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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22736
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Hi, Ive just bought a 2009 s class blue efficiency. I was

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Hi, I've just bought a 2009 s class blue efficiency. I was driving back home on the motorway and noticed a loss of power when changing down a gear, 6 to 5 doing 80mph. I drive s class and e class Mercedes as a chauffeur, I know it could be a faulty valve in the gear transmission. The loss of power stayed for the rest of the journey, about 20mins, and then gears would change but slip to 5 and skip 4. On restarting the car again the problem had gone and the car was fine. It's happened once again but corrected itself when restarting. I've had the car 2 weeks. Any ideas?

you're quite right to suspect a faulty solenoid valve here

I'd take a look at the colour of the transmission oil it should be a dark red colour if its black then its needs changing together with its oil filter. Also check its level with the gearbox warm and just after you've moved the selector through all the positions leaving it for a few seconds in each one and then back to park.

You can top up the box through the side fill plug if required and make sure you use the correct oil for your car as the fluids are not all the same. Some manufacturers recommended a change of the type of fluid - Speak to the local dealer and parts dept - they will advise you on the updated fluid to use..

If that’s all OK then check all the electrical connections of the gearbox, remove each connector in turn and spray with contact cleaner. Check that the wiring to the gearbox is in good condition with no chafing or frayed wire

If this is OK then check the selector lever linkage / cable under the car for any broken parts and or missing bolts as if this is coming adrift then it can rub on the car body or just bind up

Then - you should top up if low and re try it - or, if its level is ok - and condition is ok - then the next step is to drain it off - open it up and clean the filters inside -- refill and retry it..

Often - just doing the above cures many troubles with these - and you certainly can't condemn the unit without doing the above...

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