Jerking backlash behavior in 190D with manual transmission >>> VIDEO <<<
 

Remember your first driving lesson? You put your foot on the gas without resting your heel on the floor. You pressed down, which made the car move forward away from your foot, which made the car slow down, which made your foot press harder on the gas, which made the car accelerate again. Back and forth, chugging down the road.

The Behavior:
The above description is an exaggerated version of what my 190D does, mostly in 2nd and 3rd gears. I can minimize this effect through careful driving. It mostly happens when the engine is producing very little power, or slight braking, such as when I approach an intersection and start reducing power, or when I reach the top of a hill and reduce power for the decent. I see the same behavior when the cruise control is on; if anything it is worse with the cruise control.

The Car:
The car is a 1985 190D 2.2 (originally an automatic) into which I have recently installed the transmission from a 1987 190D 2.5. It still has the original rear flex disk, but I installed the front flex disk that came with the transmission. I also installed new motor and transmission mounts; the ones specified for a manual transmission.

Evidence:
I have found there is quite a bit of backlash in the drive train. I did this by jacking up one of the drive wheels, and measuring the amount of movement at the circumference of the tire in various gears:
1st 1"
2nd 1.5"
3rd 2.0"
4th 2.5"
5th 3.5"
R 0.75"
There is very little apparent backlash in the drive train and differential when viewed from under the car. Based on this information, I assumed the backlash was occurring mostly in the front of the transmission or the clutch. Last week, I dropped the transmission and inspected further. I found quite a bit of play in the clutch disk (3/4" at the circumference without any visible compression of the clutch disk springs).
video: http://dougtheelectrician.com/mercedes/clutch_backlash.avi
I bought and installed a new clutch disk, and guess what? Exactly the same symptoms, and nearly the same backlash measured at the rear wheel. I believe there is a very slight improvement, but it is hard to be sure.

I also have two other cars with manual transmissions which do not exhibit this behavior: A 1987 Caravan 5-speed, and a 1983 MB 240D 4-speed. I also jacked up and measured these cars, and they have very similar measurements to those above, although the Caravan's backlash doesn't change much from gear to gear and seems to be mostly in the drivetrain after the transmission.

My theory:
I am an engineer, so I am aware of the control feedback loop which travels from the driver's foot through the accelerator linkage to the injection pump, the fuel, the engine, the clutch, the transmission, the driveshaft, the differential, the CV joints, the wheels, the tires, the movement and speed of the car, the eyes (detecting the speed of the car), the brain, and finally back to the foot. The problem could be in any of these elements, although I think I have eliminated the transmission, clutch, drivetrain and brain (the cruise control does the same thing).

My appeal:
I would like to hear from folks who have a similar car and who don't have this problem at all, from folks who do have this problem, and especially from folks who have solved this problem.

Related thread:
My thread from last week about different clutch disks: http://www.peachparts.com/shopforum/diesel-discussion/272048-diesel-clutch-different-gas-clutch-why.html

Axles? I have a fair mount in my car as well. Does the 190D have a rear diff mount?

What is a rear dif mount?

Diff mount
 

OK, I think you are asking me if my differential is isolated from the frame with some kind of rubber mount. No, it seems to be bolted right up to the frame.

Reverse
 

I just found out another interesting clue. It is easy to make this behavior happen in first gear, but impossible in reverse.

so, im a little confused by exactly what the driving characteristics are problem wise. Can you tell me if this is correct, are you saying that in 2nd and 3rd, there is a problem with the smooth application of power?

Also, Im not sure that the backlash that you have measured in the drive train is really a "symptom" as related to power application. At most, you are talking 3.5 inches on the outside of a tire and you are measuring ring and pinion lash in the rear end, and gear lash in the transmission. The tire would be magnifying any movement as you measure the outside point of a large disc, plus movement like that is pretty standard on most cars IMO.

In fact, you've demonstrated it on your other cars, so that really rules it out as the cause I think, UNLESS you can find a worn U-joint or flex discs

From what you have written, could it be a binding throttle linkage problem? a sticky part where you can't give it just the right amount, and either give it too much or too little causing the car to buck? There have been lots of conversions that ended up needing a simplified throttle setup to address that. Maybe its on the throttle end and not in the drivetrain.

