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  #1  
Old 08-23-2017, 08:27 PM
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Location: Surrey, BC Canada
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HELP: 1995 C280 722.424 transmission troubleshooting after rebuild

Hello. Im looking for some help narrowing down the list of suspects.
So the story goes:
Bought the car with bad reverse clutch seal, so eventually reverse has failed in a big way.
I pulled the tranny off and took it apart, the only damage was the exploded reverse clutch, I inspected all other clutch packs and brake bands and they were fine.
Decided to go ahead and rebuild it, I used genuine MB seal kit and MB valve body seal kit to satisfy my OCD. Installed new reverse friction plates, springs, spring plate and circlip. Same time replaced B1 brake band and some cheap plastic pieces. Measured all clearances as per assembly manual, everything checked out. Same time rebuilt engine top and replaced head gasket (had a small oil leak in the back).
If anyone interested I have a youtube channel with these videos.
Installed the tranny back ...
On first test drive it drove fine until the tranny warmed up, it started to slip on take off.
I thought it was the B2 brake band slipping, also a common thing to do in this case is to replace B2 piston pin with a longer one, they come in 0.8 mm increments.
So dropped the tranny and investigated. Measured the B2 band travel, 5.6 mm (spec is 5.5 -6.0 mm), on a closer look I found that the B2 piston clearance was a bit too big to my liking – 0.007”, so I got another piston and a new B2 brake band installed. Put it back together and installed back into the car.
On the test drive it did exactly same thing as before, as soon as it warmed up it started slipping on take off. As it drives it shifts better then before, no kick from 1 to 2, shifts up and down no problems, kickdown works OK.
I don’t know enough about the inner work of a transmission and how the transmission works in every detail, so Im lost. Need to come up with a plan for troubleshooting to narrow down on this problem.
So far I suspect:
1. Low oil pressure on warm transmission at idle?
2. Slipping one way clutch? I had it completely apart, except the actual sprag bearing. Looks like need to test if it slips in reverse as well?
3. Does anyone know what the spec is for 722.424 working pressure, cant find this info ?
4. How do I measure B2 piston pressure?

any help would be appriciated, as I really want to get to the bottom of this problem.


Last edited by andy_1100; 08-24-2017 at 02:37 PM. Reason: add pics
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  #3  
Old 08-25-2017, 03:23 PM
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Location: Surrey, BC Canada
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Finally got some free time to get under the car to connect a pressure gauge. I think I found the problem.
These are working pressure readings:
Cold engine start @ P, 800-900 rpm , modulator vacuum line connected – 100 psi
60 deg engine @ P, 700 rpm, modulator vacuum line connected – 65-70 psi
70 deg engine @ D, 600 rpm, modulator vacuum line connected – 40 psi
70 deg engine @ P, 700 rpm, modulator vacuum line disconnected – 50 psi
70 deg engine @ P, 1000 rpm, modulator vacuum line disconnected – 75 psi
70 deg engine @ D, 600 rpm, modulator vacuum line disconnected – 30 psi
Drove the car for 10 min to warm up the transmission
90 deg engine @ N, 700 rpm, modulator vacuum line connected – 45 psi
90 deg engine @ N, 1000 rpm, modulator vacuum line connected – 75 psi
Drove the car for another 10 min to warm up the transmission
90 deg engine @ P, 1000 rpm, modulator vacuum line connected – 60 psi
90 deg engine @ R, 600 rpm, modulator vacuum line connected – 25 psi
90 deg engine @ D, 600 rpm, modulator vacuum line connected – 20 psi
90 deg engine @ P, 1500 rpm, modulator vacuum line connected – 100 psi

I found some info on 722.XXX transmission, it says I should read 75-90 psi on hot transmission @ D.
Another reference tells me for gas car I should read in Park @ 1000 rpm with modulator vacuum line removed:
E320................174-203 psi
E420 & S420.....160-174 psi
S500 & SL500...186-215 psi
I assume working pressure for C280 should be close to these figures. I'm supprised the car actually drives. Clearly, working pressure is too low, this explains why it slips when is getting hot. The valve body is coming off for inspection. I guess I’ve messed up assembling it.

Anyone has good reference material for valve body rebuild? All I have is not the easiest to read and see the pictures to make sure all valves are installed in the right order?

