722.118 transmission B2 servo/piston

[davidhoward]

I am having trouble with my '81 300D 722.118 tranny that is peculiar. It works fine when cold, and for as long as I leave it in D while hot/running. But when hot, if I shift out of D or shut the car off I have major slipping in 1,2,3 (reverse works perfectly). I've found that when this occurs if I nurse it up to speed, as soon as it shifts into 4, all is well again. I can speed up and slow down with no slippage in any gear. Alternatively, if I let the car cool for an hour or so, all is well.

From the ATSG manual I know the B2 piston is 'engaged' for 1,2,3 and released for 4. I also know that the B2 piston has some sort of valve built in (but don't know what it's purpose is, or how to test it).

I removed the valve body and used compressed air to measure the B2 band free play. It was out of spec, so I purchased and installed a larger pressure pin (26mm ~$13). Now the free play is in spec. In order to install the pin, I had to partially remove the B2 piston. The B2 piston did not appear to be broken --- no metal pieces or broken plastic on valve.

I did notice something odd when I applied air pressure to engage B2 --- occasionally it would only partially engage, and air would come sputtering out through the hole used to dis-engage B2! This never happened when I used air pressure to dis-engage B2.

It didn't say in the manual what PSI should be used to engage and dis-engage, so I used several pressures between 60-80psi. It didn't seem to make any difference. Unsure of what was normal behavior for B2 piston/valve
I reinstalled the valve body, etc. and test drove the car. The problem still exists.

Now I'm suspicious that there is something wrong with the B2 piston and/or it's valve. Before I remove, inspect, repair/replace it I'd really like to know if I'm on the right track. I'm sure a new B2 piston is pricey, and I'd hate to replace it and still have the same problem.

David Howard

[Pete Burton]

you're on the right track checking the B2, David. Seeing as how you pressurized the piston and you knew enough to check and adjust the B2 brake clearance, I expect you can handle the B2 piston just fine. Sometimes these crack, sometimes they get metal bits impregnated in the outer teflon ring and sometimes the smaller, aluminum body gets a groove worn into it. Plus, originally an 81 had an aluminum bushing pressed into the case that the aluminum diameter of the B2 piston rode on. Well, you can guess what happened to a lot of these. I had a trans that had a sticking B2 piston. It would hang up applying, but would always return. When I pulled the piston, there was a nasty groove worn in the aluminum body and the original aluminum bushing. They make a nylon replacement bushing that fixes that problem. You could probably smooth out the groove in the B2 piston if there was one.

BTW, where exactly do you apply air to activate the B2 band? Do you know the points to apply for the other band and clutches? I'm currently rebuilding and that info would really be handy.

[davidhoward]

OK, so I opened up the tranny, removed the valve body, removed the B2 servo cover, applied a bit of compressed air to the dis-engage port, and viola B2 piston is out.

I've attached some pictures to this post to show the state of the piston and cylinder (as well as good views of bands, pressure pins, etc).

The cylinder has a grove worn in it which is clearly visible.

The piston look OK, but it has a single split ring metal seal. I imagine it leaks like mad when it's aligned with the grove in the cylinder.

How the heck can this be repaired?!

What supposed genius designed this? The ring is obviously going to wear a grove in the aluminum, as it's a harder metal, and since it's split, as the grove gets deeper the more the piston will leak.

David Howard

[Pete Burton]

I see you have a different trans than I thought you had, so sorry about my incorrect statements about it. I think you need to ask someone with expertise with this type. Is that split ring really metal, or a teflon seal that has metal dust impregnated in it? Reason I ask is I have seals that look that color, but they are actually teflon. There has to be something else there I would think. Just doesn't seem right that it would be made with just that split ring and ever seal.

[davidhoward]

The split ring is solid metal. It feels/appears to be of the hardened variety.
I agree, it seems like a ridiculous way to seal the piston. If it's not fully compressed it will always leak a bit, and as the cylinder wall wears it will leak more and more.

I'm wondering if they might make an oversized piston/ring and maybe I have to bore the cylinder out so it's smooth again. It's also possible that the cylinder wall is a pressed in sleeve, but I haven't looked at it closely enough to tell.

Either way, it would require removing the transmission from the vehicle to fix.

If neither of the above are options, I'd say this tranny has had it. No rebuild will fix this. I may have to start looking for a junk yard tranny which is is better shape .

P.S.
Interestingly the exploded parts view image of the 722.118 tranny I got from the MB part dept. looks very different from what I pulled out of my B2 (which matches the ATSG 722.1-722.2 manual). Maybe MB changed the servo assembly. I'll check with them tomorrow morning.

David

[Pete Burton]

I'd be pretty surprised if MB DIDN"T change that servo at least once. They changed the B2 servo on the 722.3 at least twice. If that's really a hardened metal ring on an aluminum bore, you can bet teh farm that lots of other people have experienced this, and there are fixes available. However, I've been pretty frustrated myself trying to accumulate GOOD information about my trans. I hope you have better luck. You might try Doktor Bert, another member on this forum, as I do believe he has done work on a 722.118 trans.

I wonder too, if a thinwall, hardened steel sleeve could be pressed into that bore, and matched with a piston like the one on the 722.3 that has a wide, outer teflon ring. - But that's just speculation, and you need hard facts. Hope you find the right source. It's out there. You might also try one of the moderators of the tech forum, Steve Brotherton. He has a Euro transmission shop. Good luck!

