Need help diagnosing transmission problem - PeachParts Mercedes-Benz Forum

Dudernio · #1 04-12-2016, 05:05 PM

While driving at speeds above 55 mph, suddenly the transmission will "disengage" for lack of better terminology, i.e. no power to the rear wheels, and the engine RPM's will suddenly climb to 4000-5000 while my foot is still on the accelerator. Then a second later it resumes back to normal operating conditions. The problem gets worse the faster I go above 55. Driving around at slow speeds, even going up steep hills at 40 mph no problems.
Some other things going on that might be clues: there is a leak in the transmission so I'm always adding fluid. Also, the first time this issue presented itself while driving on the freeway, I pulled over and checked the fluid level and it was too high, about two inches higher than the full mark.
I had the car towed to my mechanic and they said it had very little fluid, which is confusing because the dipstick said otherwise. Any idea what's going on here? So they replaced the auxiliary pump seals, they said that's where the leak was, and added more fluid. Driving it again on the freeway yesterday for the first time since that episode, it happened again.
1987 300SDL

Mxfrank · #2 04-12-2016, 05:13 PM

There are a few active threads with the same problem. What's happening is that your K2 clutch is releasing. The clutch is likely toast, but it may simply be caused by a leak or plugged filter, as it takes a lot of fluid and pressure to engage the clutch. The first aid thing you want to do is check the level and top off if necessary. If that doesn't do it, change fluid and filter and add a viscosity improver additive. And when that doesn't work, start looking around for a used transmission or a rebuild.

Dudernio · #3 04-12-2016, 06:15 PM

Thanks, That makes sense. My intuitions thinks its a fluid and pressure related malfunction as well. The high level on the dipstick is perplexing. I want to drain the pan and start with a known quantity baseline and see how things go, then do the filer change as well. I'd like to drain the torque converter and cooling lines as well.

babymog · #4 04-13-2016, 12:56 PM

Pull the pan and look at the pan bottom and filter. Sometimes a worn transmission will have so much friction material in the pan, it tends to get picked up and clog the filter at higher RPMs. Shutting down and letting the silt settle a bit will sometimes get you home at low RPMs.

If this is true, you will have a gray silt coating in the pan. This probably means that the transmission has been slipping and is worn out. With a clean filter and new fluid, it might last a bit longer if the clutches aren't slipping, might not.

babymog · #5 04-13-2016, 01:35 PM

Pull the pan and look at the pan bottom and filter. Sometimes a worn transmission will have so much friction material in the pan, it tends to get picked up and clog the filter at higher RPMs. Shutting down and letting the silt settle a bit will sometimes get you home at low RPMs.

If this is true, you will have a gray silt coating in the pan. This probably means that the transmission has been slipping and is worn out. With a clean filter and new fluid, it might last a bit longer if the clutches aren't slipping, might not.

SD Blue · #6 04-13-2016, 02:26 PM

It sounds as if the fluid level is still in question. These transmissions are very finicky about this. It needs to be checked after the fluid is fully warmed up, on level ground, while running in park. If the fluid is cold, you will have erratic readings. Only fill to the lowest mark.

Also, since this is intermittent, check the kickdown switch behind the accelerator pedal. I had a problem on mine where carpet fibers were getting in the way and causing the switch to stick.

tyl604 · #7 04-13-2016, 03:06 PM

From the manual for my 81 300SD: "Check transmission fluid level with the engine idling, parking brake engaged and selector lever in position "P." The vehicle must be parked on level ground. Prior to the check, allow engine to idle for approx 1 to 2 minutes.

Measure oil level with the dipstick completely inserted and the locking lever released. Painstaking cleanliness must be observed. To wipe the dipstick, use a clean lint free cloth (preferably leather).

To fill the transmission with fluid, only pour it through a fine-mesh filter into the dipstick opening. Even the slightest impurity may cause operational troubles.

The oil level in the transmission is dependent upon the oil temperature. The maximum and minimum oil level marks on the dipstick are applicable references only if the transmission fluid has reached its normal operating temperature of 176 F. If however the transmission fluid cools down to 68-86F, which is the normal shop temperature range, then the maximum oil level will be approximately 0.2 inch below the minimum mark on the dipstick. We stress this point because an oil change is normally performed when the transmission oil has cooled down to shop temperature.

The fluid level must not exceed the dipstick maximum mark with the fluid at operating temperature. Drain or siphon off excess fluid if required. Then push dipstick all the way in and swing locking lever downwards.

BillGrissom · #8 04-13-2016, 03:55 PM

With transmissions I have refilled after a rebuild, I start checking level after about 6 of 8 quarts. It initially shows on the stick, but not the same front and back of the stick because that fluid is from slinging & sloshing. As you get closer to the final level, you get more stable readings. So, just because you see fluid on the stick doesn't mean that is the true level. And always wipe the stick each time.

dude99 · #9 04-13-2016, 03:59 PM

Have you checked to see if the output bolt on the yoke on the back of the transmission is tight?

babymog · #10 04-13-2016, 04:00 PM

I have heard the leather wipe for the dipstick, and think you could probably wipe it with sandpaper and be fine.

The inside of the transmission is extremely sensitive to contaminants, which is why rebuilding and even changing the filter must be done with extreme car and using lint-free cloths is how most of us work.

The dipstick however, is upstream of the filter, and a too-small-to-see piece of lint will not get past the filter, and won't block enough filter area to make a difference. Take reasonable care to keep it clean and use clean cloth to wipe it.

Just my opinion, not trying to start anything, ... if I was I'd say something about synthetic oil. ;~)

Ceristimo · #11 04-13-2016, 08:13 PM

I always wipe the transmission dipstick with my (clean) fingers, and then wipe my fingers on a cloth to be sure I don't get anything on the stick.

Regarding the transmission issue:
It seems there are A TON of these threads coming up lately. As others have said, engaging 4th requires a lot of pressure, and even the smallest thing in your tranny not being up to par will cause 4th to be all kinds of flaky.

I went through the same thing a year ago (Lost 4th gear....) and the fix was a full drain (including torque converter) and dropping the pan to replace the filter, then a refill + 15oz bottle of Trans-X.

It's been almost a year now, transmission hasn't had a single problem since *knock on wood*.

Not saying that your problem might be as easily fixed as mine was, but for the cost of some tranny fluid, a new filter and pan gasket and the slight inconvenience of being underneath a car with tranny fluid in your hair (wear goggles!) this is certainly worth a try if you ask me.