Merc AW169 2011

Mickey666

scot22_2 said:

Many thanks angrycrow.  I'll pursue that line.  Further evidence my wife had just driven for an hour in the dark with the heating own, so would have been heavy drain.  Might take it to my garage for check. Will update if anything found.

Unlikely.  When the engine is running all the electrical power is supplied by the alternator, including the power required to recharge the battery after starting.

However, a failing battery can certainly cause problems these days with cars full of electronics and computerised control systems so I’d definitely investigate the state of the battery, especially if it is fairly old.  A garage can load-test a battery to check its health, which might be a good idea.

My previous car was a Mercedes and the auto gearbox started intermittently playing up and would not change out of first gear.  It happened more frequently on cold days but was always ok after I stopped the car, switched off the engine and then restarted - the old ‘turn it off and turn it back on again’ fix beloved of computers.
The garage diagnosed a new battery was needed, about which I was sceptical, but it did indeed fix the problem.  I was told that when a battery ages and starts to fail, it’s voltage will drop significantly under heavy load such as starting a cold engine.  When the voltage to the gearbox and engine ECUs drops too much they decide it is a fault and go into ‘limp mode’, hence the gearbox not changing out of first gear.  In my case, switching off the ignition reset the error mode and because the second starting was on a slightly warmed engine it required less power from the battery so the voltage drop was within limits, the ECUs didn’t go into limp mode that time and the gearbox worked correctly.

After fitting a new battery I never again had the same problem.  A new battery fixing a gearbox issue might sound counter-intuitive but modern cars are far more than just a bunch of mechanical parts these days.  Introduce computers and software into the equation and all sorts of ‘funnies’ can occur.
Best of luck!

Scot22

However, there is a duty of care.  Not much use getting car going and then having a dual carriageway breakdown and potential dsnger.  Surely a responsibility to deal with cause.  Not very happy if you call them out again for same problem.

Loanranger21

Scot22, if you have your Merc serviced by a MB main dealer, not your back street MB garage that pretends to be a proper MB garage,  then you will get the 12 month breakdown cover free of charge. A van badged up as MB will come out to you and will have the codes to diagnose the fault correctly.  It is the RAC that offers this MB breakdown but all badged up as Merc. I am a MB owner of 13 years standing, one vehicle.  I agree that the battery is likely to be the fault. Mine needed its first new one at about 110k miles.  

Scot22

Thanks everyone for sharing your knowledge.  Damp wires dried and battery too up cured problem.