[davidhoward]

Originally Posted by davidhoward

I'm wondering if they might make an oversized piston/ring and maybe I have to bore the cylinder out so it's smooth again. It's also possible that the cylinder wall is a pressed in sleeve, but I haven't looked at it closely enough to tell.
Maybe MB changed the servo assembly. I'll check with them tomorrow morning.

David

No such luck. MB doesn't have an oversize ring. It's not a pressed in sleeve. MB has not altered the servo assembly.

I'm going to try a hone to remove the ridge, and hope that the leaky B2 piston ring gap isn't going to leak so much as to prevent it's operation. It's a long shot, but at this point what else can I do?

I'd really like to replace the ring with one that was oversize, and teflon, but I don't have a clue where to start looking for one.

Anyone know of a shop that makes one off rings to customer specifications (and doesn't cost an arm and a leg)?

David

[Pete Burton]

I would hold off on trying to use a hone. I understand where you are coming from, but I still think it's worth it to be patient and try to find a better solution. It will be difficult to hone with the trans in the car and much more difficult to avoid contaminating the system with fine pieces of aluminum. I'd be willing to bet good money that this is not unique -keep seartching, and good luck!

[C32AMG]

The 722.0, 722.1 transmissions used three different diameters B2 piston during its evolution.

The first was 75 mm then in the late 70s it was 88mm and finally and 92 mm. How deep is the worn section in the bore? What does it feel like when you move your finger over it?

To determine if the case is salvageable insert a new ring into the bore, in an unworn area measure the end gap., know place the ring were the wear is , a reading of more .012’ means replace the case. If you can’t find one look into the possibility of sleeving the bore and using the 88mm piston, or borering / power honing the bore and using a machined early or late 722.3 B2 piston or a 772.4 piston.

[davidhoward]

The darkened circle as seen in the previous post feels smooth to the touch for about 3/4 of the cylinder, but on the top 1/4 I can feel a noticable ridge. I suspect this is where the piston was sticking, and probably where the split of the ring was. It doesn't feel very deep, but I'm sure my fingers are not calibrated for tenths of mm or thousandths of inches .

I will proceed as you suggested to measure the ring end gap in and out of the notched area with a feeler gage.

By the way, it is an 88mm piston/ring in this tranny, so it does sound like I might be able to bore/hone it out to fit the larger piston from the later transmission if need be

Since I will have to be removing this tranny from the car I am also looking into a '82 240D with a blown engine ($200) that I might be able to scavange a tranny from. From what I can tell so far it should be a drop in solution.

I'll post the on-going results and pictures when needed of this continuing saga...

[davidhoward]

I measured the end gap of the (old) servo piston ring both outside .009" and inside the groove .019".

It's a long ways beyond the .012" limit stated in a previous post .

I know it was sugguested I use a new ring, but I measured the old one and it was 88mm (+- 0.10mm accuracy of my caliper) when compressed so it doesn't appear to be worn much if at all.

I did manage to find someone on the internet who makes custom piston rings to spec for about $25 a piece: http://www.dol.net/~dave.reed/rangcast.htm
They can make them with the ends step or angle cut as opposed to the standard straight cut.

So i guess my options are remove the transmission and:

1) get another transmission/case - => $350

2) bore/hone B2 cylinder slightly and get custom ring made - <$100

3) bore/hone B2 cylinder significantly and get later model transmission B2 (92mm) - I'd need to buy an servo piston and ring from MB and get a machine shop to bore the cylinder. $???

4) bore/hone B2 cylinder significantly and get a sleeve pressed in for 88mm B2 piston/ring I already have - I'd have to find a machine shop bore the cylinder and press in a sleeve which would then need a oil port cut in it. $???

I like option 2 or 1 in that order. What do you think?

[Pete Burton]

I would get a used transmission and put it in. If it works well, problem basically solved. If it works so-so, you can live with it, get another or even rebuild using the removed trans or both. I suppose it depends on your tolerance for having the car out of commission, and your interest in doing internal transmission work. For me, I have an old truck I can drive, and having rebuilt an automatic once before (and enjoyed it - a chrysler 4 sp electric shift, from a minivan), I decided to experiment. So I disassembled a spare trans (that had failed previously) and I decided to try the rebuild myself. I'm about 80% done putting the best of 2 together - we'll see if it works.

Whats the word on this project?

I have a GOOD working transmission out of a 240D (automatic) that I need to get rid of. I will make you a killer deal and we can arrange something if you want me to deliver it or vice-a-versa...

I believe this unit has somewhere around 140K on it? I sold the engine to a guy out of the car (someone converted it it gas powered) and he is happy with it. Runs strong.

The tranny was known good and shifted firm. Judgement was by a shop tech.

Let me know

[davidhoward]

Brandon,

The status is.... I've reassembled my tranny. I'm making short trips around town in the car, and when I make long trips, I have to wait an hour or so before I can drive again.

Bottom line, I'm looking for another transmission --- I figure if I am going to remove this one from the vehicle anyway, I might as well replace it with one that is in somewhat better shape than try to band-aid this one.

I'd be interested in hearing more about your offer, please email me with contact info --- david_howard@yahoo.com

Originally Posted by davidhoward

I'd be interested in hearing more about your offer, please email me with contact info --- david_howard@yahoo.com

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