Porsche Enthusiasts Club Forum :: View topic - Brake Light Failure

Both brake lights out, so check fuse and pedal switch - both good. One pin on the switch is at 12V, the other is not connected to ground. So far as I can see that wire should feed back through the harness to the light clusters and go to the bulbs, which feed it to ground. i.e, the other terminal on the pedal switch should be ground, not floating.

On the light cluster side I've cleaned up the grounds, the rest of the bulbs are a fair bit brighter, and the brake bulbs are definitely getting GND, the round plugs have also been cleaned.

Circuit so far as I can make out is GND > Bulbs > Long harness section > Pedal Switch > Fuse > Battery +ve.

That would imply a connection directly from the pedal switch to the brake light signal wire, but the multimeter shows no continuity.

I've seen mention of a relay for the brake lights but I believe that only applies to oval dash cars?

Is there anything I'm missing there, or are there any common points of failures I've not seen?

Author:	Gryphon [ Thu May 05, 2016 10:00 pm ]
Post subject:	Brake Light Failure - Interpreting Current Flow Diagram
After having no joy fixing my dead brake lights last weekend I've been getting my head around the current flow diagrams. Actually pretty straight forward, but has confirmed that there should indeed be a direct connection between the brake pedal switch and the rear clusters. I've highlighted the area I've narrowed the fault to: What I don't know is what crossing over to the grey area represents, and what the numbers 'A16' 'B20' and 'F12' mean... are these points I could probe the wire? I'm grasping at straws here to put off pulling out and cutting into the harness! For reference, where I'd got to in my previous fault finding: Gryphon wrote: Both brake lights out, so check fuse and pedal switch - both good. One pin on the switch is at 12V, the other is not connected to ground. So far as I can see that wire should feed back through the harness to the light clusters and go to the bulbs, which feed it to ground. i.e, the other terminal on the pedal switch should be ground, not floating. On the light cluster side I've cleaned up the grounds, the rest of the bulbs are a fair bit brighter, and the brake bulbs are definitely getting GND, the round plugs have also been cleaned. Circuit so far as I can make out is GND > Bulbs > Long harness section > Pedal Switch > Fuse > Battery +ve. That would imply a connection directly from the pedal switch to the brake light signal wire, but the multimeter shows no continuity. I've seen mention of a relay for the brake lights but I believe that only applies to oval dash cars? Is there anything I'm missing there, or are there any common points of failures I've not seen? Thanks!

Author:	Brett928S2 [ Fri May 06, 2016 12:03 am ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
Hi I had some brake light problems on my 928 lately... Not sure if your model is the same but... Like you I "thought" that the live went to one side of pedal switch then when pedal pressed it sent 12 volts to rear brake lights.... But It does NOT.... 12 VOLTS come FROM the fuse board TO one side of brake light switch.... WHEN pedal pressed the 12 volts goes to OTHER side of switch... 12 volts then goes TO bulb control unit (on a 928 its a black box, near fuse board) THEN the 12 volt live goes to REAR of car to brake lights....then to ground... Cannot see your diagram by the way... As a general rule though, IF you have a dash warning light for brake lights (either failure or blown bulb) you are VERY likely to have a bulb control unit somewhere.... Hope this helps , as I may be wrong for your model .... All the best Brett

Author:	Gryphon [ Fri May 06, 2016 7:22 am ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
Thanks Brett. Fixed the image link - oops! I don't have any dash warning lights, but what you describe is exactly what I'm seeing with the multimeter. It's also what I see in the 85/2 wiring diagrams (which I presume is for oval dash, mine's square dash). If mine does have a bulb controller that means I'm back to not knowing what's wrong From the Current Flow diagram above it's not intuitive that I have one, but I suppose passing into the grey area could be going into the control module. More googling needed I think! Anyone familiar with the square dash setup?

