Focus Mk2 - brake lights permanently on and other quirks

shietpilot

One for the Ford heads,

My 2007 build Focus mk2 acquired a fault today. I was greeted with the check engine light on whilst stuck in traffic.

Came home, scanned the codes and here's what came up:
P0504 - Brake Switch A/B Correlation

The problems:

If the light switch is in the OFF position and I press the brake pedal:
- brake lights go on
- dash lights, side lights, reg plate bulbs go on

If the light switch is in the ON position 1:
- dash lights, side lights, reg plate bulbs go on
- brake lights are on

If the light switch is still in the ON position 1 and I press the brake:
- dash lights, side lights, reg plate bulbs stay on
- brake lights get brighter

So now I'm thinking - ah, easy side light wire is shorting to the brake lights and that's why either turns on the other.

Here's where it gets tricky.

Engine off, key in my pocket, light switch ON in position 1 and I press the brake:
- dash lights, side lights, reg plate bulbs are on
- brake light is brighter
- ignition turns on

WTF?! :mad:

I plugged out the brake switch and the brake lights still come on when I turn on lights to position 1.

Plugged it back in and then I unplugged the light switch. Pressing the brake pedal still turns on dash lights, side lights and reg plate bulbs.

Anyone heard of this before? The ignition part really scares me though...

Cordell

http://www.focusfanatics.com/forum/general-technical-chat/150103-exterior-brake-lights-stay-right-center-left.html and this http://www.focusfanatics.com/forum/9136977-post33.html

So either you have the wires mangled and shorted together somewhere, or something went very wrong in one of the car body ECUs.
If you want to solve it yourself you need to get a service manual with the electrical diagram (usually Hayes has it), and identify the brake lights wires and see where they are getting power from. Disconnecting the brake switch from the pedal will solve nothing, as it does not switch the lights directly.

Hal1

Possibly as simple as a brake switch? http://www.obd-codes.com/p0504 the good news is they are cheap but only get it from ford.

martinr5232

Did you have any bulbs replaced in the back lately ??
Try taking the bulb out one side at a time see can you figure out which side is causing the problem.

Hal1

They are on that's the problem, the switch is knackered.

martinr5232

Hal1 wrote:

They are on that's the problem, the switch is knackered.

The switch is not the problem because they are two totally seperate circuits.

Its either wrong bulbs fitted in stop and tail lamp or stop and tail bulb fillaments fused together in the bulb.
Maybe wiring fault but i doubt it.

shietpilot

Bulbs are old and I haven't changed them at all since I bought the car. I will try take out bulbs individually tomorrow.

I don't think it's the brake switch though because if I remove it the side lights still turn the brake lights on...

I removed both rear brake bulbs about 10 minutes ago and checked resistance between 2 connectors on each tail light. There was a MIN, MAJ and GND.

There was almost 0 resistance between MIN and MAJ. I'm assuming MIN is for the normal lights and MAJ is for brakes...

shietpilot

martinr5232 wrote: »

Its either wrong bulbs fitted in stop and tail lamp or stop and tail bulb fillaments fused together in the bulb.

You're an absolute gent, my friend!!!

Not the first time you have saved my A$$!

Rear nearside light/brake light had the 2 filaments touching each other inside which meant power from brake light was powering the side lights and power from side lights was powering the brake bulb.

The ignition issue:
The brake light circuit on the Focus only works if ignition is on. I guess ignition being on powers on a relay which enables the ignition to work.

This means power from sidelights was going through brake lights and when I pressed the brake lights it was reaching the ignition switch and in turn powering up the relay which turned on ignition.

Happy ending :pac:

Are you anywhere in Dublin, Martin? I'd drop you a box of beer. I was already taking out light switches, brake switches and removing plastics from under the steering column

jca

I came across this in a jap import diesel Corolla, I first noticed the clock was dimming whenever I pressed the brakes as the brake light circuit was feeding into the sidelight circuit and the clock "thought" the sidelights were being switched on. The ghosts in the machine started when the engine would keep running as long as I had my foot on the brakes.... I only finally sorted it when I was parked outside my local mace facing the window and spotted the sidelights coming on and off with each press of the brake pedal...