Hey guys, i have been browsing this forum for a new months as a non-member and after speaking with the guys at SL Shop, i was prompted to sign up to ask some questions. I will start this by stating that I am mechanically and electrically competent, but my knowledge in the Mercedes realm is 0! All of my skills have been learnt on more modern Japanese cars (year 2000 onwards) and lately 2010+ Jaguars so I’m a little out of my depth and finding good resources very hard to find!

My father-in-law has a 1989 Mercedes 300SL, The SL Shop have confirmed this is an R107.

This was his dream car and when he started his own business, he promised himself on day he would buy one. Eventually he did and it’s been his baby ever since, its kept in a garage on a battery maintainer under a cover and only came out during special occasions.

Sadly, in September 2023 he passed away after a short cancer battle, leaving his wife responsible for the car. She has no mechanical knowledge so maintenance of the vehicle has fallen on my shoulders and we want to keep the car mechanically sound and keep it in the family as a tribute to him, it's an honour for me to be trusted to work on what was his most prized possession but obviously I feel weight on my shoulders.

The longest standing issue appears to be the lack of a working speedometer. I removed the gauge cluster over the weekend to replace some burnt out bulbs and the odometer gears as I was under the impression that these would be damaged, I swapped them anyway but they were fine and the unfortunately the speedometer still fails to function.

The speedometer appears to be electronically driven rather than the earlier models which I believe were cable driven (attached is an image of the rear of the cluster).

It does have the original stereo. There are no ABS lights on which would indicate to me that a wheel speed sensor had failed and the gauge has worked, but maybe 5 years ago. The only clue I have is my father-in-law saying that he has had “a little bit” replaced several times, but after some period of time it breaks and the speedo breaks again so he just gave up replacing that part and gauged his speed on the engine RPM, but sadly he is no longer here so I cannot ask him for more information. I have looked through his receipts and cannot find anything relevant. The steering wheel central bolt took a blowtorch and a 5ft breaker bar to come loose which suggests to me it had never been previously removed so i would think that this little part that's previously been replaced is something other than somewhere in the gauge cluster.

I was wondering if anyone here had any advice as to what checks/diagnostics I could perform to maybe prove if the cluster is faulty, or to check for a failed sensor somewhere or any parts we could purchase from The SL Shop to try to rectify the speedometer issue.

My current thinking is that it could be the Gearbox speed sensor https://parts.theslshop.com/mercedes...sor-554295170/

The SL Shop have of course suggested their Gauge Rebuild service, but this is over £1000. We don't mind spending this kind of money, but i don't know if the gauge is actually faulty, or its some kind of fuse or sensor elsewhere in the car. As i say, my Mercedes knowledge is basically 0, as is my classic car knowledge so i really am struggling to find good information.




Any replies would be very much appreciated.

 

Attached is a document that shows how to diagnose the circuitry to isolate the component that may be at fault. On many of the old Mercedes circuit boards for various devices simply replacing capacitors and diodes can restore function. Also, resoldering joints can solve problems. After this you may need a professional such as the SL Shop.

 

Thank You very much, at least that's a diagnostic process i can try to see which component is faulty. I also wasn't aware that as this is an automatic model, that the speed sensor is part of a larger harness, so thank you very much for that document. I will try this over the weekend

The instrument cluster did not show any signs of any previous repairs and with my father in law saying he had has something replaced previously, i suspect it was the sensor rather than any cluster repairs, but we will see, i will update this post with anything i find!

 

If you have more money than knowledge and you really want to make it a tribute, take it to the SL shop.

it will come out better in the end for you if you choose to have them, give it a inspection and tell you what you need to work on, and then you have them do what you can’t.

 

That would be the ideal, but not possible. Money is available to be spent on the car, but we're not in a money isn't a consideration type of scenario. Realistically the car will never be sold, but of course it only makes sense to sink so much money into it at the end of the day, its not a concours show car, just something very sentimental to the family

Im pretty decent with mechanical and electrical work on newer cars, but this car was made the same year i was born, so everything i know is pretty much irrelevant here. Luckily, some things are simpler due to it being older technology and i believe the diagnostics pages posted above will at least show me if the issue is the cluster or the sensor. Fingers crossed its a sensor as the cluster is expensive to replace/refurbish due to its age! Just need to find time to test the theory.