JamesSmithie

I was driving on a flat stretch of highway in my 2012 GL350 bluetec in top gear at about 65mph on clear sunny day in West Texas. Then the highway went up a long hill with a significant (about 8% I would guess) slope and I attempted to maintain my speed. The auto transmission apparently tried to downshift from top gear, but did not downshift and therefore the vehicle decelerated slightly and I let my foot off the gas and cruised up the hill at a lower speed of about 45mph. This happened several more times on the road trip under the same circumstances, but I noticed that I was able to use the paddle shifter on the steering wheel to successfully manually downshift to maintain the speed going uphill that I had on the flat approach to the hill.

What may be causing this issue of not automatically downshifting when going up a steep grade? I am guessing that it is related to the transmission. I am aware that besides transmission-related causes, there are many other potential causes related to the engine, but am inclined to rule those out because the engine is in great condition.

DarthVaderpa

Try first to "reset" the adaptive shifting points of the Transmission Control Unit (TCU):

1. Turn the key to position 2.
2. Press the gas pedal all the way.
3. Continue to keep the gas pedal pressed for at least 10 seconds.
4. Turn the key to OFF, position 0. Don’t remove the key.
5. Release the gas pedal.
6. Wait 2 minutes. During this process, the key is left in the ignition in the OFF position.

Once you perform this procedure, the Engine Control Unit (ECU) and Transmission Control Unit (TCU) will work together to learn how you drive by monitoring your driving pattern. After you follow this programming method of the TCU/ECU, drive the car for 15 min. Do not race the vehicle unless that’s how you always plan on driving.

I would also recommend to service the transmission with new oil and filter.

DennisG01

That sort of sounds more like a limp mode (reduced power). A CEL isn't always lit up right away. If you don't have any luck with the reset, go get it scanned. Maybe EGR? Just guessing there.

Max Blast

Limp home mode locks the transmission, and whatever gear it’s in. My CDI would go to limp home frequently when the tumble flaps short-circuited and triggered a code with no check engine light for some reason. What I do remember from that hair-raising experience of trying to negate Tehachapi pass in sixth gear was that the paddle shifters did not work.

the other fail mode is transmission protect mode when the vehicle speed sensor fails and you are locked in second gear.

you need to have an Indy or a Mercedes tech read proper Mercedes codes, and perhaps reset the adaptations using a proper Mercedes scan tool

DennisG01

No, limp mode doesn't always work that way (locking the tranny to whatever geat it is in). Gear shifts can still work as normal - as can still being able to use paddle shifters. I can't say what would cause it to work a certain way sometimes and a different way other times, though. "Typically", though, limp mode is more about limiting throttle.

BlackML550

Until you scan it, you're guessing. Most likely its a transmission fault but it might also be a referred symptom from something else like an ABS sensor. My trans locked up in low gear off the back ot a speed sensor problem. It wouldn't change gear at all. Happened once and it turned out to be an ABS sensor on the left rear wheel. Cleared the code to get home and its never happened again.

The trans diagnostics are quite comprehensive. Buy yourself a half decent scan tool for probably no more than $300 and you'll not only see what codes you have, but which ones you don't.

Max Blast

Yes, scan with proper tool but be prepared to find nothing. This was the frustration when my CDI kept going to limp mode - no cel, no stored or current codes.

what I learned from that debacle was that on the diesel, an engine condition will trigger a tranny locked in gear, and a tranny fault will dump all boost. So one affects the other.

Tsumi

I've experienced my R500 locking into a gear when towing up long inclines. I assume my transmission was overheating and it was going into limp mode because of it. Mine does not have the auxiliary transmission radiator, which will be rectified soon, and we'll see how it performs after that.

eric_in_sd

A scan tool would tell you the transmission fluid temperature.

It should be simple to connect an aftermarket transmission cooler. Probably want to get one with a thermostatic crossover valve. The transmission fluid likes to be in a specific temperature range.

Tsumi

The OEM auxiliary transmission cooler already comes with a thermostat built in. I just bought one from a X164 since our cars all use the same radiator setup. If that doesn't provide enough cooling I was going to get the Weistec transmission oil pan.

DennisG01

Hmmm. That sounds odd. I suspect there's another problem in the tranny - I can't imagine any reason why a car's tranny would overheat going up a long incline with no heavy trailer behind it. It certainly can't hurt to add a cooler, but in this case it sounds like it's a "band-aid" fix - it would be better to fix the underlying/real problem else it will come back to bite you. Start by taking it to a reputable shop to get a full flush done.

Tsumi

First time I was towing a 2500 lbs boat (total weight of at least 3500 lbs) in 90+ degree weather up an at least 6 degree incline for about 10 miles or so. It locked into whatever gear it was at at the time and wouldn't shift until I came to a stop, at which point it locked into the first gear. There was some reports of smoke but I think that was the result of the rear main leaking oil onto the hot transmission. Transmission was fully rebuilt 40k miles ago and got a fluid change just before the trip.

Second time I was towing a 4500 lbs boat (about 6000 lbs total) in 70 degree weather. I noticed it locked gears as well but this time I was on a much shorter incline and was at the top by then. It only does this on inclines while towing and never on flat land or downhill.

DennisG01

OK, towing is a different story as it puts much more load on things. A tranny can definitely overheat in situations like that. Sometimes dropping into a lower gear to keep the RPM's higher and the fluids moving can help. But, yeah, w/o a cooler when trailering things can go south.