foxint

Guys.

Great threads and fantastic information. I am a 928 new-guy with limited knowledge, but lots of questions, but limited money, living in the Australian Bush.

I read with interest the thread “Did Mercedes manufacture the complete 928 automatic transmissions”. From this I understand yes.

I have a 928 ROW 1982 RHD car with 3-speed auto. I used to have a 1983 280TE Mercedes with a 4-speed automatic transmission. Excellent car…nice box.

I am quite happy with auto. I could only find the above post and those for upgrading to a 6-speed manual.

In a nutshell are there any Automatic upgrades? I am thinking the easiest is to put a 4-speed box (that the later cars had) or a better more modern box. Seems the smart money is looking at the Benz wreckers and parts places…so not wanting to re-invent the wheel…what would I be looking for?

I am guessing that a Benz 722.3XX box is the closes to the 928 4-speed??.. I looked at the photos and it (I am no expert) that the 928 4-speed has 6 Bolt pan - Large Case?? Meaning a 722.3 (4 Speed 6 bolt Large case) should be OK...I am only guessing.

Sorry if this has been asked, but I did a search and drew a blank.

Dan

FredR

Dan,

Fitting a 4 speed transaxle unit is not too difficult - a chap did this over here some years ago and it worked well. However he did use a 4 speed transaxle unit taken from a wrecked 928.

As I understand the Porsche item is a stock MB unit with a modified valve body. The main thing to note is that later transaxles [MY 90+] had the PSD system fitted to the diff and that would be of limited use as it would have to be operated as an open diff so the optimal unit would be an earlier diff with the limited slip option.

Whether or not you can graft a stock MB box into your car remains to be seen but for starters MB did not use the transaxle concept [or so I understand] so presumably you would need a specific Porsche 928 diff unit no matter.

As I am aware you cannot graft the 3 speed diff unit onto the 4 speed tranny but I am far from sure on that one.

DeWolf

Keep driving the auto as is and keep saving the coins for a manual swap.

foxint

Hi Fred

Thanks for the reply. As my wife said…I have wasted most of my Saturday reading about the bloody car…

You have confused me with 2 things:

1. Fitting a 4-speed transaxle is not too difficult
2. Limited Slip diff.

1. I thought that the transmission and the transaxle were 2 separate (although connected) items. My thinking is the Benz part is the transmission and the Transaxle is separate. I would have thought that the old 3 speed box is unbolted and the new 4-speed box is bolted back on. Although I have read that a new torque tube is needed (not sure if it is longer or shorter).

2. Fitting a Limited Slip Differential would be great. Thanks for putting it out there. The wife will ask why, if the old one works…ha-ha. This is a whole different operation. Should it be done at the same time? Or is my humble 1982 base model not suitable for the upgrade? Possibly too expensive and not worth it??

So really if I understand the operation – and I am far from understanding the mysteries of the 928 – but my thinking is:
- Retain the old diff (1982 so it is not SLD, unless I got lucky and the Aussie cars got one, I was lucky with the brakes, so I am told the 1982 ROW had S brakes)
- Remove the 3-speed transmission
- Bolt on the new 4-speed
- Plus, all the other stuff I have no idea about. I will eagerly await instruction from the masters.

My rationale in looking in the Benz parts bin as it is cheaper. So, if someone knows the best Benz 4-speed box, that would be great…. From my confusion and excitement, I think…I stress I think….it would be a 722.3XX or a 722.4XX box. My gut feeling is it is the 722.3…. but that is an uneducated guess.
Thanks
Dan

FredR

Dan,

The transaxle is the total assembly of the auto box and the diff- one without the other is useless [obviously]. My understanding is that the 3 speed diff is not cross compatible with the mounting of the 4 speed diff but someone else needs to confirm that my understanding is correct- I was advised such by Porsche but...?

If you source a tranny from a 928 more than likely it will be a complete transaxle unit

Fitting the 4 speed transaxle unit into your model year as I am aware is a straight forward drop in affair. I cannot remember whether the 4 speeds have the same torque tube configuration or not - again someone will know the answer to that one. For some reason I believe they are cross compatible but...?

The 928 automatic with an open diff is very good at smoking a rear tyre [one of them that is] and going no where quickly. I have only had the later PSD system fitted to my 928's- quite brilliant. With stock skinny rubber I could smoke both wheels but with modern 295 rubber you press the go pedal and the thing hooks up no matter unless on wet surfaces which is pretty much never over here!

DeWolf

Hey Dan,

What part pf the country are you in?


