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Porsche specialists buying, selling, servicing | 911virgin | Uxbridge

Rear Fog Light Wiring Logic?

L

LM964

New member
Joined
18 May 2009
Messages
28
Been messing around wondering why my rear fogs are not working. What I do know is that (in the UK) rear fogs only come on when a) headlights are switched on AND b) front fogs are switched on. You cannot have only the rears on and not the fronts.

I do have front brake ducts fitted in place of front fogs. Before I dig around any further I am begining to wonder if I need to check the front fog wiring, bridge/jump the now disconnected connectors there then try again.

I thought too that rear fogs (if fitted) were a mandatory MOT check for working. I guess not...

Anyone else with front brake ducts experience the same? I know the relay is good as the reversing lights use the same relay and they work fine. I checked all fuses the other day. Maybe I'll check again.

Cheers
 
The two are linked through their individual relays. You must switch the fronts on (lamps there or not) to energise the relay before you can switch on the rears.
 
Ah yes, have a feeling i pulled out the FF relay as 'dead weigbt'. Probably neex to put back in and try again. Thanks!
 
This is what I did to get the rears only..

Rear fog independent

1. R32 comes on with lights
L12 connect to L13 - new connectors needed

2. isolate supply to fogs
cut L24 to L34 link

3. R32 supply side
Tap off link L11 to 1.5 violet wire from J23 to F21
Install R32 relay grey
 
Thanks Ras, Dropped front fogs relay back in and hey presto. All good. Some thiking on this though;

A) Rear fogs presumably not part of an MOT check unless front fogs are fitted.

B) I know that if rear fogs are used/working then you cannot additionally wire them up into any part of the braking system.

So, from the above, if the fronts are removed and the rears not checked (or considered not mandadtory - as they can only work once the fronts are switched on), then there is nothing to say I cannot switch these over for brake light use (function only as additonal brake lights).

TDIQuattro - I would follow your steps if I used the car as a daily runner and more likely to need rear fogs in lousy conditions. Think though that with the pretty useless original brake lights, making use of of redundant rear fogs would be handy for me. Tried different LED's and tbh the brightness/lumens was still useless.

Marcus
 
Rear fogs are mandatory for an MOT for vehicles manufactured after 1st April 1980.
 
Thanks ras. Seems then my non-working lights were missed in the previous MOT after i removed the ff relay.
 
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