Can you describe the way it acts again?

Can anyoen confirm there is no rear diff mount? Even without a mount there should be bushings. I would guess these need to be replaced.

I think I know what you mean, I have a few manual 240Ds so I have something to compare. My Euro 85 (125K) will do that "springy take off" just as you describe but mine doesn't sound as bad as yours. It feels like weak clutch springs to me :confused:, my 79 (with 50K on clutch) will do it if I dont have RPMs up:eek: Its not a big deal and doesn't happen until I get sloppy with my shifting. My wife complained about it once, she pays more attention now:D

engine bucking continued
 

winmutt: There is no rubber shock mount for the differential. It bolts right to the frame without any shock isolation. It is isolated from the driveshaft through the rubber flex disk, and from the wheels through the CV joints in the axle.

dropnosky: The problem happens to some extent in all gears. It is not very noticeable in 4th and 5th gears because the engine cannot apply much force in these gears. It is not usually a problem in 1st gear, since most of the time the driver is accelerating fairly hard in this gear. To cause this problem in any gear, all the driver has to do is make an abrupt throttle change. At this point, I agree with you that it is probably not the drive train causing this.

I am starting to think this is a throttle linkage problem, or perhaps some strange inertial interaction in the injection pump or governor, but I don't see where it could be. All the linkages seem tight to me. It was at one time an automatic, and there was a linkage from the bottom of the injection pump down to the auto transmission, which I have removed.

I once had a similar problem in a 1965 Buick Special, and I discovered that one of the motor mounts was completely broken. I could stand in front of the car with the hood open and the engine running, and by "goosing" the throttle linkage with my hand, could make the V6 engine rise several inches out of its mounts. Because the throttle linkage was routed in from the side, this motion would reduce the throttle which would cause the engine to settle back down, which would increase the throttle, etc.... I don't think this exact thing is happening, since the motor mounts are new, and the throttle linkage, from the engine to the firewall, is a cable, not a rod.

Stevo, I just replaced the clutch. I am thinking how to make a video of this behavior. Maybe in the next couple days.

My 240D does something similar. I don't think it's a PIO. There's a window of throttle input through first gear that will not produce bucking, step outside and it bucks a lot, till you get to maybe 20 MPH. The damper on the IP linkage is toast, I just threw it out, might not hurt to replace it though. I think it's the motor bucking on soft mounts- if it suddenly generates a torque at the tranny output shaft, I think it starts rocking and bouncing off one or both of the mounts, probably enough to retard the drivetrain on one side of the oscillation. I always figured it was bad mounts.

It could be an oscillation based on bad input filtering. I don't know how or if there is any input filtering on the 190D. The 240D has a dashpot in parallel with the input linkage, so it's sort of one sided damping, keeping the input lever from rebounding if the throttle is suddenly released, but no impediment to a sudden throttle application.

I may be wrong, but pops told me that audi came up with throttle input damping to deal with bucking on front wheel drive cars. No direct linkage, but a spring / dashpot arrangement.

I had a pretty severe back and fourth jerking in my cars drive train after the conversion. It would only occur when i was above fifty, or backing off the throttle in fourth gear. Turns out that i did not line the two drive shafts up correctly even though i marked them. When i lined them up correctly it stopped completely.

I am sorry to say, I still haven't resolved this problem. Does anyone have any more ideas? Bump!

anti-jerk valve
 

I explained this problem to my local MB guru, and suggested we should take a drive. He said we knew exactly what the problem was without needing to experience it. Apparently, there is an "anti-jerk valve" installed on the injection pump of diesel MB cars with manual transmissions. There are three solutions:

1) just live with it by careful driving
2) find a injection pump from a car that was originally a manual.
3) find an injection pump shop to add the anti-jerk valve (expensive)

I am not sure how to take it, but my wife says sometimes she could use an anti-jerk valve too.

One last question: Does anyone know the German for "anti-jerk valve"

i want a picture of this anti-jerk valve! :)

240D manual dude here. ... (and it likes to buck from time to time. ... i just thought it was part of the experience.) who knew mercedes engineers had a valve for that?

I'd like to see a picture of an anti-jerk valve. Never heard of one on this forum before, except for the banning system typically exercised from discussion on the open forum.