Date registered: Feb 2005
Vehicle: 1989 W201.029/M103 3.0
Mentioned: 0 Post(s)
Quoted: 4 Post(s)
No it won't damage anything, in fact that's what the manual selection is there for. You can also use it to reduce speed going down hills or any time you prefer that the car doesn't shift into the highest gear.
It makes sense to use manual 3rd selection in stop/start traffic, I do it routinely in a similar vintage 190E (meaning similar gearbox control system between mine and yours). I find it a problem if you're on and off the gas in traffic, you accelerate then suddenly traffic stops, so you go off the gas and the transmission upshifts to top gear in D which doesn't help you slow down, it almost pushes you forward and you have to rely on the brakes. Holding it down to 3rd prevents the upshift and helps you slow back down again.
As I said, that's really what it's there for. Kickback for overtaking at speed is done automatically in D and doesn't require manual shifting. The manual 3rd selection is pretty much specifically for driving in heavy traffic.
FYI manual 2nd selection is for high loading situations, like towing up a hill or hard acceleration. Using these gearboxes like a semi-automatic with full manual selection all the time does make them jumpy and jerky. They seem to me to be engineered with specific use selections, like manually select 3rd for "this situation" (what you use it for, heavy traffic), manually select 2nd for "this other situation" (hard acceleration from rest, up a hill merging into traffic). Going all the way back to 2nd in traffic really knocks the poor car around a bit, and selecting 3rd for hard acceleration just doesn't change anything or do anything for you.
As far as your 420SEL tranny going out I'd say you're on the money with servicing history. The B-W trannies used in the 80s-90s Mercs (722 series) is pretty bulletproof in just about every version, and they're sized really more by car weight as loading indicator than engine size. I've got the light one in my light car (722.4) but my same engine in an E-class has the heavier version tranny (722.3), because increased weight means more torque load in regular driving. The key to auto longevity is really that, routine conditions that the car experiences for long periods. Out of the box they're fairly bulletproof but the big way to kill them is failing to regularly change oil and filters, ie. service them. And have them looked at soon after they start playing up with shift flares and whatnot. They can handle extreme variations well beyond design limitations no problem for short periods, but driving all the time with dirty oil or flooring it around all the time with minor issues, that's what kills them. Service regularly and keep in good order, they'll take anything you throw at them. The ones Merc puts behind their saloon V8s is as tough as something you find behind a big block muscle car, unless you put a Messerschmitt engine in your car, you're not going to break it.
driving a fast car should feel like falling off a building.
Last edited by vanir; 08-15-2014 at 04:29 AM.