Gearbox woes come to an end.

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New Member
Aug 1, 2023
W209 320 CLK
Decided to change differential when I had the rear subframe out renewing the suspension links for MOT. Everything went to plan, reinstalled subframe and prepared car for MOT. Sailed through MOT, garage is about quarter mile from house. Took car for a test drive after MOT and immediately the Engine Management Light came on and I couldn’t get past second gear. Checked the forums for the DTC P2069, P2502 , and N15/3 P0700. All info pointed out the issue was the speed sensors. Further research and recommendation was to change the conductor plate. Suffice to say two conductor plates and a valve body later and I had the same problem. Changed the Transmission control unit still the same problem. Checked the new abs sensors I had installed everything OK. Lot of head scratching, then decided to go back to beginning and the only other item I had changed was the differential. When I bought the diff I was very careful to insist the “recycled” part came from a CLK320 petrol. The part numbers matched so that’s why I installed it. Mistake number 1 – the part numbers I checked turned out to be the casing number, hence why it slotted in. This is the point I started to ask the forums about ratios and whether the wrong ratio could affect the speed sensors. Turns out this is exactly what the problem was. I have now reinstalled my old diff, and test drove this morning, all gears back no eml. Happy days are here again. So every days a school day and this is what I have learned today:

Changing differential, ensure the ratios match. The ratio is stamped on the rear of the body on the lower right hand side. If it is rusted away like mine do your research on what it should be and don’t trust recyclers. Check the ratio against the original, this is very simple to do, count the number of turns on the input shaft to turn the output shaft 360 degrees. My original was approx 3 and quarter turns (3 times 360 plus 90 equals 1170, divide this by 360 degrees = approx 3.25:1, should be 3.27:1) I compared this against the recycled part which was over three and a half turns and was stamped 3.45:1.

Don’t assume every speed sensor DTC will be fixed by changing the gearbox conductor plate there are other issues that can throw up the engine management lights. I expect a defective abs sensor could also have an effect, as it appears the TCU and ECU must compare the gearbox speed sensors and the abs sensors before deciding to change gear. The recycled diff was altering the expected speed from the rear wheels hence confusing the ECU and subsequent generation of the DTC’s and EMLight and going into limp mode.

Solus pro is a good all-round tool and generated the DTC’s, but unfortunately I followed the wrong route. A warning that there was a mismatch in the expected rear wheel speed sensors and the gearbox speed sensors might have helped find the solution quicker.

I hope my experience can help others and good luck to anyone trying to keep an old car on the road.

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