've replaced the bulb (double filament) and traced the earth all the way round and there's no breaks or weathering. It's securely fastened to the inside of the boot.

Unsurprisingly Halfords didn't have a clue what could cause it, but a friend suggested a relay switch. Are they correct? If so, is this a DIY job or a cheap garage job?

Any help is much appreciated.

DoaM

A relay / switch unit sounds feasible ... when the brake pedal is pressed the tail light is turned off and the stop light is supposed to come on. (Assuming both lights are shown within the same location and there's not a separate brake light). You'd need to look at a wiring diagram to be sure though ... check a Haynes manual or similar?

Does this issue affect both brake lights?
Does your car have a high level brake light? (e.g. top of the rear hatch). If yes, does this come on?

dcouponzzzz

Thanks for replying DoaM.

It only affects one break light, the left light still brightens when applying the brakes and the high level break light still lights up (despite it being hanging off slightly).

I'll see if I can pull anything from a Haynes manual.

Any ideas on pricing for a garage to take the job off my hands?

DoaM

No idea ... I just used my electrical/electronics background to approach the issue logically.

scotsbob

dcouponzzzz wrote: »

Thanks for replying DoaM.

It only affects one break light, the left light still brightens when applying the brakes and the high level break light still lights up (despite it being hanging off slightly).

Two out of three brake lights work. Therefore switch, fuse and relay are ok.
You say in your first post you replaced the bulb and checked the earth wire. Have you checked the current through the live wire?

marlot

This is still most likely an earth, bulb or bulb holder problem.

If you're handy with a multimeter, you can trace the wiring back. If not, an hour or so of a decent auto electrician should find it.

Are you absolutely sure the new bulb is inserted the right way around?

twhitehousescat

marlot wrote: »

This is still most likely an earth, bulb or bulb holder problem.

If you're handy with a multimeter, you can trace the wiring back. If not, an hour or so of a decent auto electrician should find it.

Are you absolutely sure the new bulb is inserted the right way around?

correct ,,,,,,,,,,,,,,,,,,,

angrycrow

Get a spare length of cable, connect it to an earth point in the boot then feed it through the rear of the car next to the wiring connector for the lights. Temporarily secure it to the base plate (earth) for the rear light cluster. If the problem goes away the issue was the earth to that light cluster, most likely poor connection in the multi plug onto the light cluster. If this fixes it just make your temp earth wire a permanent earth wire. This is a common fix on certain Peugeot rear lights.

Strider590

I would say a bad ground/earth point, they tend to use the cars chassis/body as a ground, it saves using a high power negative wire and helps to prevent corrosion, unfortunately from recent experience, Ford are pretty terrible at it.

Rover_Driver

As the problem is affected by the headlights, check their Earth connection as well.

facade

dcouponzzzz wrote: »

Hi All,

So my MK1 2004 Focus (5 door LX 1.8 petrol) has a brake light problem. When the headlights are switched on, everything looks fine, then when I apply the brake, the right light goes out completely.

Definitely sounds like an earth problem to the side/brake bulb

If the lights are off the brake light earths through all the sidelights, so it probably works but a bit dim (I bet the front sidelights glow a bit too)
If the lights are on and the brake lights aren't, the rear sidelight earths through the brake lights, but if the sidelights are on and the brake light is on there is no earth path.
If it was an earth fault with the whole cluster, the brake light should earth through the two indicators, and at least make an effort to light up