Mysterious Transmission Roughness - PeachParts Mercedes-Benz Forum

sasquatchgeoff · #1 01-20-2012, 10:45 PM

I don't know a lot about transmissions. This has been going on with my 201 since about March of last year. It first happened when the car sat overnight in subfreezing cold in a Bend, OR hotel parking lot.

Crank the car up, idle for a minute or so, put in gear and drive. It will smoothly shift into 3rd, and then downshift smoothly back to second when pulling up to a traffic signal. Proceed through the intersection, and then the shift into third is a huge "clunk" (rough) - I can hear what sounds like a throw-out bearing whining under the shift lever. The shift into fourth is equally rough, still the whine like a bad throw-out bearing.

When this first happened, I thought the trans was going out, pulled into a Jiffy Lube (I was 250 mi from home) and did a transmission fluid transfusion (the Jiffy Lube method) - no filter or pan gasket change, just the fluid. It was due, and besides I was feeling pretty neurotic.

Drive away from Jiffy Lube and all is fine. Drive the car for the rest of the day making sales calls with no incident whatever.

The the next day after arriving home, I pull out of my driveway and it happens again. Pull the car over to the side of the street and turn it off. Sit for a few seconds, and then crank it back up, put into gear and drive away - again with no issues.

Since then, when the weather is below 50 F or so, this same behavior happens. I simply pull over, re-crank, and problem is solved. Sometimes when it is really cold, it takes a couple of rounds of this. Apparently once the engine gets warmed up, There is no issue.

The really weird thing is that it doesn't happen in hot weather, unless it happens to be a real cool morning - so my theory is that it is temperature related. But the re-cranking solving the problem in the interim suggests a sticky valve in the trans maybe? Like I said, I don't know a lot about transmissions - but I am eager to learn.

I tried many things, including adjusting the Bowden cable to no avail.

Is there some vacuum diaphragm mounted on the fender well that triggers clutching in the trans? I know there is an adjustment to counter flaring or hard shifting there, but I have no issues other than when I have cold weather - otherwise the trans performs very well.

Any help appreciated - thanks in advance.

geoff

Diesel911 · #2 01-21-2012, 12:53 AM

Did you ever change the AT Filter?

Cold = increased ATF Viscosity

Cold may also = decreased Vacuum; maybe if the Pump is worn

If it had been Me I would have changed the AT Filter when I got home if for no other reason then to rule it out as a cause of the problem.

It might be interesting to T in a Vacuum Gauge on the Vac Line that goes to the Transmission Modulator and compare the Vacuum when cold and Engine hot. That might show a Vacuum Pump issue.
Then there is the Cap on the Modulator that is supposed to seal and hold Vacuum. Maybe it is not sealing well when cold.

Of course it is entirely possible there is something sticking in the Trans.

Diesel911 · #3 01-21-2012, 01:01 AM

This is an out of the Box one; what would happen if the Bowden Cable was sticking (if that is possible).

I once had the Throttle on my Motorcycle stick due to moisture freezing inside of the Cable. The return Spring did was not enough to shut it off and Engine ran away; until the Bike and I hit the ground.
Fortunately I was only in low Gear.

sasquatchgeoff · #4 01-21-2012, 01:12 AM

I am thinking the modulator cap maybe. The modulator is on the injection pump right? it has the green vacuum damper correct?

The vac pump is relatively new - installed brand new at 247K. There is 90K on the trans and the filter has never been changed - only the fluid - which is always clear and-decent looking. My bad on the filter. I know I should have changed it by now. I have this inordinately morbid fear of a leaky pan gasket. When I get to that, it will be interesting to see whats there.

The thought of cold fluid being more viscous has me intrigued. But how would pulling over and shutting down remedy the situation?

sasquatchgeoff · #5 01-21-2012, 01:26 AM

Not sure that I want to fiddle around with this adjustment - reason being is it seems to do fine when things are warm and toasty. First order of business is to change the filter, then check vacuum.

I really appreciate the help.

vtmbz · #6 01-21-2012, 09:39 AM

IT doesnt sound like its moving fluid very well in cold weather. I had one similar transmission that would get all crazy when I parked it nose up a severe driveway all day. When leaving, it flared and complained until the fluid got resettled.

A possible scenario for the better shifts after pulling over might be that the transmission has an opportunity to move fluid where it ought to be.

Like you I dont know squat about automatics, but I vote for a defective front pump or torque converter.

Crazy_Nate · #7 01-21-2012, 09:45 AM

Check for vacuum leaks - they seem to get worse with lower temperatures. Also, the shifting firmness will increase at lower temperature as mentioned above. I don't think you can do anything much about that.

Diesel911 · #8 01-22-2012, 08:13 PM

Quote:

Originally Posted by sasquatchgeoff

I am thinking the modulator cap maybe. The modulator is on the injection pump right? it has the green vacuum damper correct?

The vac pump is relatively new - installed brand new at 247K. There is 90K on the trans and the filter has never been changed - only the fluid - which is always clear and-decent looking. My bad on the filter. I know I should have changed it by now. I have this inordinately morbid fear of a leaky pan gasket. When I get to that, it will be interesting to see whats there.

The thought of cold fluid being more viscous has me intrigued. But how would pulling over and shutting down remedy the situation?

On mine the Modulator is on the side of the Transmission and the Vacuum Valve that regulates the Vacuum to the Modulator is on the Fuel Injection Pump.
But, I have a lack of 201 knowlege.

Below is a pic of the Modulator Valve; red Arrow points to the Cap.