Since buying my SL55, my mechanic (ex Mercedes/AMG) have fixed a number of issues on the car. The biggest problem was a lack of power and a weird gear-changing delay that ended up being the TCU (or at least replacing the TCU solved the problem perfectly!)

We've done plenty of other repairs/servicing such as the gearbox plate, blow off valve, various pipes and so on.

However, about 6 months into this year of repairs, a fault occurred that has stuck with us ever since. Did WE introduce it, or did something fail whilst repairing other bits? We don't know.

Basically, the car over-revs, but it isn't strictly an idling problem.

Let's say... the car is warm, so no choke interference or anything like that. The car is sitting beautifully at 600rpm. I rev it to 3000 rpm, and now it has come down and is sitting at 1000rpm. And it stays there.

The fault also manifests itself when driving along; if you take your foot off the pedal, it pulls along at 1000rpm rather than slowing down. When it changes down from 3rd to 2nd or 2nd to 1st/Disengaged when you come to a stop, quite often it will 'blip' and pull the car forward as it goes into the lowest gear. Other times it will 'cough' at exactly the wrong time and actually stall, giving a battery warning. Sometimes when you come to a complete stop, the car will 'judder', essentially like it's trying to pull away and you're stopping it, this lasting for about 4 seconds until the gearbox finally decides "oh, you must be at a standstill" and disengages. After the judder and eventual disengage, only now are we back to idling at the 'correct' speed of 600 rpm.

We are 99.9% certain it isn't the gearbox itself, having serviced it, changed the TCU and even replaced the plate. We did the last of these because we had a suspicion that the sensors aren't reading the same rpm - in fact, this is still the case... on the STAR screen that shows the 3 different sensor readings, one will be at 600rpm and the other 1000rpm...

So... anyone have any ideas? If it isn't the sensor in the gearbox plate, might it be the other sensor?
We've always thought "Loose airpipe", but have checked everything in the engine bay and cannot find anything loose.
It's not the accelerator pedal - we've changed that.
One thought is that perhaps we have a dodgy connection somewhere - i.e. it is an electronic fault, somewhere near to the alternator - because the problem started after working on the alternator - i.e. before the work it didn't have the problem, but as soon as everything was put back together, it DID have the problem. We are thinking about 're-tracing our steps', and taking everything off all the way down to the alternator and then reconnecting everything, but does anyone have a 'short cut' - i.e. rather than testing everything, is there a likely culprit?

Obviously there are no error codes, so to a certain extent we're going in blind on this one...

Out of interest, should the rpm sensor readings be the same, or is it 'normal' that they're different? And if they shouldn't be different, what is your recommended action considering we've put a new plate in the gearbox?

I'm sooooooo close to having the SL55 I paid for, so if ANYONE can offer any suggestions at all, they would be very, very welcomed.

Thanks!
Joe

 

Since you have already looked at the some of the areas that might cause erratic behavior of this sort .... my first guess would be failing gas pedal (failing Hall sensor in pedal is common) a more typical failure of the pedal is for it to go out of range at full throttle (causing limp), but being flakey at the bottom does happen as well..... second guess is poor connection between Throttle body and it's harness (this is a common problem, but normally has other symptoms like car goes limp and ESP light comes on briefly).