722.120 Transmission Modifications:
 

I am convinced one of the best features of my 116.120 is the 722.120 transmission. I built the unit myself and went to the trouble to order all the selective plates for the clutch packs and adjustment struts for the bands.

This did take some time to get all the parts together, but it made for a much better functioning transmission.

I also use Type 'F' Fluid in these units because it has a higher frictional coefficient which gives a more solid shift. We used it at the dealership exclusively up until the time I left in 1992.

My 1978 300 SD used to take off in low gear all the time. However, when I rebuilt the transmission, I used a 1979 valve body, which now shifts differently altogether.

At idle against the service brake, the transmission is now in second gear. This minimizes 'pull' in gear and gives a smoother feel to the idle.

As soon as the car starts rolling, the transmission shifts to 'low' and gives great acceleration. After several adjustments to the vacuum modulator, the shifts are immediate and very solid. There is no indication the trans is about to shift, it happens very quick.

I had always favored the '6 bolt' variant of the 722, but after rebuilding my 722.120, I must say I am impressed at the rugged construction, especially the planetary units.

Just thought I would say something nice about the 722.120...Bert

What did you use for documentation on this transmission? Others have commented that the ATSG manual is not very good for the MB.

I am glad you mentioned the Type F. It sounds like a good idea to me but there is always a controversy when this is mentioned.

Did you install all of the parts as they were or did you drill holes or anything like that?

Are there any high wear areas in these transmissions besides the soft parts?

Twitch,

The 'Type F' argument has been going on for years now. It's ironic that aftermarket 'performance' transmission fluids, like that marketed by B&M, is based on the Ford Specification. B&M also provides this fluid with the GM 700 R4 Hydramatic which calls for Dexron-Mercon as a factory fluid.

MBZ changed the frictional properties of the bands in the 722.120 transmission sometime around 1980 to give a 'softer' shift, based on customer complaints. However, keep in mind that a crisp, instant shift, promotes clutch and band longevity because it reduces 'flare' and the subsequent heat of slippage.

When I serviced my 722.120, I used the ATSG Manual, which was fairly decent.

I made no modifications other than adding (6) springs to the Direct Clutch Piston. You will notice a number of small 'pins' in the piston onto which a small spring is placed. In most 722.120's there are (12) springs, but there are enough pins for (18) if I recall correctly.

I believe these were omitted by MBZ over the years to make a softer shift.

Basically, I have several transmissions for parts and I placed a spring on every pin. This requires more pressure to overcome the springs and gives amuch crisper shift into direct (4th) speed

What I also did was take the time to reduce the clearance in both clutch packs to the minimum specification by utilizing the selective fit steel plates. I also took great pains to get the proper band adjustment. This pays some real performance dividends when the unit is properly built.

The planetary units in the 722.120 would be at home in a 2.5 ton truck. They are larger than the GM 400 Hydramatic and even the modern GM 4L80-E Transmission. Hard parts and bushings seem to last forever in this unit.

There are two major problems I have found while working on these transmissions. The first is Direct Clutch Failure which is brought on by lowering modulator pressure (backing out the modulator adjustment) to satisfy the customer who complains of a hard or suddent shift.

The faster, more immediate the shift is, the less heat and 'slippage' the friction elements are exposed to. You don't want the shift to 'jerk' but it should give no indication it is about to take place either.

The next problem is Direct Clutch Failure (3-4 shift flare or slip) brought on by low vacuum from leaks in the system, depriving the vacuum modulator of proper signal.

You should have relatively high vacuum signal at the modulator valve on top the injection pump, around 10 to 12 in/Hg. The vacuum should diminish gradually as the throttle is opened to around 2 in/Hg or 0 in/Hg as full throttle is reached.

What I did on my 116.120 to minimize this proplem is chang the vacuum routing to make the modulator the first item in the vacuum circuit. I installed a 'T' fitting with a one-way check valve on the line coming off the vacuum pump.

In this way, vacuum goes to the shift modulation system (the plastic valve on top the injection pump and the actual modulator on the transmission itself) before anything else receives a vacuum signal.

The one-way check valve is positioned downstream of the modulator valve so vacuum is maintained after shutoff. Keep in mind the modulator valve on top the injection pump is a controlled bleed, so a small amount of vacuum is lost through it all the time.

I then increase modulator pressure (clockwise) until the shifts 'feel' good and crisp, but are not harsh. I have found this is more of a trial-and-error effort than a specific adjustment.

However, I have personally seen these 722.120's with over 500,000 miles that still looked good on disassembly. When I opened the one in my Dad's old 300 D with 475,000 miles on it, the MBZ Star could still be seen on the friction elements and the bands.

The only reason we pulled it was because of external leaks resulting from hardened rubber seals. Dad didn't want to spend the $175.00 on the overhaul kit that contained the (8) clutch plates, so we just replaced the rubber seals

I hope this information is helpful...bert

Herr Doktor-

Your information on the W116 300SD is always helpful.

I have a a 722.120 on my bench at this moment awaiting its chance to go back in my 80 300SD. I bought the car from California months ago with a known bad tranny. It turned out to be a bad front pump- the torque converter fingers had sheared off (cause or effect?). Anyway, I have swapped in a good pump and torque converter from a 722.112 off a 280SE. I have not attempted to rebuild the transmission as it was working properly before and only has 125K miles on it.

