pappy92651

The 928 four speed automatic A28 (722.3) is very robust. I have a habit of breaking things that are robust. The transmission that I just killed had been rebuilt 9 months ago. One of the planet gears in the planetary carrier cracked and went flying off. It didn't know it had failed until I was almost home, while I was down shifting, with no warning at all. This assembly was the only part that had not been rebuilt. There are people who know how to rebuild transmissions and there are people who know how to BUILD transmissions.

The person who rebuilds my transmissions says that he has never seen a planetary fail. He visually inspects them and reinstalls them. That's actually a strong testament in favor of a good design. For most applications this is probably OK. Other transmission builders who deal with AMGs (Mercedes)see broken planetary gears much more frequently. They actually rebuild the assemblies as a standard practice to prevent failure. Hmmm??? Mercedes power:weight Vs. 928 power:weight. What happens when you start adding power to a 928? The weight doesn't go down but the forces applied go up!

If you are running more HP than normal and have very good traction this can be a problem. The transmission professionals tell me that the 722.3 transmission can safely handle over 600 HP. Torque is another matter. If a driven part is weak or out of spec torque will hasten it's demise. Heat is the other common enemy.

What can you do? If you drive your automatic hard, run higher than normal power, and manually shift the AT, you might consider all upgrades the next time your transmission is out. You need to discuss this with your builder. AMG offers hard parts for this purpose. The parts are not cheap! My brand new planetary assembly cost over $950.00 and came from Germany.

If anyone wants to know which upgrades I am installing I would be happy to provide more detail and reasons. There are professionals on Rennlist who can build high performance 928 automatic transmissions. If you add more power to your stock AT reliability issues will eventually confront you.

goliver

Rotten luck. Good info. for the rest of us though. I for one would like to learn of all the tricks that go into the new tranny.

Regards,

Thaddeus

So, what are the symptoms of bad planetary gears? A horrible grinding noise and immobility?

pappy92651

When the planetary assembly fails the symptoms depend on which gear is at fault, sun, planet, or ring. The degree of damage also determines whether or not you lose all functions. For instance if you lose a few teeth from the sun gear you might not even notice if you are cruising in 4th because the carrier is locked. I lost a planet gear so I was in two forward gears at the same time. There was a whining noise but other than that I was able to drive the car forward because one of the bands was slipping. The fluid got very hot in the space of 1200 feet as I continued limping toward the garage. I was taching about 4000 RPM at 10 MPH. This would not have gone on for very long. It was a judgement call to try and get it home by driving slowly.

A reliable transmission is subject to many of the following;

1) Replacement of all hard parts that are out of tolerance
2) New or rebuilt planetary assembly
3) New bands and clutches, NOT relined
4) New or deglazed steels as required
5) New modulator
6) Updated servos
7) New or completely rebuilt torque converter
8) Replace all seals, lip rings, and gaskets
9) Run a synthetic transmission fluid which is stable at high temperatures
10) Add a suitable external transmission cooler

Find a professional who really knows the car. He can rebuild the transmission and adjust it in his shop. All fluid levels and pressures can be verified so that trouble will not come to visit you for a long time.

The tricks may improve performance but may increase stress on the car as a whole. Some of them are to add a high stall speed torque converter, change the valve body for first gear starts and early down shifts, remove the safety lock outs, install a kick down bypass switch for 1st gear starts, adjust the modulator pressures for firm shifts, and use custom racing fluids which decrease band lock up speeds. These are just a few of the tricks. Most of them will create more heat and increase shock loads. Emphasis should be on creating a solid transmission before you make these changes.

My car has the following: External trans fluid cooler, (new) 2700 RPM stall torque converter, kick down bypass switch, Kohler steels, custom valve body for late up shifts / early down shifts, Amsol synthetic racing fluid, AMG planetary assembly, and 2.54:1 gears with LSD.

This next set of changes should put me in the 4.3s 0 to 60 MPH class.

perrys4

Hope this isnt too stupid of a question but do you think the new torque converter played a part in this failure or was it going to happen in time anyway? Unrealted, I was at the Montage last week and as I was leaving for home a clean Gray OB passed me in traffic. Seems there a a lot of 928s in Laguna. Seems everytime I wander down there i see one or more.

pappy92651

Hello Ron,

Good to hear from you again. Yes and Yes, it's likely that the torque converter accelerated the death of an already worn out planetary assembly. Any flaw in the drive train is going to be aggravated by increased torque. Adding 285/30ZR-18s to the rear didn't help either. All that torque had to go somewhere. I suspect that the driver had a lot to do with the failure.

928s in Laguna Beach are multiplying like mice. All of a sudden they are considered to be a prestige car (again). Lots of white haired old ladies are driving them. Then again lots of white haired old ladies are driving Corvettes in Laguna! It's weird.

