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2006 C4S 997 (tiptronic) brake lights not working at all

vikdasor

vikdasor

PCGB Member
Member
Dear All,

Hope someone can shed some light on the issue I am currently having with my rear brake lights which have totally stopped working at all which I only realised couple of days ago as pointed out by another driver while in traffic.
All the other lights in the clusters operate as they should but not the brake lights. I did buy new genuine brake switch and changed that today but the result is the same.
There is also no warning signal showing on the onboard instrument panel and after carrying out an icarsoft diagnostic check there are no warnings related to the brake lights.
The car is well cared for and always been garaged with 59k on the clock.
I have checked the new switch and the old switch with multimeter and they seem to be operating perfectly well. This has left me puzzled and I was hoping that someone can point me towards the right direction.
The car is also booked in for its 2 year service at end of August but I would hopefully like to resolve this issue before she goes in to the dealers.
Any advice will be highly appreciate.

Thanks
Vik Dasor
 
"There is also no warning signal showing on the onboard instrument panel and after carrying out an icarsoft diagnostic check there are no warnings related to the brake lights."

Are there any codes stored for any modules? ( I *think* the way the brake lights are wired on the the 997 is the VES (Vehicle Electrical System) detects the brake light switch, and tell the RCU ( rear control unit) to activate the lights - but I am not able to check that out at the moment) - so were there any codes for either of those?

If it was a 987 boxster - I would be thinking "water under the passenger seat affecting the rear control unit" - but given it's a 997 - thats much less likely - have you had the passenger seat out of the car recently?

How about a very flat battery ?

The fact they are all not working and there are no dashboard errors does suggest the switch - from memory - when you install a new switch you have to manually extend it to it's fullest - then install, it then "calibrates" itself against the pedal.

Perhaps disconnect the wiring, plug in the old switch and work it by hand to see if you can force any response but pushing it "all the way" ?

Perhaps someone near you has PIWIS diags - that will tell you more that icarsoft ( can read switch states and I think activate the lights to check the phsical connections)

Not sure if the icarsoft has the requisite "live readings" but some diags have the ability to report on the switch state - useful to focus the investigation in the appropriate area
 
Many thanks for your reply.

No codes stored at all.
I did nearly have a flat battery yesterday, which was replaced yesterday with a new battery. (not sure if the old battery would have caused this issue)

the icarsoft diagnostic tool tells me that the switch is working. It shows not actuated on the system but when the brake is pressed it shows as actuated and it also shows the delivery of voltage at 11.9v when the brake is pressed at live readings.

the seat has not been removed at all.

it could be a calibrating issue but I am not sure.
 
Thinking out loud…. So excuse the disjointed responses

Likely not a calibration issue of you are getting live reading of actuation.

There are however two switches int fhe one unit on a 997. One detects “touching the brakes”
The other detects “pressed hard enough to put the lights on”. (Touching the brakes switch is normally closed, brake lights is normally open). Just a thought that the correct one is being tested.

Flat battery - one that is flat for a while can cause the the car to go into transit mode. Usual symptoms there are thing a like interior lights only working with ignition on. Heated seats not working, frunk interior light not working. I have never heard of it causes brake lights to stop working and if it was doing this before the battery went flat then unlikely to be an issue. - a handover reset from a PIWIS diags system is required to reset from transit mode if that is suspected.

Other candidates. Ignition switch has been known the cause odd electrical problems. Perhaps give that a good wd40 drink and engage in some “key wiggle “ to see if you provoke a response.
 
You are absolutely right. I’ve just checked with my multimeter one is an open loop and one is a close loop.
it could also be that the new switch I have could be faulty too.
I will certainly get back with an update.
 
The closed loop detects “touching the brakes” - used for cruise control “off”. The open loop is the “brake lights”

Not convinced of a double fault (old and new switch) but I guess it could happen.

Good luck.
 
Update: As it may help someone diagnose the issue in the future.

The control module under the (left) passenger seat was the culprit. Replaced that and all was back to normal. There was no water damage on it but I can only guess that could have been age related issue.
Part number replaced was: 99761826006
Process was quiet simple to replace: remove 4 bolts on the seat, tilt seat backwards, take care when tilting backwards so you don't damage the rear or sides of the seat with any belongings that are on the rear seat, remove black cover under seat which clips out, it reveals the control module with 3 different wiring connections, carefully unplug them and the module is in your hand, replace with new one and the fitting is reversal of the removing.

Note: In some cases the replacement module may need to be set to your car spec but I did not have to this.

Also thanks to forum member (Malarcy) for his input.
 
thanks for posting resolution - interesting it was the rcu - not heard of one failing without it being water ingress related but I guess anything can develop a fault.

In the spirit of posting info for anyone finding this - I checked and I was wrong re the brake light switch connection - it is connected directly to the rear control unit, not the VES - so everything tallies with the symptom and the effect - but the failure without an external cause is a new one on me.
 
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