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G-wagen automatic gearboxes

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grazza
Posts: 987

G-wagen automatic gearboxes

Post#1 » Tue Dec 03, 2013 1:40 am

Hi, can anyone answer some questions about the G auto gearbox please?

I would like to know if the qualities of the auto are unique to the G and not other MB models, being the ability to be tow started and lock in 1st. I guess the tow start has to be pretty unusual. Dual oil pumps?

Also is this true of all G gearboxes, up to the latest W463 and W461's?

I have been told that all MB autos can be locked in all gears, even the sedans. I thought this was rare nowadays, even in 4wd's. I think many 4wds share their autos with non-4wds and the lock-in-1st ability is not a given anymore.

Any further info on the G auto is welcome.

thanks.

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AlanMcR
Posts: 6024

Re: G-wagen automatic gearboxes

Post#2 » Tue Dec 03, 2013 9:50 am

As for the 722.6 found in a huge number of Gs from 1996 -> mid-2000's:

1) cannot tow start (the second pump was last seen on the 733.5?).

2) can be limited to chosen gear or less, meaning that you can hold it in 1st.

3) the lock up torque converter only engages in 4th and 5th.

4) electrical and mechanical limp modes lock the transmission into 2nd gear.

5) shifts harder and higher when in low range (low range signal wire is fed to TCU).

6) has hidden hill-descent feature if locked into 1st & low range.

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Chris
Posts: 3836

Re: G-wagen automatic gearboxes

Post#3 » Tue Dec 03, 2013 10:14 am

There are 3 flavors of G automatics 720.1 (1981-1990), 722.3 (1989-1997), 722.6 (1995-present).

Here is the dirt on these 3:

1) In 1971 mercedes began production of a 4 speed automatic with torque converter, they also made a 3 speed version for a couple of years which was exclusive to powerfull large displacement V8 cars.

In 1980 MBZ extensively modified this automatic transmission to suit the G and van platforms by integrating an electricaly activated solenoid which keeps 1st gear engaged, no passenger car automatic can hold 1st gear. Transmission case and bell housings have breather 'snorkels' for water fording, deeper oil pan ensures constant oiling at extreme angles and internal components are more robust with extra friction discs, special valve body etc. due to the G's weight. They called this the 720.1.

The torque converter has a torque multiplication factor of 1.8 or 2, so first gear ratio (normally 4:1) is nearly 8:1 under 1800 RPM.

2) 722.3 may be the best non electronic automatic Mercedes has made. Smoother shifting, slightly more 'luxurious' in feel than the previous unit. G-specific versions have the same special features listed above, like the 720.1 it is a vacuum controlled 4 speed torque converter transmission but optimized to compliment the more efficient "modern" engines emerging out of MBZ in the early/mid 80's.

Mercedes used this auto behind its V12 luxo-barges with great success, every 722.3 is easily modified with V12 bits (friction discs/bands etc.) to survive well over 700 horsepower.

*The operators manual for both G-wagen and passenger cars say you can pull start both of these 4 speed autos (720.1, 722.3), and I have tried this on a couple different vehicles, but in my experience this is not practical. MBZ says to get the vehicle up to constant 40kph on flat ground in neutral (builds up pressure in ATF because rear pump always spins) then drop into gear and hope the TC locks up and rotates the motor. There is a risk of over-pressurizing the ATF and popping seals, keep it under 40kph!!!


3) 722.6 or NAG1 is an electronically controlled (CAN-BUS) 5 speed torque converter transmission with overdrive (0.83:1). It is THE best automatic MBZ has made, and it may be the best in the world. Super reliable, seamless gear changes, nearly indestructible, and it can be modified (with AMG/V12 bits) to handle serious HP.

From what I have seen the 722.6 found in all G-wagens is physically and functionally identical to any/all passenger car NAG1/722.6, the only notable differences I know of lie in the number of friction discs, small displacement vehicles have fewer discs then large displacement vehicles.

Holding first gear is now the job of the TCM (transmission control module). There are 2 aftermarket TCMs available that I know of which would allow swapping this gearbox into anything, however these TCMs are not yet plug and play, the end user may need to fine tune shift points/pressures/speeds.


There are other automatic transmissions (722.4, 722.5, NAG2) that I have not mentioned above. These are smaller/weaker and intended to be used in light passenger cars, definitely not suitable for a G.

The 7G offered in modern luxo-Gs was known to be problematic but seems to be getting more reliable as production goes on.

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grazza
Posts: 987

Re: G-wagen automatic gearboxes

Post#4 » Tue Dec 03, 2013 1:00 pm

Excellent info, thanks.

I am now running a 722.351 since an overhaul of my 720.1 did not go so well (something to do with the valve body)
I am not sure if this transmission is out of a G or not, its something my mechanic sourced.
If it does not have the G features mentioned (bigger oil capacity, tow start, etc) then I am thinking I may want my 720.1 back in, since I regularly take my G offroading.

I know it was easier and neater to fit to the OM606 since it uses a cable instead of linkages.
I am still trying to get the right shift points right (cable adjustment) but when they are better, the shift is very harsh.

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grazza
Posts: 987

Re: G-wagen automatic gearboxes

Post#5 » Tue Dec 03, 2013 1:04 pm

I am also thinking of the 722.6 in future, as I know someone doing this to their G soon, with an aftermarket TCU.
But is this the right gearbox for offroading?
It would make the long distance driving to the play areas a lot nicer...

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AlanMcR
Posts: 6024

Re: G-wagen automatic gearboxes

Post#6 » Tue Dec 03, 2013 1:36 pm

Correction: the 722.6 uses a rod between the shifter and the transmission.

It won't tow start the way the factory made it, but I have pondered how hard it would be to put a small electric pump and one-way valve in the pan to pump up the hydraulic pressure. That would be more technically difficult than carrying a spare starter, but much more compact.

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grazza
Posts: 987

Re: G-wagen automatic gearboxes

Post#7 » Tue Dec 03, 2013 10:49 pm

AlanMcR wrote:
6) has hidden hill-descent feature if locked into 1st & low range.


Hi Alan,
Can you elaborate one this? Does it vary the slip based on descent speed?
I thought HDC used ABS.

speedkills
Posts: 336

Re: G-wagen automatic gearboxes

Post#8 » Tue Jun 01, 2021 7:31 am

I'm curious about the hidden hill-descent feature as well, as I see no sign of it on my 2011 G55. Even cutting all fueling would be nice while going downhill in 4L and 1st gear, but instead I find when I get over 1000rpm it surges forward like I tapped the gas and does that back and forth 1000rpm-1500rpm or so as long as I am foot off the throttle downhill. I wonder if this is a problem with my truck or if all of the supercharged models do this?

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AlanMcR
Posts: 6024

Re: G-wagen automatic gearboxes

Post#9 » Tue Jun 01, 2021 9:52 am

I have the earliest TCU for the 722.6 in the G. There were at least four later versions that I know of. It may be that they took that feature out of later versions.

On that note. If anyone has a damaged G TCU that they can part with, I'd love to have it. I'm hoping to extract the newer versions of the firmware to create an upgrade path for older models.
...Alan

speedkills
Posts: 336

Re: G-wagen automatic gearboxes

Post#10 » Tue Jun 01, 2021 9:56 am

How did it work? Can you describe it in action? Like is it just not upshifting regardless of the revs, or is it cutting all fuel to introduce as much pumping losses as possible to slow the truck down? Or is it actually activating the ABS? And how was it activated? Literally anytime you are in 1st gear in low range, or did you have to do something else?

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