Doctor Diesel

Taking the plunge

Web05My present car is a Mercedes-Benz C 220 CDI, with the torque converter automatic gearbox. After 50 years of manual gearbox driving, I took the plunge and changed to automatic life, and I am thoroughly sold on it, and on the car itself. However, I do have a concern regarding which I am hoping you can help me. When driving along in ‘D’ position, you often have to stop, for whatever reason. At that point the engine is surely still trying to turn the input to the gearbox, but the brakes are preventing any corresponding response from the gearbox output. Does that not create some mechanical tension in the box which, I would think, is not good? Maybe that is designed in as acceptable – or should I be shifting into Neutral, as I would with a manual ‘box? The situation is complicated a little by the stop/start system, but when I am creeping up to a roundabout, for instance, I tend to turn that off because the number of restarts can become quite ridiculous. So please ignore the stop/start system issue for the purposes of my question! Regards.
Ken Dickinson
P.S. I have been reading Diesel Car since the days of editor John Kerswill, and am still thoroughly enjoying it.

My word, you have led a sheltered life Ken! I’m surprised that you haven’t picked up more about automatic gearboxes and how they work over all the years that you’ve been a reader. I’m so glad that you like the C-Class, but you must not be concerned! Torque converter transmissions have been around for decades, and basically the torque converter is a fluid link, where effectively the force of one turbine rotor impels transmission fluid to drive another turbine rotor, although these days most of them employ a mechanical “lock-up” device that saves energy loss when cruising at steady engine speeds. But when the car is driving the engine, as you might say, “on the over-run” there is merely a slushing about of liquid and a bit of energy loss, much of which would otherwise be lost in engine friction or a need for greater braking effort. Nothing to worry about whatsoever! But your ideas of “slipping into neutral” are worrying, as this is summat that tha’ moost not do never lad! Look in the owner’s manual (that’s the big thick book that you’ll find in the glovebox) and you’ll see that it’s something not to be done – in fact pushing or towing a torque converter automatic does present some problems, and people like the AA and RAC would expect to lift the driving axle off the ground if a tow was needed, or more likely they would use a small breakdown car transporter anyway. I’m glad that you are pleased with the transmission though.

I think, if you look back over recent copies, there were some hints on driving automatics economically in The Extra Mile not so long ago. But the most recent transmissions are very economical, mostly down to having lots of ratios in the ‘box – seven in the case of your C-Class – which means that you’re nearly always in the right gear for both performance and, when on a light throttle, fuel economy. BMWs and Jaguars now have eight-speed boxes, and some Mercedes E-Class models have amazing nine-speed automatic transmissions! I have totally ignored the stop/start system, as you requested, and which is precisely as it should be. Half of them don’t work properly anyway, and they are really only any use for saving fuel in heavy traffic, and people who habitually suffer heavy traffic find that the battery isn’t up to stop/starting regularly, without any long runs. ‘Nuff said.
Doctor Diesel

One Response

  1. The automatic gearbox with the fuel wasting torque-convertor and “not-quite-smooth” up and down chnages is rather old-fashioned.

    The gearbox Mercedes etc should have built is the stepless type found in the Prius.

    Here is an animatiion of how the Prius drive system works

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