chieftam

I must caveat that this is the first MB I have owned, so maybe I am just not used to a normal MB action. My question regards the action of the transmission, in my case in the GL320, especially when driving around town. At in town speeds, when letting off of the accellorator, seemingly to "coast" up to an intersection or a red light, the transmission keeps downshifting as you slow, raising the rpm of the engine with each shift. My previous or other vehicles, and including the 6-speed in the C6, tend to go into a freewheeling motion, picking up 1st gear when you stop, but he GL seems to hit every gear going down, again, providing compression braking along with the normal slowing. This happens even if I am using the brakes. I could understand this if it were a hybrid, with regenerative braking, but I would think that this compression braking would be a bit harder on the normal action of the transmission, and with the continual raising of the engine rpm, it can't be providing positive impact on fuel economy. Is this a normal operation and function of the MB 7-speed auto? Comments and thoughts appreciated.

Comfy_Couch

This is my first MB as well, and I have had the same experience. At lower speeds (i.e. below 30 mph), and in particular after a cold start, the downshifts are sometimes harsh, making it feel as though I have tapped the brakes. I also hear a crunching noise sometimes when the transmission makes the final shift from 2-1 just before coming to a stop (see my post earlier today). I also have a 320, but I don't know if this issue is unique to the diesel version of the GL.

I love my GL and have had no major issues, but I do wish the tranmission were a bit smoother. Perhaps some others can chime in to let us know what their experience has been and whether this is a fixable issue (or something we'll just have to learn to live with).

Brocktoon

Might it have something to do with the fact that unlike other diesels, the 320 has an air throttle that closes on deceleration and the fuel is cut? Does it feel like that is going on?

chieftam

What it feels like when I let off the throttle to slow at an intersection is like I have a manual transmission and I downshift to the lower gear, and let out the clutch causing the engine to rev and slow the vehicle down a bit, then you downshift to the next lower gear, let out the clutch and do it over again until you stop. Like was said, it is more pronounced when the vehicle is cold and you have just started out.

Brocktoon

I think what I suggested is what you are feeling, because if you downshift in a standard diesel, it revs up the turbo, calls for more fuel,and speeds you up. The downshifting feeling is a rare bird in a diesel.... I'll see when I get mine.

Urabus

Downshifting/engine-braking will have little impact on fuel consumption.

Ausmbtech

They do downshift while slowing to ensure you are always in the correct gear when you get back on the throttle, to reduce brake wear and also to reduce fuel consumption.

When these engines are in overrun mode above 1200-1300rpm then they have the fule injectors switched off comlpletly.

If you do feel any SIGNIFICANT jerking during downshifts (predominantly 3-2, 2-1) then I would definately take it to a dealer for further investigation as there are various remedies depending on your exact fault.

stlduckhunter

I have a 320 and have had it in service twice for rough downshifting---3 to 2 and 2 to 1st. They downloaded new software on two occasions and got it right the second time---no more rough shifting.

Comfy_Couch

Do you have more details on exactly what they did? Did they just re-flash the transmission again?

chieftam

I'll have to have the dealer check it when I take it in for service. It isn't what I would call "significant", though as mentioned, it is more noticeable when the vehicle is cold. When warm, you can tell it is downshifting, and it slows you down, but it isn't obnoxious. I'm just glad Comfy_Couch mentioned that it is like he tapped the brakes when cold. I have started out several times, started to slow for an intersection, and had it downshift roughly just when my wife was starting to put on lipstick or make some other adjustment, and she would jump on me for hitting the brakes! I made her read that post last night! All of a sudden, I'm not an idiot!!!!!! Thanks!

bselph

I too have a 320 and am new to diesels. Mine does the same down shifting but it is not rough. I asked my dealer about it on my 1st free service and they said it was normal on the 320. I now have 5K miles and don't notice it much anymore. I guess I am used to it.
One thing I have noticed is that sometimes my idle stays at 950rpm at a stop instead of dropping to 620rpm where it should be. I can't seem to figure out any commonality on this symptom. It seems to be totally random. Any ideas?

StevethePilot

(This is for the exceptionally knowledgeable Brocktoon as well)

What I've found is there are many situations where I might in another car ride my brakes but in my 320 I ride my throttle. One instance is my son's school, where it's a slight downhill around to the back where he's dropped off. I slow to (close to) the 15MPH limit and instead of riding my brakes (if nobody in front of me is slowing or stopping) I gently ride the throttle.

This isn't something you can do on a big hill, but it's a skill you won't generally find use for in a gas car (or at least none that I've driven) with an automatic transmission. It makes me feel a little more in control as I'm going through parking lots, that kind of thing, that I'm right on the throttle if I need it, but going slowly enough that I'll stop immediately with the brakes.

I have 34,000+ miles on my car. Bought it May of last year. I think I'll be driving the wheels completely off of the beautiful beast.

STP

StevethePilot

Divide by 10? Unless I'm reading what you're saying completely wrong, I don't think my engine goes to 9500rmp! Them's race car revs, yeah?

