freeing the gear at red lights(auto) - PeachParts Mercedes-Benz Forum

Hocky · #1 02-09-2004, 06:36 AM

I am in the habit of freeing the gears when my car is at the red lights, more often when its waiting for the surburban trains to cross left to right and right to left(this takes longer if more trains come along at peak hour). If the road is flat, I just free the gears and there is a slight clunk noise and there is a bump like something hit from behind felt. Is this a normal feeling when you change from drive to N or P at standstill? If not normal what can be wrong to cause this? If I carry on this thing will it hurt my trans or engine or drivetrain? My Camry has no clunk or feel of any jerk when this is done. The stationary gearchange is smooth with no jerk at standstill. It feels a lot better. Anybody?

#2 02-09-2004, 07:48 AM

by going into neutral, you are forcing the transmission to grab 1st or 2nd gear again when you put it in drive, so you create just a little extra wear on at least one of the internal clutches, which I would expect to be a more significant factor than anything which happens in the motor or torque converter when the transmission is left in drive. The only exception might be a car with an underdesigned cooling system, like my '83 Toyota Celica, in which shifting to neutral at long lights or train crossings sometimes allows the motor to idle just enough faster to keep the temperature close to normal with the AC on when the temperature is above 85F.

chazola · #3 02-09-2004, 08:17 AM

I believe shifting into N for longer waits is suggested in the manual that comes with the car.

I don't like the feel of the 'clunk' you get though.

AtlBenz · #4 02-09-2004, 08:43 AM

Quote:

Originally posted by Hocky
If the road is flat, I just free the gears and there is a slight clunk noise and there is a bump like something hit from behind felt.

bad flex disk maybe?

Jackd · #5 02-09-2004, 10:11 AM

shifting to N or P for shorts period of time (eg. red light) makes no sense. it only cause more wear when the tranny is shiffed back to D. Leave it alone, this is what a ANTOMATIC transmission is for.
Your "cluck" Flex disk or transmission support or rear axles joints or engine mount.
JackD

Ali Al-Chalabi · #6 02-09-2004, 01:50 PM

If you are just going to be stopped for a minute or so such as when waiting at a red light, its probably better just to leave it in drive.

However, if you are going to be sitting more than a couple of minutes, such as waiting for a long train, it is a good idea to shift it into neutral.
This will keep the transmission cooler by not holding the turbine stationary while the torque converter housing turns at 600 rpms with the engine. Putting it into neutral releases the input shaft to the tranny, which in turn allows to turbine to rotate with the engine and release the loading and heat generation of thr torque converter.

Hocky · #7 02-10-2004, 05:27 AM

I will rephrase my comments. If left in gear, normally D, I can feel the pressure in the brake pedal of the car wanting to move forwards and I have to step harder sometimes making it uncomfortable if it is a long wait like for trains and at large intersections. If I change to N, there is no actual clunking sound, but u feel the whole car relaxing backwards on changing the gear to N. Then I can step off the brake if the road is level. The feeling is normal now. If the road is sloping, I will step on the parking brakes.

Ali I think you meant live the car in gear and not in neutral in your first para.

400E · #8 02-10-2004, 07:48 AM

What model are we talking about?

And what is your idle speed, both in Drive and in Neutral? Sounds like it might be too high.

I agree with Ali and Chazola. Quote from the W124 owner's manual:

"For brief stops, e.g. at traffic lights, leave the transmission in gear and hold vehicle with the service brake.

For longer stops with the engine idling, shift into 'N' or 'P.'"

Ali Al-Chalabi · #9 02-11-2004, 05:31 PM

Hocky, you are correct, I edited my post.

The behavior you seem to be describing seems normal.

Hocky · #10 02-12-2004, 05:47 AM

Sorry 400E and all others too. I meant to say that my car is the 1994 Euro C180 auto. It has 196,000kms on the clock already. Next changes will be either the rear shocks or all 4 OR the rear rotors, the front rotors or all 4. Will keep these changes a while longer due to $$$$ short. Brakes are good, but they squeak on the right side on braking. Pads are from last year only. Rotors wearing a bit thin. Brake sensors not lighting up yet. Rear shocks have signs of fluid leaks.

pentoman · #11 02-12-2004, 07:08 AM

Which reminds me, isn't it bad for automatics to 'freewheel' in Neutral? Or is that just with the engine off?

Rusty

#12 02-12-2004, 07:51 AM

used in many '80s BMWs and certain Volvoes (mostly the diesels) will burn out their internal clutches if their motors are reved up while in neutral or park after being warmed up -- this requires a bunch of special procedures for some states' smog tests, and also leads to disasters for kids who like to keep working the throttle pedal while sitting at stoplights. Freewheeling with the engine off depends on how the particular transmission has its oil pumps designed and driven.

Ali Al-Chalabi · #13 02-12-2004, 10:26 AM

On a Mercedes transmission, there is nothing wrong with it being in neutral. Very high RPMs in neutral will create high static pressures in the troque converter and should not be done, ie, not a good idea to repeatedly rev the hell out of your engine in nuetral for prolonged perios of time at red lights.

I remember some 80s auto ZF transmissions on some BMWs that would hold rediculous fluid pressures in neutral. This would cause internal leaks through seals if left in neutral all the time as this high pressure would damage seals. Those specific transmissions should not have been designed that way, though, at least IMHO.