Hi. I have a 2014 1.25 Fiesta Zetec.

My simple question is how does the headlight switch actually control which lights get switched on/off? Please read on....

I wanted to upgrade to auto lights so I got a replacement switch with the extra auto lights setting. I realise I do not have the rain sensor/light sensor in my car. I did plug the new switch in and found that it worked just like my original lights with the addition that when you put the switch in the auto lights position the dipped headlights came on (same behaviour as the dipped headlight setting).

I worked out which pins on the back of the switch control each light setting so I could isolate the ones for auto lights.

Now I then disconnected the pins on the socket for the auto lights (pin 3) and the day light running lights when the switch is in the off position (pin 1).

With these two pins disconnected when you select auto lights on the switch or set the switch to 0 the dipped beam headlights come on. The other 2 switch positions work correctly.

I then additionally disconnected pin 2 (dipped headlights). Again turning the switch to the dipped headlights turned them on. The only thing that didn't happen was when I took the keys out and opened the door there was no lights on buzzer!

I used a continuity checker on the switch to quadruple check the pins I disconnected are for the auto lights, dipped headlights and position 0 on the switch.

So back to my question. How can the lights come on when the pins are not connected?

When I reconnect the pins switch position 0 goes back to being daylight lights and auto lights stays as dipped headlights etc.

Any ideas on how this is happening?

Many thanks

tony

​

Are the lights not controlled by the BCM module? It might need certain signals from the switch to operate the lights correctly.

Why disconnect wires? If the new switch is working like the old did then just don't switch it to auto, does having the switch in auto turn them on and off with the ignition? So it's like a basic auto lights.

Are you wanting auto lights via the sensor? As that needs the correct sensors, windscreen and looms then switching on via IDS.

Lot of time, effort and money that could be saved by moving your hand from the wheels few inches to the light switch when needed.

I find the auto headlights don't turn on early enough anyway. I'd recommend just using the switch manually.

The light switch will be connected to the bcm and it is only sending a position over the combination of wires. By disconnecting some of them you will just mess up the position as the combination has changed.

There would be far too much current draw going to and from the light switch for it to be controlling the power source directly.

headlights controlled by bcm module,

pin1 off postion

pin2 headlights

pin3 autolights

pin4 sidelights

pin5 dimmer earth

pin6 dimmer

pin7 headlamp leveling

pin8 front fog

pin9 headlamp leveling

pin10 switch ilumination

pin 11 earth

pin 12 rear fog

Thank you everyone for your replies. I seem to have got the right wires that match Ian's pin list. I guess I am still not really understanding how they work with the BCM module.

What I was hoping to do was to add my own light controlled relay ( http://www.amazon.co.uk/Photoelectric-Switch-Sensor-Module-50mmx25mm/dp/B00BLZ93T2/ref=sr_1_1?ie=UTF8&qid=1449151883&sr=8-1&keywords=12v+photocell) to the wire coming from the autolights pin 3. Basically the relay would make the circuit if the attached sensor detected low light. The reason I disconnected pin 3 was to check that it indeed controlled the lights on and off.

The issue seems to be that even with pin 3 disconnected if you turn the lights to auto they come on. The same is true if you disconnect pin 2 and turn the switch to headlights on the headlights come on. If you leave pin 1 and pin 4 in place the side lights and daylight running lights work correctly.

So I still have my 2 questions:

1. How do the lights still come on even when the circuit is not being made?

2. How can I add in my light sensitive relay into the wiring to make the lights only come on when the relay latches (normal behaviour of the module I bought)?

Thank you so much in advance if​ anyone has the answer to either or both of these questions.

Tony​

Probably a failsafe, so if the BCM does not detect a short between off position and ground it will default to turning lights on.

(Assuming it doesn't detect a short from one of the other positions).

That might be it. Thank you. I will check in the morning and let you know. It would fit the behaviour.

The switch is providing an earth path on each position for the bcm, not feeding live to bcm or lights

I would think the auto position of the switch would enable the auto sensor via the BCM which then turns the lights on and off, I doubt the auto position is allowing a live feed to the sensor, modern car electrics aren't as simple as that anymore.

Thanks again to everyone.

I think I can still make the sensor work. The relay on it has 1 common and 2 outputs (1 normally closed and 1 normally open - they switch when sensor trips on low light).

If I connect the pin 3 (auto lights) from the switch to the common and the normally closed output to pin 1 and the normally open to pin 2 then when it is daylight the BCM will think I have selected position 0 (off) on the switch. When the sensor trips the BCM will think I have selected position 2 (headlights).

I just need to find 12v ignition on power wire to connect the light sensor module.

Does anyone see an issue with this approach?

I was wondering how many amps (or M amps) is going through the switch. I am guessing very little as it is just coming from the BCM?

Many thanks

Tony

It is only shorting to ground so any current required for the bcm to detect the circuit is not open should be negligible.

Heres the switch diagram

How is this sensor wired? 5 wires eg, +, -, com, n/c, n/o or 4, eg the + and com are joined internally?

Just that the relay needs to switch an earth not a live, fair play for trying but it's a lot easier to move your hand to the switch.

Ian.

Thank you. This diagram really helps with the wires.

Please could I please ask another favour. I wanted to fit an auto diming rear view mirror. I have found a way of fitting it on the current screen mount. However I cant seem to find the correct wiring diagram for it. I have 3 wires coming out of it. One is black, One is solid orange, one is solid purple. i was hoping you might have the wiring diagram?

I assume black is ground. I think the other 2 are reverse and power but I dont know which is which. I have tried powering them but I may have fied the circuit board as I cant get it to work.

Many thanks

Tony

Mark

Good point. Luckily 5 wires so the sensor power is completely separate. The switching relay is isolated.

Any thoughts on an easy to access place to take power from on the drivers side? I was wondering if there might be a socket in the a pillar as I don't have the rain sensor or auto rear view mirror wiring that goes into the roof the pins on it might be available. That would save me cutting into existing wiring if I could connect directly into this socket..

Many thanks

Tony

The mirror you have doesnt sound like the correct one for your car, the colours I have are different, 2 blue and ablack yellow

Ian. Thats odd. I got it on ebay and it is the new style mount with the wiring going the ball joint. The seller said it was a fiesta but maybe it is another ford car. I would have thought they would be standard. Do you happen to know what your 3 wires do?

I would have thought the mirror would just have a connector part of mirror and harness plugs into mirror, I may be wrong, the diagram im looking at shows 3 pins on harness to mirror, pin1 blue power, pin2 black yellow earth, and pin3 another blue reverse