GoRideSno

Here's some anecdotal info to add to the mix FWIW.
I bought my S4 knowing that I may have to replace the transmission since the cooler had been leaking into the radiator. The car was driven by the car lot with decreasing trans fluid until it just wouldn't go anymore. I bought the car and replaced the radiator and trans cooler, changed the trans fluid and drove the car. The converter seal was/is definately in need of replacing till this day. I now have 5k+ miles w/ at least 500CHP and another 5k with around 400chp. I've launched the car repeadetly on drag radials and have spun the other tires, 265 18s, to nearly 80 mph. I always manually shift, always. I haven't had any sort of transmission failure to date.
There is also supposed to be a car in the dairy land running 26psi and with a stock transmission with 140kmi.

Andy K

pappy92651

Hello Andy,

I'm willing to bet that once this trans is rebuilt properly it will be bullet proof. I don't know how the previous owner drove my car, but judging from the condition of the underside it was hard and fast. Nothing remained but a few pieces of stock exhaust. I still think that the 722.3 is a tough tranny. You sure have tested yours.

hans14914

I am interested in your tranny shop. I have a 1985 AMG, and i am trying to keep a list of people who have access to parts, and are familiar with their inner workings. Would you please post, or email me the contact info. Does the guy work on the rest of the car or just a trans shop?
Thanks
Hans

perrys4

Pappy, curious if the guy you are referring to is the same guy greg uses to rebuild his trannys.

Bill Ball

pappy - great info. I hope I don't get into a rebuild soon, but I do have what should be a simple question now. My 89 S4 tranny has worked fine, but this week it no longer drops into 1st with the selector in 2 when I come to a stop. It used to drop into 1st at about 3-5MPH. It will kickdown just fine, so I don't think it is the kickdown cable. Sure, I could install a kickdown bypass, but I generally have no need for it. It's a stock daily driver that I occasionally do open road racing in and the stock tranny operation has served me well until this happened. All is fine except doesn't dropdown or start out in 1st anymore with the selector pulled to 2. Any ideas? Is there a selector adjustment? The selector works for all other gears and 2 does drop it from 3 to 2 and hold it in 2 as it should, so I'm a little puzzled. Thanks for any advice.

pappy92651

Hello Hans,

The person who I trust the most is Steve Cattaneo. His business is called Master Tech Transmission and it is located in NY: http://www.mastertechtrans.com/
Steve is incredibly knowledgable and is willing to talk about your requirements. He goes beyond normal levels of detail. Call him if you have questions. He is a transmission specialist. Other people on the list will attest to his credibility.


Hello Ron,

Yes, it is the same person that Greg uses.


Hello Bill,

I don't think your problem is an adjustment. If the transmission goes into all of the other gears then your selector cable is probably set correctly. With the car raised you will see an arm on the drivers side of the transmission with an adjustment nut. If the selector is in second the arm should be at the far end of it's travel. By rocking the arm back and forth you should feel a pre load from the detents inside the case. Each position of gear selection should feel this way.

If the selector is manually placed in 2nd, the car should start in 1st gear from a dead stop. If it does not you most likely have issues internal to the transmission. The fact that the transmission down shifts under load is an indication that the kick down solenoid is working properly. Try the adjustments, but I would head in the direction of a transmission service just to see whats happening inside. Take a look at your filter and the quality of the oil in the pan. I have opinions about what might be wrong but I would rather have you take the transmission to a specialist. The problem might be remedied by a full transmission service. The valve body might require a rebuild. I hope that the problem is easy to fix. Neither service requires removing the transmission.

You might give Steve Cattaneo a call for a second opinion.

Pappy

Bill Ball

As I feared, it seems internal. It is about due for a service. I did have a flush, filter, Redline about 30K miles ago. Thanks for the restrained advice.

Bill Ball

Just went out to lunch and the 1st gear downshift is back (temporarily). I'll do a tranny service. Thanks for the input and advice on rebuilds, which I'm sure I will need later if not sooner. Overall I have been very happy with the MBZ 4 speed.

pappy92651

BTW, this is what happens to planetary gears when the shafts wear out and you apply more than normal torque repeatedly (see attached). Film strength at higher temperatures has much to do with transmission longevity. Some lubricants have better thermal stability at high temperatures. The Kinematic viscosity can drop to less than 2 cSt at flash temperatures. This can lead to metal on metal contact under load. My higher than normal stall speed hastened this process. My driving style was also a component. The transmission now has a separate cooler, heavy duty hard parts, and synthetic fluid. All of my old hard parts were scrapped! The pan was filled with finely chewed needles and gear teeth.