Clock spring

TC

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 replace it.

Paul McKechnie

This is really useful, because your advice confirms what I read on a MB web forum.

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 direct.

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.

TC.

TC 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.

Paul McKechnie

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 problem. 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 materialised. 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 aga>TC

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!

TC

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 working order.

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 Assistance.

TC

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.

Paul McKechnie

lock-to-lock

Mercedes-Benz