Scott

elgy

My understanding is that the internals are MB but the case is Porsche... so it would not be a simple swap for a MB box. Others know better than I and I'm sure they will contribute...

Alan

Fred - not really that (un?) limited, you can install a bolt instead of the hydraulic actuator and dial in the fixed amount of slip you want, so in some ways it could actually be more versatile than a ~40% fixed limited slip.

Alan

FredR

As I am aware it is a stock MB box with a Porsche modified valve body inserted into it although where it was inserted I know not. At a guess I would suggest the factory of the tranny manufacturer but knowing Porsche....?.

James Bailey

The 3 speed and 4 speed have different length torque tubes and shifter /kick down linkage too. The late 4 speeds 87> have a larger torque convertor and housing which had a larger bulge between the rear seats for clearance ( some have used a large hammer to clearance an early chassis !)
The Mercedes front mounted transmission is NOT going to attach to the Porsche transaxle differential.......
I would agree with your wife if it still works why "fix " it ??? Besides most of the late Porsche automatics start out in second gear !!!!

Ninespub

Alan: My PSD went out when the accumulator exploded about 12 years ago. The shop owner said he was going to put in a "bolt" and set it up and I never really knew the difference after that. Can you explain (in layman's terms) where the "bolt" is and how it is "dialed in" as a fixed limited slip? About 10 years ago, i bought another used PSD unit and never installed it and am thinking about doing that now. but if there's no real gain, why mess with it now..........

James Bailey

There are very different opinions about how well the Bolt fix actually works.... Anderson did it for a race car BUT soon changed it out . Greg Brown thinks it is a VERY BAD idea. The fully functioning PSD does way more than simply act as a limited slip under acceleration. Like assist in high speed cornering !!!

FredR

The PSD system is the kind of thing that under normal circumstances plays little to no role in most real world driving. On the other hand in certain exceptional circumstances it can and quite probably would make the difference between wipe out and nothing untoward happening.

As for setting the unit up with a fixed manual setting I would be left wondering how anyone would know at what point to set it to get the specific amount of pre-set lock up required. Perhaps your mechanic knew what he was doing but it sounds rather irresponsible to me- it would be rather interesting to know what some of our professionals think of what was done and whether they would do such. If you have a replacement PSD unit why the heck would you not fit it?

As to doing this on a non PSD model year [pre 1990] retrofitted with a PSD type diff, that is an interesting point of discussion. My instincts tell me it is not a particularly smart thing to do but to be fair I am not at all sure on that one.

Alan

Its just a variable clutch limited slip. When the clutching force is dynamically controlled - you can do some nice things - including using it at high speed with little diferential speed between the wheels.

However in dumb (fixed bolt) mode its not so different than any other simple limited slip. So - of course in that mode you don't get the advanced features...

Yes you have to get it set up right but I assume there are tests you can do - e.g. ~parked w/ one rear wheel on ground & measure torque to spin the other rear wheel, compare to 40% LSD and adjust as desired: same, more, less.

Alan

foxint

Hi Guys

Thank you…great comments and great advice.

DeWolf – I am in Orange, it is in the middle of NSW. We are now famous for Wine and Watches. That is all. Most trades and hardware is far more expensive here, because they can screw the locals. Being a realist, when my Volvo 960 broke and after naively giving the experts money, I told the mechanic his last 1,500+ bill not fixing the car and his advice of talking it to Sydney - told him in no uncertain terms, I was not paying. I am taking Kate the 928 to Sydney tomorrow for a check-up, injection adjustment etc….

In the thread “Did Mercedes manufacture the complete 928 automatic transmissions”. I understood from this (I may have this wrong) that MB made the box lock stock and barrel (not the diff/transaxle).

If you source a MB box it is cheaper than buying from a Porsche person. Also, I understand that it is the 722.3XX box or could be the 722.4XX (the .3 is the 6-bolt large pan and the .4 is the 6-bolt small pan) …. but I will ask the experts here, to confirm. Since I am in the bush and the “experts” are a-holes.

No: the 3-speed is not broken (and this is my first 928), it seems a little less than the startling performer that I read. I must be honest, my 2005 V6 3 litre Toyota Camry seems more responsive.

My thinking was if the MB box is reasonable, this may make my driving experience nicer.

After reading your responses (thanks), it would seem opportune to do the 4-speed and LSD conversion. LSD would be safer especially if I find myself on the local roads that are total, repeat total rubbish….

Thanks…any specifics would be most welcome.

Dan