I recently purchased a '97 E300D with a 4 speed automatic transmission plus an overdrive gear. Should I keep the car in 4th gear all the time or always use the overdrive? I've heard some people say not to use the overdrive gear because the engine rpm's are so low and that will cause problems such as carbon build up. I asked someone at the dealership and he looked at me like I was crazy (said use the overdrive) but I got the feeling he didn't know what he was talking about. Usually, I always have it in 4th unless I'm on the interstate; then I'll use the overdrive as long as I'm going fast enough to keep the rpm's at 3k or more. Does anyone have thoughts on this?

Scott

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Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

You should read your ' operator's manual' to find this out. On a car that new it probably has either or both rpm sensor/load sensor and will not go into overdrive under inappropriate conditions.... but your manual will tell you when you should have it in one gear or another...

The owner's manual tells me to use the overdrive. However, Mercedes Benz isn't going to install an overdrive gear and then tell you not to use it because it will hurt your engine from the carbon build-up. I'm not worried about improper use of the overdrive under stressfull conditions such as driving up moutains. I wouldn't use it in those conditions regardless of whether the transmission had a sensor to prevent it from shifting into overdrive. What I'm concerned about is whether overdrive should be used in normal day to day driving around town and on the interstate.

Some people believe diesels are meant to work and the higher rpm is better but then again maybe there isn't a problem in using the overdrive and letting the engine operate at a low rpm. In other words, is it better for the car to cruise at 65 mph at 3,200 rpm in 4th gear or 2,500 rpm in overdrive? I just don't know and was hoping someone might be able to provide some insight.

Thanks,

Scott

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Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

You talk like these are all " gas" mb people that made your car and wrote the manual....
They have a little diesel experience.. .. and I do not think they would allow the trans do anything which would be harmful to the life of the engine.
that book was written by people with access to everything about that engine.... so I would believe the operators manual... unless it has been updated...

It's an automatic right??? Put the thing in the "od" position and let the tranny decide which is the appropriate gear. If you get into mountain type driving, and the tranny begins to hunt between gears, then select the lower selection to save wear on tranny internals.

Scott,

Don't mind these fellows. They are usually quite a bunch of good guys but can get testy when a question is basic.

With regard to the question concerning the overdrive, be assured that you can drive it all day long at 65mph with the engine turning 2500 in O.D. Like Diesel Power suggested, if you run into a hill, and step into the pedal, then the transmission should automatically kick down out of overdrive and into third gear. When it does this, the available torque to the rear wheels goes up proportionally to the change in gear ratio. This usually results in the car increasing in speed because of the additional torque available. Then, if you are on cruise, the cruise lets off the power and the trans immediately shifts into O.D. again. This process will repeat itself until you get to the top of the hill. In this case, it is better to put the vehicle in third and leave it there.

Now, the situation changes when you want to go 90mph (I picked 90 as an example only). I do not know the hp available for that specific vehicle, however, there might not be enough horsepower available in O.D. at an rpm of 3460 (the resulting RPM at 90 mph in O.D.) and the vehicle might not get up to 90 mph. In this case, you would have to use third gear with an rpm of 4400. However, if the transmission is setup correctly, it would automatically make this change for you. But, transmission setups are normally quite fickle and an understanding of horsepower vs. speed by the driver can only help the situation.

Brian Carlton
1984 300SD

Use the overdrive except when using the engine for braking on steep downgrades or when pulling a trailer.

It's there to save fuel and give better acceleration by use of a higher ratio rear end -- just think of it as a 5-speed automatic and forget it.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!