The clock spring is a part that sits just below the steering wheel. Usually
between the steering wheel and the turn signal switch. It is make from a
thin mylar strip which has printed circuit traces on it. It is coiled up
much like a clock spring . It's purpose is to carry the electrical signals
to the air bag and horn button. As you turn the steering wheel it winds up
and unwinds so there is a good electrical contact at all times. They
started being used when air bags were put in the steering wheel. The old
brass ring with a copper brush just wasn't reliable enough for such an
important device as an air bag.
SRS is sublimental restraint system which is the air bag.
Sinse the clock spring is a mylar strip with printed circuits on it and it
is flexed every time the steering wheel is turned, then yes it can cause the
SRS light to come on if one of the traces should break. With the exception
of accidents, the clock spring is
the most likely cause of the SRS light comming on. The only fix is to
This is really useful, because your advice confirms what I read on a MB web
So far, my independent specialist has tried the following;
1. Testing driver's air bag.
2. Checking the seat belts.
3. Changing over the control unit from a donor vehicle.
The situation is not helped by the fact that he is unable to carry out a
diagnostic check, the reason being that the computer is unable to access the
fault; even a MB dealer couldn't diagnose the problem. It seems that the
current Star Diagnosis software, introduced in 1996 when the 210 Series
first appeared in the U.K., is not 'back engineered' to handle the 124
Series. This is a separate issue that I am taking up with Mercedes-Benz
Getting back to the SRS problem, the MB dealer suggested a wiring failure
for which they wanted a 'blank cheque' in order to carry out a manual wiring
check. Given the residual value of my 1995 E200-124, it would have made
more financial sense to drive it into the nearest brick wall!
Finally, can you give some idea of how much work (i.e. labour hours) is
involved in changing a clock spring?
Thanks for your help.
I personally have never replaced a clock spring in a E200, but I have
replaced many in domestic cars. It cost about $200 on a Dodge Caravan or
other domestic car. That will be Parts and Labor. Now I just replaced a
fuel tank sending unit in a 95 Chevy pick-up. It cost the customer about
$560. I looked it up the same job on my 500 SEL. It would be less than
$200. So Domestic cars are more expensive to repair than MB in some cases.
Maybe someone else can give you a closer estimate.
I regret to advise that I've only just discovered that the specialist
independent had already checked the clock spring and found it to be in
Given the fact that the SRS light came on shortly after a full lock-to-lock
manoeuvre and a short blast on the horn, I really believed that your notion
fitted the circumstances.
I haven't a clue what to do now.
Thanks for trying, which is more than I can say for Mercedes-Benz Customer
Possibly, when he checked, he did it in a static condition (not being
turned) All the clocksprings I have seen go bad were intermittantly at
first. When I check them, I check each trace with the front wheels off the
ground and turning the steering wheel from lock to lock. I still miss bad
ones at first some times, but catch them later.
Keep us informed.
An excellent, interesting and most enlightening discussion.
Tho. my Merc is too old for all these life preserving toys, my GM
model runnabout is not. The information discussed here is obviously
'need to know', which is what makes things appear to me (and obviously
to this dealer) to be one of the dark arts. Once again however, its a
mechanical failure that causes the fault.
What is most alarming about this slightly sorry tale is the computer
It seemed that the promise of in-car processor power being able to
tell the fault in English even, in a few seconds has never really
I shudder somewhat to hear that dealers have to resort to swapping
parts off other cars (and no doubt that has been another customers car
at some point somewhere).
I remember a dealer fitting a whole new clutch to my fathers Volvo
once, to try and cure a strange noise fault only to discover that it
was a vacuum hose come off. He still had to pay for the clutch.
Beware the 'blank cheque' repair!.
Thanks again... Rob.
Just to reassure you that it wasn't a MB dealer who changed over the control
unit from a donor vehicle. Actually, the independent specialist I use took
the control unit off their own 178,000 mile 'loaner' (I've driven it myself
so I know the car involved).
I'm not using this example to deny that what you suggest doesn't happen!
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