Last edited by andy_1100; 08-25-2017 at 03:29 PM. Reason: correction
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  #4  
Old 08-25-2017, 03:38 PM
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Location: Modesto CA
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Re: Pressures

Three items come immediately to mind:
1) Front pump very worn.
2) Defective modulator/modulator valve.
3) Leaking rear pump (if installed).
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  #5  
Old 08-25-2017, 04:41 PM
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Before the reverse failed the tranny worked fine, except the slow reverse engagement, but that was due to a rapptured seal I assume.
When i got the car about 3 years ago, I changed the ATF cause it was burned. Took it for a drive the next day and I had a slip in 4 gear 2 or 3 times. Came home and added Lucas transmission fix, after that had no problems at all, just a little kick going from 1 to 2 gear.
On rebuild:
- Valid point, but Front pump didn't show any wear or anything abnormal.
- Modulator valve seems to be OK, no vacuum leak, no oil coming. Transmission had no problem shifting before the rear clutch failure.
- No rear pump installed.

It feels like there's a big oil leak somewhere. I might have messed up the valve body, i'll have to take it apart and check if i got all valves installed in correct order.
Im going tomorror to a wrecker to pull a transmission from a 1995 C280, and i'll take the valve body apart to compare to mine. Hope to find the problem.
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  #6  
Old 08-25-2017, 08:11 PM
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The vacuum modulator reduces line pressure when load is light ( high vacuum ) and increases it when load is high. Your tests are with vacuum line attached but your specs are with the line disconnected so we can draw any conclusions from your data.

Buy the $ 30 ATSG service manual ( don't bootleg it as they put lots of effort in to these things. )

Did you air check this trans after it was built / before valve body was installed? This is a way to check for major internal leaks.
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  #7  
Old 08-27-2017, 10:49 AM
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97 sl320, i was measuring working pressure, and i did measure with and without modulator vacuum line, didnt see much difference.

If you have any details how to test the transmission for leaks using compressed air could you send it to me, please. aiptech1@gmail.com
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  #8  
Old 08-27-2017, 04:49 PM
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To air check a transmission you leave the valve body off, use a diagram to identify the various clutch / band apply ports on the case then apply air using a rubber tipped blow gun. Some transmissions ( not make specific ) need a plate bolted to where the valve body would go as sometimes passages are not round.

When applying air , a slight hiss and a clunk as the element applies is normal. If you get a whoosh, that isn't normal if you are applying air to the correct port.

This isn't used to check pump capacity, the goal is to find something leaking excessively like a cut clutch piston seal / failed piston check valve.
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  #9  
Old 08-28-2017, 02:00 PM
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Hello 97 SL320. Thanks for the reply, I do understand the theory behind testing with compressed air, I tried to test B2 piston...not mush suscess. Problem is - Non of the rebuild manuals indicate where to apply the air.
I got a second transmission from a wrecker on the weekend, i might practice on it....
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  #10  
Old 08-30-2017, 01:18 PM
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Location: Surrey, BC Canada
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might have found the problem...

so yesterday i pulled the valve body off my transmission to swap with a valve body fom the transmission i got from a wrecker, and what i see.....the big paper gasket that should be under the valve body plate is installed on top of the plate, between the valve body and valve body plate. Rooky mistake on my part...
Pulled the valve body plate and installed the gasket on the correct side..under the valve body plate.
Today will install the my valve body back and will see how it goes...keep my fingers crossed this is the only mistake i've made

this is the valve body plate with removed valve body on the tranny i got from wrecker, the papar gasket is under the plate
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  #11  
Old 08-31-2017, 01:20 AM
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Issue is solved. The transmission works as it should, 70-75 psi working pressure on hot transmission idling in "D".
The only concern i have is when idling in reverse working pressure shoots to 280 psi, and i cant find any info if this is correct.
Otherwise it drives nice, no kicks on shifts.
Case closed.
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  #12  
Old 08-31-2017, 08:46 AM
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For some transmissions (in general), reverse pressure is higher because internal gear train leverage is higher. ( not overall ratio ) RE: The way reverse is achieved has higher internal stress than forward gears.
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  #13  
Old 08-31-2017, 09:20 AM
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97 SL320
Thanks for the info.
Now ...oil and filter change after an overhaul....after 20k km?
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  #14  
Old 08-31-2017, 02:04 PM
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If you want to have a look in the pan for excessive metal / material, that would be fine otherwise just run it.

I just noticed, did you use Permatex Set & Seal on the pan gasket? If so, that is for machined metal surfaces where tight contact precludes the use of silicone. Rubber pan gaskets just need a thin wipe of oil so it doesn't drag as the bolts are tightened down.
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  #15  
Old 08-31-2017, 03:30 PM
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Ok. Ill just drop the pan in 5000 km

No i didnt use any sealant on my transmission, i just wipe mating surfaces with brake clean and install.
This is a photo of the transmission i got from the wrecker, its a bit messy. Theres sealant all over it. I might keep it and rebuild later.

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