But someday I may rebuild it. You seem to have a great familiarity with theses 4-bolt trannys. Are the internals of the 722.112 (also 4 speed) identical to the 722.120? Is it just the valve body that is different? Are there any "special"tools needed in a full rebuild of a 722.1xx? I have rebuilt a Ford C-4 so I am not completely terrified of the task.

Rick
Deltona, FL

You will find it very similar to a C-4 with an extra band. I built mine with no "special tools," but you will find the seals on the forward and direct clutch pistons a bit tricky to install. Just be patient and keep working until you get the piston to slip into place.

The entire rotating assemly, including both drums, the center support and all (3) bands are simply pulled out of the case as one unit. All service work to this assembly is done on the bench, meaning you separate the drums and center support after you have it out.

There is a long plastic wedge that holds the (3) bands in place during assembly. Once the valve body and front pump have been removed, you simply grasp the input shaft and slide the entire unit out.

You will need a special "4-Prong" socket to remove the yoke on the output shaft.

Also, one of the servos (the smallest one) has an 11" long spring that is more than your thumbs can handle. Several tool companies offer a lever that hooks onto the case and can be used to compress the servo spring during disassembly. This is a very generic, universal tool and I highly recommend the tool and a helper. Both of you should be wearing full face shields when attempting disassembly of the servos.

Each servo has a small strut with a ball on each end. They look like little 'bones' and they are available in all different lengths. Make sure you keep them with their respective bands. They are critical to proper band operation.

The only thing in the extension housing (rear part of case) is the rear pump (allows for push starting) and the governor assembly. Since there are no rubber seals back there, I usually leave the rear housing in place and clean the case in the usual fashion, washing everything out with clean solvent followed by compressed air.

There is no way around it...bands for this transmission are outrageous. I paid around $160.00/each for the (3) bands and I could get them only from my local Mercedes-Benz dealer.

Another option is good used bands. I have found a bunch of these transmissions at self-service wrecking yards in Fresno. I always pull the bands when I can find a good set that show no wear.

I recently bought a complete 722 out of a 1979 300SD from Pik-N-Pull on Jensen Avenue in Fresno for $106.00. This one happened to be a genuine MBZ Remanufactured Transmission. These finds are still out there if you have a sharp eye.

I found a pretty good deal on the overhaul kit for this transmission too. I can get a gasket/seal kit including all the steel rings, valve body gaskets, lip seals, 'O' rings, external seals and (8) friction plates (bearing the MBZ Star) for around $175.00. The kit is all german-made by 'Lemfoder.'

Steel plates are a dealer-only item and they are available in selective thicknesses.

Contact Transmission Parts Incorporated in Fresno, CA. for the ATSG Overhaul Manual.

Best of luck...Bert

Curiosity has me... what info do you have in your head about the later 722 five speed auto with overdrive? :D

Is it a lockup torque converter? Does it require any sort of computer interface or is it standalone like the older 722s we run? What about overall length from the bell to the mount and bell to the end of the output shaft? I looked at a few exploded diagrams between the four and five speeds and they seem pretty different.

This unit was brought to my attention by another member on here as a potential swap.

Any info is much appreciated! :)

Herr Doktor-

Thanks for the 722.1xx info. Doesn't sound too horrible to renew but I think I'll wait until the one I have fails. I too have been looking at stockpiling 722.1xx trannys when Upullit has them on sale for $40 and they look like they have a breath of life left in them.

Do you have any familiarity with the 722.3xx series that I have in my wife's 83 300D? I have a supposedly working spare on the shelf with 190K miles on it. As that one and the currently working tranny both have close to 200K miles on them and the reputed life is 250K I may want to do a preemptive rebuild to save from having to pull the used one out in a few miles anyway.

Is the 722.3 similar? Need any "special" tools and is it relatively akin to a C-4?

Rick
Deltona, FL

I have never tried swapping that unit. I do know the 6 bolt (722) will easily replace the 4 bolt with a driveshaft swap.

Let me see what I can find out for you...Bert

I always referred to the transmissions as a '4-Bolt 722' and a '6-Bolt 722' as a generalization. The later 6-Bolt is actually easier to work on than the 4-Bolt. However, the 4-Bolt is actually a heavier duty unit.

If you compare the size of the planetaries in the two units, the 4-Bolt is much sturdier looking. However, both transmissions would be a great unit.

I would like an overdrive in my W116.120 behind the 617.950 engine. I think that would be a great modification, especially for highway cruising...Bert

I agree that would be a nice modification...........

I am up to the 4,000 mile mark on the 116 now and everything is going very well. I have, however, encountered a rather annoying characteristic I have never before experienced with the 722.120 transmission.

On occassion, it will not downshift into low range when leaving a stop sign or traffic signal, but will remain in the second speed range. Sometimes, if you apply a bit more throttle pressure than usual, it will downshift and work normally.

Since the condition seems to correct itself at times with increased throttle pressure, I believe it is not electrical but rather a valve that is sticking.

This surprises me since every single part in that transmission was cleaned and inspected, but anything can happen.

I will study the schematic later this week and post when I have taken corrective action.

Hope this thread will prove useful to someone...Bert

Bart, I know this is an old thread but there is a switch under the accelerator pedal that when pressed hard forces the transmission to kick down into first gear otherwise you will always start in 2nd gear in a 116. There's a solenoid on the side of the transmission that controls this.