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Discussion Starter #1
So my 89 420 SEL is exhibiting some very strange behavior, I have done alot of reading on this and I cant seem to find anyone who has had the same issue with their reverse gear.

It only works when its cold out. The colder it is outside, the faster it goes into reverse. If its under 32 degrees outside, it goes into reverse instantly, if its over 45 degrees outside, it will never go into reverse... This seems opposite to what everyone who loses reverse deals with. Its usually when its warm out it works fine, but if its cold it takes a long time for it to go into reverse. When it first happened, it was warming up from last winter and I just thought the B2 brake band had gone out. But now this winter as its cooling back down, my reverse gear is slowly coming back to life. Im inclined to think that perhaps my brake band is ok and there is something up with my valve bodies. Could that be the case??? If so Its much easier and cheaper to repair the valve bodies than to have to tear into it and redo the brake bands and clutches...

Any help would be greatly appreciated.

-Andrew
 

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Well this is kind of a shot in the dark, and I'm not a transmission expert.

Colder fluid is thicker. Is this only an ambient temperature thing, or does it have to do with the temp of the transmission itself.. For example if it's cold out and shifts fine into reverse, does it suddenly not go into reverse once the car is warmed up?

If so, maybe you have a massive internal leak. The cooler thicker transmission fluid may be creating somewhat of a seal, and allowing enough pressure to build up and give you reverse. As it gets warm, it will thin out and perhaps leaks too much to build up any pressure in reverse.

Dumb question. Did you check the fluid level, and how frequently has it been serviced?
 

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Discussion Starter #4
Im going to side with a leak thanks...

My guess is that one of the o-rings or seals on the reverse valve has gone out. Anyone have any technical experience doing this? Im not by any means afraid of tearing into it. I have a full shop with the necessary tools to pull and dismantle a transmission.
 

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Once it warms up, what happens then?
 

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Discussion Starter #6
It stops working when warm, and by warm I mean both warmed fluid, and warmer outside temps. Really it only works when the actual transmission fluid is at 40* or below. So even on a cold day, if the fluid gets warmed up through use, it wont go into reverse anymore.

Im really thinking that perhaps my problem is that a seal on the reverse valve has a catastrophic leak in it. Which would be simple to replace. Its been a good 20K miles since I last changed the fluid, but it does not smell burnt and its still nice and pink.
 

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It stops working when warm, and by warm I mean both warmed fluid, and warmer outside temps. Really it only works when the actual transmission fluid is at 40* or below. So even on a cold day, if the fluid gets warmed up through use, it wont go into reverse anymore.

Im really thinking that perhaps my problem is that a seal on the reverse valve has a catastrophic leak in it. Which would be simple to replace. Its been a good 20K miles since I last changed the fluid, but it does not smell burnt and its still nice and pink.
Ok, the piston is getting stuck in the bore.
 

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Discussion Starter #8
So what does that mean for me? Is that fixable by simply refreshing the seals and cleaning out the gunk? Or am I hosed?
 

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Sounds like one of two things. Thicker fluid would compensate for a bad seal, to some extent. On the other hand, I doubt it would make enough difference for such a narrow temperature spread to go from "instant" gear shifting to not shifting at all. (I'm assuming temperatures are expressed in Farenheit. If not, then it's probably the harsh desert environment that has done in your transmission.)

Seizing can happen if the piston is a metal that expands more than the bore, as aluminum pistons do in iron cylinders, if the piston does not have room to grow.
If it is a piston stuck in a bore, it means that there is very likely a layer of "crud" (if that's not too technical a term) built up in the bore. This is usually the only way a piston will get tighter in its bore with long use. Usually they get looser from the piston wearing on the walls of the cylinder, but if the piston is only used once in a while, the fluid can rest and deposit residue on the walls, and then the residue hardens with heat. I've seen similar things happen in brake cylinders.

If this is what happened, then most likely, you'll only have to open it up and clean it out.

Then, too, a hydraulic piston has to have somewhere for whatever is on the other side to go. Perhaps there is fluid trapped on the other side and the escape passage closes up when it gets warm? (Just a WAG.)
 

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So what does that mean for me? Is that fixable by simply refreshing the seals and cleaning out the gunk? Or am I hosed?
No not hosed. I think if you can take it apart, lightly sand the piston with some 1600 grit while holding it under the running water in the sink, and clean it, put it back together. It will work again, for a unknown amount of time.


As metal is heated, be it by a flame or fluid temp, it expands. Expand too far and it gets stuck.


But you need a valve body.



Let me see if I can find that picture.
 

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Hi,
Could be the brake clutch#3 worn out. It sit right behind toque converter so you can't really do much. You can try oil additive from Lucus and cross your fingers. Does your tranny making any weird noise while driving at low rpm like 1500 or so. If it does then your clutch is gone.

BTW, this is commonly happened if drivers don't let the vehicle come to complete stop before putting it in R or P.
Good luck.
 

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Discussion Starter #12 (Edited)
Other than the fact reverse wont work unless the fluid is very cold, the transmission works perfectly, smooth and solid shifts with no slippage anywhere. And when reverse does work, it holds strong and doesnt slip out. I assume that you all mean that the B2 Brake band piston has stuck correct? Anyone have the Special tool to press it back in? or is that necessary? Also, which valves do I need to concern myself with on the valve body? Im going to guess the Lock-out RV1 valve, shift valve B2, and release valve B2? And also the reverse valve shutoff plunger?
 

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From what you're describing, the piston gets stuck after the transmission warms up to operating temperature.

If you're working on it while it's cool, then chances are that it won't bind when you try to pull it out. It should be easy enough. Take it out, clean the piston and the bore thoroughly, and reassemble.

Another question: Have you tried getting it to operating temperature in reverse and seeing if it sticks and won't come out of gear? If it pops right back out while hot, then that would indicate a different problem.
I don't know what it would indicate, but it would probably be something other than a sticky piston.
 

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Lol. B2 brake piston.



Even better then a picture, l0ch0w.

How about the .pdf to fix it?
 

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Discussion Starter #15
I have that whole manual, I must have missed that page -_-

Thanks for the help! Ill be tackling it some time after thanksgiving. Ill post up my results and take some pictures of the repair :)

I did some more research, and the b2 and b1 brake bands arent used for reverse, they are simply released when reverse is activated. If I were having brake band issues, I would have other issues like not being able to go into 4th gear, or use my 1st gear and such.

Only think Im having a problem with right now is finding the part numbers for those little guys... I guess ill call the mercedes dealership tomorrow.
 
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