nursecosmo wrote:
Timmaah! wrote:
I get judder (shudder) and jerking under very different conditions. Jerking occurs after acceleration such as an onramp when leveling off at 65 mph where it bucks randomly for a few seconds or sometimes more. If I accelerate with overdrive off and level off at 65 mph and go ahead and set the cruise and wait for around 5 to 10 seconds before hitting the overdrive button there is never jerking. I don’t think it is due to air in the fuel because I have never been able to take even one stroke on the primer (except for priming after filter change) even after sitting in the garage for 3 weeks. My guess is that there is a profound inability of the computer(s) to coordinate the EGR valve, the EGR flow control valve and the fuel flow quickly and accurately enough. Obviously, if installing a lift pump cures the problem then something IS going on with the fuel flow and my guess is consummately wrong.
I get judder at 54 mph in 3rd gear with just the right amount of boost. I need to be going up hill at 52-53 mph and hit it just right to build boost without downshifting, then it judders through the entire drive train. It’s so dramatic that if feels like she’s gonna blow. It’s only done it once by accident and twice on purpose when trying to repeat it.
I offer the following explanation: Just after ignition on the power stroke when the angular position of the crankshaft is in just the right position, the impulse torque can overcome static friction between the clutch surfaces in the torque converter allowing the torque converter to slip. This unloads the drive train beginning the judder cycle. A very short time later as the impulse torque falls off during the same power stroke, dynamic friction between the torque converter clutch plates overcomes the impulse torque and the torque converter grabs. This suddenly reloads the drive train completing the judder cycle. Forced unloading and loading of the drive train occurs twice a revolution producing a dynamic impulse judder (or shudder if you prefer) that corresponds exactly with the beat of the engine.
Timmaah, you are describing two different problems. The first is probably fuel supply related. The second is lockup clutch chatter. It happens exactly as you describe and is NOT normal or healthy for a transmission. Clutch chatter is distinct from torque impulse shudder. In the case of impulse shudder, the clutch does not slip, but rather, stays locked to the drive train. It is usually only felt at very low rpm under full load. It involves the engine rapidly accelerating at the peak of the power stroke and then decelerating at the end of the power stroke, thus creating some low frequency harmonics. Because there are only two power strokes in one RPM as you have already mentioned, it can be very pronounced in a powerful engine like a 4 stroke turbo diesel. The reason that we will never see it in the (unmodified) Automatic CRDs, is because of the low RPM at which it occurs, at which we are unable to achieve a full load without the TC spinning the rpm up.
Yes, absolutely, I know that I am describing 2 different problems. The reason I described 2 different problems is that when I read different descriptions that people write I get the impression that some do not necessarily know there are 2 different problems. I did not know for sure there were 2 different problems until I had both problems.
You write, “In the case of impulse shudder, the clutch does not slip, but rather, stays locked to the drive train.” I want to be very clear at this point. I am not saying that the torque converter unlocks and locks causing judder (I may have confused the wording on this in an earlier post). I am saying that the torque converter clutch slips and grabs while in the lock-up position under impulse torque creating drive line judder. As far as I can tell this is our only point of disagreement. I don’t care if you disagree with me on this. We are just going to have to agree to disagree.
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