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BMW X3 3.0D 2006 - Fitting towbar electrics, bypass relay - Fixed #

Posted by Bradley Kingsley on March 31, 2014, 8:39 pm

Year of Manufacture *: 2006?
( Optional ) Engine Code or CC: 3 litre diesel

Hi all

Just fitted a towbar on above car. After fitting bypass relay I found all sorts of things wrong. Tail lights come on instead of brake. Relays bouncing all round in erratic manner.

I removed all my wiring again to find the tail lights are the same bulb as the brake. What I mean is there are 4 x21w. 382 bulbs on the rear clusters. These come on with tail lights and when the brake is pressed one on each side gets brighter.

I believe the canbus system is deliberately adding voltage to these two bulbs to shine the brake lights. And the system is working correctly? Am I right?

If so the only way I can see to fit a bypass relay is to use the switching voltage/earth? From the brake pedal. (or by the genuine loom?)

Does this theory sound right or have I got other issues?

Bradley

Re: X3 Fitting towbar electrics, bypass relay.

Posted by Alan Lange <iframe src='https://members4.boardhost.com/ukautotalk/info/user=ukat3293' width='330' height='300' border=0 style='border:none;'> on March 31, 2014, 9:10 pm, in reply to "X3 Fitting towbar electrics, bypass relay."
Alan Lange

Hello Bradley, yes you are right. Brake light gets 12 volts, tail gets 5 or 6 volts from memory. I usually wire from the number plate lights as this should be 12 volts & try using the high level brake light wiring. (you can't get left & right tail lights wired independently though).

Re: X3 Fitting towbar electrics, bypass relay.

Posted by Bradley Kingsley <iframe src='https://members4.boardhost.com/ukautotalk/info/user=ukat3153' width='330' height='300' border=0 style='border:none;'> on March 31, 2014, 9:46 pm, in reply to "Re: X3 Fitting towbar electrics, bypass relay."

Many thanks Alan,
Never thought of using centre brake light!

You saved me some hassle there.

It's good to have my theory backed up.

Re: X3 Fitting towbar electrics, bypass relay.

Posted by Shaun Dale <iframe src='https://members4.boardhost.com/ukautotalk/info/user=UKAT2719' width='330' height='300' border=0 style='border:none;'> on May 8, 2014, 8:51 am, in reply to "Re: X3 Fitting towbar electrics, bypass relay."

Hi Bradley,
I presume you've done this now, but for the future and the benefit of others "smart bypass relays" are available that will pick up the PWM signals from the combined function tail/brake light bulbs and split it out to separate signals. The downside is they need a supply run from the battery to power them.
Shaun

Fixed #

Posted by Bradley Kingsley on May 28, 2014, 8:58 am, in reply to "Re: X3 Fitting towbar electrics, bypass relay."
Edited by board administrator May 28, 2014, 1:25 pm

Yes Shaun I have sorted it now. I used the centre brake light and number plate wires as signal wires for the relay. However I do always run a fused live from the battery or fuse box feed anyway. Thanks for reply brad

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