Member Since: 15 May 2013
Location: Chester
Posts: 7633
So what's different? Brake switch - same... Rear lights - same (if updated to D4 lights) ..... Can only be the firmware then, can't it?
And if so, then maybe all your D4 brake switches are already fubared
30th Dec 2016 1:05 am
defector
Member Since: 23 Feb 2009
Location: Greater Manchester
Posts: 1420
Could it be that the led's pull a much lower current so less arcing in the brake switch therefore less failures?
Or am I misunderstanding how canbus works whereby the switch simply sends a signal to ecu to light up the brakes.
30th Dec 2016 3:33 am
discoteca
Member Since: 08 Mar 2010
Location: Warwickshire
Posts: 1477
I could be wrong and I don't have time to pour over the wiring diagrams or old posts, but I'm sure someone here a long time ago offered a good explanation which was:
There isn't any canbus anywhere near the brake light circuit, it's a real battery voltage and real current draw circuit, a hard wired circuit with traditional switching if you like. D3 has filament bulbs so draws more current than D4 LED brake lights, this current causes the rubbish brake light switch to wear out over time and arc across its contacts in a D3 but not in a D4. That's why you see the issues in D3s and not in D4s.
The brake switch circuit is also hard wired to one of the ECUs so the car knows when you are braking (can't recall which ECU) so when the switch starts to fail you get voltage spikes directly into the ECU (not dodgy canbus messages, actual voltage spikes and noise) and that is what sends that ECU into meltdown, which in turn then starts sending things out on the canbus networks that shut everything else down, resulting in what was often described as the Christmas tree dashboard.
D3s fitted with D4 lights are probably less prone, but if the switch is already worn then ultimately the result will be the same eventually.
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31st Dec 2016 1:14 am
discoteca
Member Since: 08 Mar 2010
Location: Warwickshire
Posts: 1477
Yes, if I'd have thought about that for a moment it would have been obvious, but it's late
31st Dec 2016 1:21 am
tayaste
Member Since: 15 May 2013
Location: Chester
Posts: 7633
Led brake lights is the way forward then.
31st Dec 2016 3:48 am
defector
Member Since: 23 Feb 2009
Location: Greater Manchester
Posts: 1420
Ah so, my theory was correct then
Yes i think i may follow that route and change mine to LED and put a new brake light switch in as well as preventative maintenance.
31st Dec 2016 6:05 am
sean 471
Member Since: 26 Apr 2011
Location: Sheffield
Posts: 2990
Had problems many times with brake switch and bulbs. Changed to led rear lights and been fine.
31st Dec 2016 10:17 am
PeterTh
Member Since: 06 Sep 2013
Location: Alunda
Posts: 330
I feel a bit stupid after reading this thread. Just bought a new brake light switch for my D4 to change just to be sure. Must be old D3 damage to the head...
//Peter2011 Discovery 4 3.0 TDV6 S Automatic
1999 Discovery 2 V8 ES Automatic
Earlier
2006 Discovery 3 TDV6 SE Automatic
1996 Discovery 300 Tdi ES Automatic
1975 Series 3 2.25 diesel
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
...another 7 years? 
