Brake light ( Stop lamp ) switch how are they meant to work ?

[GRAEME JOHNSON]

On and off simple Let me explain, as part of my ASR light fiasco I fitted a new brake light switch
I had been having trouble with brake lights staying on and then no brake lights at all
I bought a new switch from the dealer matched to my vin the white plunger was pulled out and moved
freely and the 2 wireing harness plugs could not be mixed and only fitted one way

When the fault codes were read the was a code for the brake light switch which was cleared we then ran the car
The man reading the codes appeared to be viewing live data on his lap top and he told me the switch was not working correctly
I was told when the brake pedal is depressed one set of contacts should be closed and the other open and when the pedal is released
this should reverse with the closed contacts now open and the open contacts closed, I can easily remove the switch and test this

but is he right ? Its a new genuine vin matched part 5 days and 3miles old The brake lights do work fine
Its my doubts over this diagnosis that makes me a bit suspect about his ETA diagnosis

So before I start pulling things apart I would like to:

• Test this brake light switch for correct operation ( how should the contacts be working )
• Fit a new contact switch to the throttle pedal (no codes but its cheap and wires could be damaged/ flexing while driving )
• And most important have a good look at the throttle linkage and the stop positions on the ETA

Any other suggestions welcome

Graeme Johnson

 

[gsxr]

UPDATE: I believe this is NOT correct:

"when the brake pedal is depressed one set of contacts should be closed and the other open and when the pedal is released"

See post #6 below. Your tech may be thinking of the idle speed contacts which are opposite (on/off/on to off/on/off, screenshot below).


What are the current fault codes on the E-GAS module, and are there any symptoms/problems now?

Same switch p/n 001-545-01-09 is used in all 124, 129, 140 chassis plus others.

 

[GRAEME JOHNSON]

I have not run the car since I had the codes read, as I say I will test the new brake switch now I know how the contacts should work
fit the throttle pedal switch it cant hurt and most important look at the throttle linkage adjustment to be sure that the eta is on the
idle stop. Thank you for the help and advice
Graeme Johnson

 

[Surf Blau]

I was told when the brake pedal is depressed one set of contacts should be closed and the other open and when the pedal is released
this should reverse with the closed contacts now open and the open contacts closed, I can easily remove the switch and test this

but is he right ?

Yes - I tested this with live data view using "HHTWIN" on my own car, as I couldn't quite understand why it said both on and off at the same time. Looking at the screen while pressing the brake pedal switched both values, ie the open one closed and the closed one opened. Which is which, I do not know though, but in general the brake light switches are a common fault, which on these cars can cause other systems (like ASR) to act up from what I understand.

 

[GRAEME JOHNSON]

Thank you Surf Blue yes I tried a new switch (genuine) because it does seem a common problem
The man reading the codes was looking at live data and I was shocked when he told me the switch was not working correctly
I will test this off the car and also with it back on the car in case something is not right with the travel of the brake pedal
Graeme Johnson

 

[gsxr]

UPDATE: Your tech may be wrong! See screenshot below... this car has no fault codes and no issues with ASR.

 

[GRAEME JOHNSON]

This is what I dread different people different answers
I will test the switch on and off the car anyway just to confirm what it is doing
and also the old switch if I can find it
Thanks to everyone for their help
Graeme Johnson

 

[gsxr]

I updated post #2 also. I think your tech is confusing the ETA idle speed switch contact readings, with the brake light switch readings. Idle speed contacts do reverse, but brake light does not. Sorry for the confusion!

 

[GRAEME JOHNSON]

We will have to wait for Surf Blau to reply now !
Its worth finding out for definate now to clear this up for all our benifit
Graeme Johnson

 

[GRAEME JOHNSON]

Just to muddy the waters a bit more-

 

[R129 UK]

I happened to be using my HHT-WIN this afternoon and did a test.

The plan was to check the status of the contacts with and without the brake lamps on and post the results here.

The outcome was very strange:

Test 1) Pedal "free" - no lamps on:



Test 2) Came about because my attempt to push down the brake pedal and illuminate the lamps did not stay until I got back to the laptop to view the screen - the pedal was weighted with a 5L can of oil but the brake lights were NOT illuminated:



Test 3) Repeated the oil can - Pedal weighted and brake lamps illuminated:

 

[gsxr]

I think the issue here is a disconnect between what HHT-Win live data displays, and the actual electrical contact.

As long as the live data shows opposite signal between pedal in normal position vs pedal pressed, the switch should be good (assuming no fault codes are stored).

@R129 UK - note that your BAS data elements are not the same as what's shown on the 124 for E-GAS/EFP.

 

[R129 UK]

be good (assuming no fault codes are stored).

@R129 UK - note that your BAS data elements are not the same as what's shown on the 124 for E-GAS/EFP.

I was trying to highlight that I found a change in status of the switch which showed 3 different states under 3 different conditions.

 

[gsxr]

Yes - the 3 states were strange.