Author:	AndyG [ Fri May 06, 2016 6:01 pm ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
I Don't know if this will be helpful as I have an oval dash but I had an issue where I lost half my dash lights and the brake lights. It turned out to be the ignition relay at fault.

Author:	Rhett [ Fri May 06, 2016 7:05 pm ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
Hi..I have had a look at the current flow diagram and have the following (which might help).. The circuit starts with fuse 6 (8A), this exits the fuse board on terminal D21 (red/white) and goes to the brake light switch. It then goes back to the fuse board (red/blk) via a 6 pin connector T6 pin 4 (this is near the fuse board). It re-joins the fuse board on terminal A16. This is internally connected to F20, where a red/blk wire goes directly to the two 7 pin connectors near the lights themselves. These connectors have a link wire between them that feed the separate brake light bulbs. The bulbs are internally connected to GND on the back plate and then to chassis. There are no further components involved, and no controllers, etc. As far as fault-finding is concerned, I would check back at the fuse board the connections on A16 and F20, as both lights are out, I think the problem is going to be either there, or down at the connectors but before it splits to separate feeds. Use the multi-meter on the red/blk side of the brake light switch and check for 12V here when the switch is operated. You should get it. If not - the switch is faulty. If you do, check at A16 for 12V (switch operated), if not - the wire back to the board or T6 pin 4 connector is faulty. Check at F20 in the same way - if not here then an internal fuse board problem or bad connection. If ok here, chase it down to the two 7 pin connectors and see what you get there. If 12V at F20 and nothing here, you have a break in the wire somewhere. Best thing to do in that case is run a new wire down and tape it onto the cable form run. Hope that helps... If you get stuck, let me know where - and I if I can help further I will Good luck!

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Author:	Gryphon [ Mon May 09, 2016 9:10 am ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
Brake lights fixed! And I'm feeling a little bit stupid Thank you for all the information Rhett, very good indeed. One question, did you get the 6-pin connector from the current flow diagram or was that bit experience? (I can't see where it might be indicated on the diagrams) I pulled out my fuse box which the PO's have had a lot of fun with - bodged wires and chocolate bock connectors everywhere! I need to sort it out but am a little worried about messing up the alaem/immob which is wired into everything Found the 6 pin connector in question: The red/black wire is indeed connected to the brake pedal switch and the other side of the connector runs directly to the rear light clusters - there's no light controller or module of any kind on the square dash 944s. At this point I realised my stupid assumption - When I started this I assumed that the brake lights would be the big red lens of the light clusters. OOPS!. They're the little ones beneath the indicators. Strangely enough, there was no continuity between the red/black wire to the brake lights and the grey/white to the others (my guess is fogs, but I haven't verified!) Actual fault - both bulbs were dead, blown in the same week, but the filaments were still visually perfect. Lesson Learnt! Check the obvious things first... Thanks for the help everyone, she's back on the road!

Author:	Rhett [ Mon May 09, 2016 4:55 pm ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
Hi there, glad to be of assistance, it's good you got it all fixed - whatever the problem. The current flow diagrams do provide the information about the connectors, see the explanation below: Attachment: wiring diagram.jpg [ 57.93 KiB \| Viewed 5323 times ] I guess it also helps that for more years than I really want to remember I have been an electronics engineer... The photo of your wiring problem is an absolute nightmare . In cases like that I always just take everything out and return it to stock before replacing anything..but that's just me (see above). You're 'back on the road' and legal - brilliant.

Author:	Gryphon [ Mon May 09, 2016 9:54 pm ]
Post subject:	Re: Brake Light Failure - Interpreting Current Flow Diagram
That is a very useful little explanation diagram - thanks again! The fusebox is a nightmare for another day, maybe if/when the dash comes out. I'm very glad I did pull it out as there was an un-fused 12V wire that was missing a stretch of insulation. The old electrical tape had fallen off and the copper was very close to the body... prime little fire starter! Patched that up and searched through the rest of the bodgework for anything dangerous, thankfully nothing else too bad.

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