I have 1 possibly 2 problems and can't isolate what is causing what.

VEHICLE: 2005 CLK500 Cabriolet. W209.475
I have started getting what I hesitate to call a misfire
At idle I can feel the engine rocking, not violently, but it is not smooth.
When driving under a light load I feel a slight rythmic bump.
It is also quite noticeable when it shifts from 1st to 2nd under light load. At that time it feels like the driveshaft is rubber for a second (for those with manual trans experience it feels like shifting too early and the clutch is bouncing on the springs.)

I have a bi-directional OBD2 reader and:
- there are no current or stored misfires
- no current or stored engine related errors
- cylinder balance test shows nearly equal change when disabling each cylinder
- normal MAP readings, O2, fuel pressure, readings all steady

ODD COINCIDENCE:
I started noticing this right after replacing a dead alternator. The new voltage regulator seems to be a bit on the high side. It regulates at about 14.6v and occasionally goes up to 14.7v. Last night I noticed my dome lamp rhythmically flickering. (The OBD2 reader doesnt have a very fast refresh rate so it wouldn't reflect super accurate instantaneous readings.) Engine rpm doesn't seem to change the rate of flicker very much.

STRAW GRASPING:
Is there any way that any of the dozen or so modules in the car might be sensing that there is too much voltage and the slight ripple be effecting the timing, fuel, or what ever and be causing the problem I'm feeling?

My next troubleshooting step will be bringing out my oscilloscope and observing the 12v to see just how much ripple there is and if it corresponds to the bumps I feel. I won't be able to drive while observing because it is an AC only O'scope and it wouldn't be safe to drive and fiddle with knows.

Any ideas would be helpful.

 

Yes a good check is the 12V for ripple. You may have a bad rectifier. The battery will be absorbing most of any ripple, so multi-meters wont detect much. You could try using the AC range on a multi-meter for a quick check for AC ripple. Should show very little AC volts. Anything over 0.5V AC may be an issue.

 

This particular problem manifested it's self after my alternator died and was replaced.

After replacing the alternator (with internal voltage regulator) i have noticed irregular pulsing in what was originally a very smooth running engine. By using an OBD 2 reader i have captured voltage fluctuations as high as 15.4 volts (rather than the nominal 14.7 volts)

Does anyone have info about how various modules react to an over-voltage situation? At night I can definitely see flickering of the dome/map lights. The flickering is not directly related to engine speed.

 

UPDATE
I replaced the replacement alternator (less than 30 days and it has a limited lifetime warrentee)

I got things buttoned-up enough to start it and observe the OBD2 readings. Before, I was getting random variations from 13.5 volts to 15.4volts. With the new alternator it only varies from 13.8 volts to 13.9 volts just idling, or at 3500 rpm, no load, or with AC, max fan, defoggers, headlights, top activation and anything I could think of. (With a fully charged battery)

I tied the hood down (I still have some coolant troubleshooting to do before I fully button everything up) and test drove it around the neighborhood. My "misfire" problem is gone, my flickering light problem is gone and even the hard but yet rubber-band shifting problem is gone. It hailed and rained here so my coolant troubleshooting will have to wait till tomorrow, butI am tentatively claiming victory over this phase of repair.

CONCLUSION:
Some module(s) didn't like more than 15v and was either momentarily shutting down or the values drifted so far out of norm that they couldn't output the proper values. That caused the drivability and shifting problem.

 

Total success. Running as well or better than before the problem!!!

Now on to the next problem...