 

[GRAEME JOHNSON]

Thank you R129UK for taking the time to do that and messing about with oil cans
I was thinking about it at work
Iam driving along foot off brake pedal I want brake light contacts open as I want the brake lights off I want the second set of contacts closed
to supply power for the cruise control I turn on the cruise control the throttle is now held open by the cruise control
I brake, I want brake light contacts closed as I want brake lights but I want the second set of contacts to open and cut power to the cruise control
as I dont want it holding the throttle open against the brakes
So I think when one contact is open the other should be closed
I really want to get this right first as its cheap and simple before I pull anything apart, I will just use a meter on the switch pins with the switch on
and off the car in case the brake pedal its self is sticking or not moving fully in some way
Graeme Johnson

 

[Surf Blau]

We will have to wait for Surf Blau to reply now !
Its worth finding out for definate now to clear this up for all our benifit
Graeme Johnson

I looked through the pictures I took while playing around with HHTWIN on my car, but sadly didn't have a pic of testing the brake light switch - I'll try to double check my findings this weekend and post the results here

 

[Surf Blau]

I happened to be using my HHT-WIN this afternoon and did a test.

The plan was to check the status of the contacts with and without the brake lamps on and post the results here.

The outcome was very strange:

Test 1) Pedal "free" - no lamps on:

View attachment 236630

Test 2) Came about because my attempt to push down the brake pedal and illuminate the lamps did not stay until I got back to the laptop to view the screen - the pedal was weighted with a 5L can of oil but the brake lights were NOT illuminated:

View attachment 236631

Test 3) Repeated the oil can - Pedal weighted and brake lamps illuminated:

View attachment 236632

I would guess BAS is more sensitive than the actual brakes (and thus lights), as its job is literally to prepare the brakes, no?

 

[Surf Blau]

Just to muddy the waters a bit more-

As a lowly non-native English speaker, I'm a bit confused as to the terms "service brakes" and "auxiliary contact", but this implies that depressing the brake pedal will close one contact and open another, right?

 

[R129 UK]

I would guess BAS is more sensitive than the actual brakes (and thus lights), as its job is literally to prepare the brakes, no?

I think so, as far as I understand, BAS detects how quickly the brakes are being applied and above a threshold it automatically applies maximum braking effort to reduce stopping distances. Quite clever really.

I will look further into it to understand how the system works in more detail.

 

[gsxr]

As a lowly non-native English speaker, I'm a bit confused as to the terms "service brakes" and "auxiliary contact", but this implies that depressing the brake pedal will close one contact and open another, right?

"service brakes" just means the braking system - calipers/rotors/lines/etc.

"auxiliary contact" is the second pair of electrical contacts at the brake switch, a redundant backup system in case the primary contacts fail. The auxiliary / secondary contacts may be either the same type, or opposite (NO vs NC), depending on the design.

 

[GRAEME JOHNSON]

I looked through the pictures I took while playing around with HHTWIN on my car, but sadly didn't have a pic of testing the brake light switch - I'll try to double check my findings this weekend and post the results here

That would be very helpfull , I hope to test my new brake light switch this weekend to see how the contacts are opening and closing
But it would be nice to know for sure what is correct

 

[GRAEME JOHNSON]

Nice sunny morning in the UK decided to look at the brake light switch to see what the contacts are doing and fit the new throttle pedal switch
Roads are empty lets have a quick run out
Start up no pedals depressed, starts no ASR light , lets turn the ASR off see if that makes a differance, ASR off triangle lights up
Drive 10 miles no sign of the light Lets turn the ASR back on, so far so good cruise control working fine speeding up with the stalk disengaging with
the brake pedal Lets try a full bore start down to 1st hold on the brakes go a very brief squeal from tyres but no triangle flash , May have to wait for
a wet day or find a quiet gravel car park so I can add some steering lock . 10 more miles covered this time with ASR on, no sign of the light
I havent touched changed or adjusted anything , I might have a few more test runs then cross my fingers and book an mot (our yearly inspection)
Intermitant the worst kind of fault

 

[Surf Blau]

I had a chance to look at the functionality of the brake light switch in my car last night; Looking at the "Actual values" in the ASR-module there is a line called "N.O. contact" and "N.C. contact", which I assume mean "Normally Open" and "Normally Closed". By default the first one is OFF (=open) and the second one is ON (=closed).

When pressing the brake pedal, these switch from ON to OFF and vice versa. Interestingly, as in @R129 UKs example further up, it is possible to change just the first one by pressing gently on the brake pedal - not something I think is relevant under normal driving, but they are clearly two separate contacts.

The first change in the video is from a "normal brake test", then I am fiddling with the contact by pressing the pedal gently:

 

[GRAEME JOHNSON]

Thank you for that Surf Blau That confirms its ON/OFF then OFF/ON
When the codes were being read the man doing the testing was standing beside the car drivers door open laptop on the roof
and he just reached in to operate the brake so perhaps the pedal was not being correctly depressed and was just changing the first contact
As I posted above it all worked fine on Sunday so I am a bit reluctant to mess about with anything now
Perhaps it is just a quirk of the way the switch works that there is a point at the very start of the travel that both contacts are closed

 

[Surf Blau]

Fingers crossed that you're right - intermittent faults are the worst!