Brocktoon

Well, I'll see what you mean when mine shows up, but whenever I rent a car in Europe, I always have a hard time adapting to manual down****ing in a turbo diesel. You seem to get a big hit of compression braking, but only for a second, then you get a big blast forward when the turbo loads up, followed by coasting.

I'm just suggesting that the addition of an air throttle would slow the engine in a more gas-like fashion. Kinda like a Jake brake, but on the intake, rather than exhaust side of the engine....

bselph

My 320 is fast but not 9500rpms fast. Sorry for the extra 0. Post edited.

Comfy_Couch

Glad to be of service.

stlduckhunter

That is exactly what they did.

dgiturbo

The down shifting is normal. It should be as smooth as if you did it correctly with a manual, and hardly noticeable without looking at the RPMs. Doesn't hurt fuel economy as once you lift off the accelerator pedal, the electronic injectors turn off until the RMP's get down to a programmed speed. (Its called deceleration fuel cutoff) In city driving, the down shifting helps me to save my brake pads.

As for the cold down shift issue, tell your dealer to change the Valve Body NOW.
A re flash is useless. And when winter comes along, trust me your wife will not be able to put on lipstick when it happens. Actually you will find yourself looking in your rear view mirror to see who just rear ended you. Search for rough down shift 7Gtronic.

N'AWLINS

Like duckhunter, I've had mine in for service twice...occasional hard downshifts from 3-2 and 2-1. Its random, and I cannot get it to replicate when the dealer has it. On both services, they claim to have reprogrammed, but really no difference yet.
Occasionally, it feels like someone just bumped into you from the rear. Very disconcerting for a fine automoblile like ours.
I'll keep listing it at every service interval until they get it right.

Danno4x4

chieftam,

Let me offer you a little insight into how your transmission works, and based on that, I would say don't worry too much about the hard downshifting for now.

I noticed the same type of shifting that you are describing when our GL was brand new... After a while, it went away, now it shifts butter smooth.

Note that based on my own knowledge of How Automobiles Work, this is a best guess on my part:

Most automatic transmissions these days are what you would call electronically adaptive. They learn how to to operate based on experience... Both with the driver and with the powertrain...

The 7 Speed transmission that the GL uses is the same transmission that many other MZB vehicles use (actually I think almost all of them use) so it was designed to work with many different engine and powertrain combinations.

I suspect that the base programming that is in these transmissions is very generic is intended to be used by as many models as possible, and that based on actual use, the transmission would "learn" how to operate with that vehicle over time.

Since the 450 and 550 use gas engines which do not have the same compression as the diesel, the base program is probably better suited for those engines at first since they build more of them than the diesels. If they had a base program that was better suited for the diesel from the factory, it would probably feel too soft for the gas engines...

The question is, why doesn't MZB have a base program better suited to the diesel... Maybe that is what the dealer is attempting to "re-flash" into the transmission computer. I would be willing to bet that they would rather just have the same base program for the GL320, GL450 and GL550 so that as they are building them, they can just grab one from inventory and toss it into the vehicle no matter which GL version is being built, and that in the end, it would adapt to the vehicle anyway.

dgiturbo

I also read somewhere on the forum that another member spoke to a MB rep in Alabama who advised that by September this year, there will be a permanent flash for all of our tranny woes.
I for one am positively hoping this is true.

StevethePilot

That just sounds dirty.

Nevada Jack

"all our tranny woes?"

My transmission could not be smoother and I am sure it was "flashed" during it's latest service if there was a update but not because of my complaints. I am sure some are more conscious than others but drive the car normally, tromp on it a couple of times a day, and don't concentrate on the transmissions performance and soon you will not even have reason to mention these "woes"..:-)

This transmission has been used since early 2006 models and is well known to dealers as well as the people in Alabama and Germany (where they are all made.) I am sure danno has his finger on the pulse of the difficulty getting it perfect for a variety of engines but whatever problems this transmission poses, they are minor and it will adjust as advertised.

Comfy_Couch

You've been fortunate enough not to have this issue, but with all due respect that doesn't mean it doesn't exist. And from what my dealer tells me, they are well aware of the issue, and are frustrated by it, too. I don't know how many others have experienced it, but I for one would welcome a fix (even the magical adaptation-over-time variety). True, "woes" may be a bit overstated, but shouldn't a transmission should really blend into the driving experience rather than calling attention to itself?

scottybdiving

I'm afraid I have to agree with Danno4x4. The torque of a diesel probably makes the downshift more pronounced. Some people may find that more objectionable than others depending on their past experience with diesels and some cars may do it more harshly due to the adaptive learning capabilities. My 320 did it more harshly when it was new but I rarely even notice it now. Add the fact that both Kristen and I have been driving diesels for many years and we are comfortable with the torquey downshift. My Duramax does this by choice when I select the Tow/Haul button on the transmission, to aid with braking.

I think the 7G is a perfect match for the diesel, more so than the gassers. It maximizes the engines torque and compensates for it's lower RPM's with smaller ratios changes between gears. This is the same principal used in the trannies of the big rigs. Otherwise, they would never be able to addequately power a 5200 lb vehicle with a 3.0L V6.