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1980 300CD Auto Trans shifting Problem

  #1  
Old 07-05-2017, 12:58 PM
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Default 1980 300CD Auto Trans shifting Problem

I am new to the forum. I would like to see if anyone could offer some assistance. My car has a 722.1 trans. It appears to be a one year only version of this trans. This trans only has a vacuum connection to the engine. There is no mechanical connection via bowden cable. The problem is the trans shifts to early, flares between 2nd and 3rd and slips into 4th. I also can not get it to kick down. Here's what I have done with the trans since owning the 300cd. I should mention the trans has 253k miles on it.

1. replaced the front seals (input shaft and "O" ring around the pump housing).
That stopped the leaking.
2. I replaced the (red) modulator valve with an expensive new one from e bay.
3. Rebuilt the vacuum pump since I only had 14 in. of vacuum. After the rebuild, I now have 18 in. Vacuum. Better but not great. I have checked all the vacuum hoses and replaced the big vacuum line going to the break booster. Also replaced all the rubber connection hoses ("Y"s "T"s and straight ones). I can pull a vacuum with my vacuum hand pump and hold it.

4. I have the transmission tuning manual from Kent at "Mercedsource" . I have gone through all the procedures described in the manual.

5. After replacing the Modulator valve and setting to the factory pressure the trans exhibited the same problem even with the vacuum line disconnected from the trans. So I turned up the pressure so the trans would shift hard at high RPM and would kick down. When I re-connected the vacuum I'm back to the original problem as described above.

6. I have tried adjusting the vacuum pressure with the small colored vacuum line connectors. I'm down to the smallest one with no real change in shifting.

7. I forgot to mention I also have replace the trans filter and fluid with synthetic ATF. The pan did not have a bunch of junk in at the bottom. And the old fluid looked good and smelled good. Not burnt.

Is my trans just worn out? Reverse works great. It goes into gear as soon as I put it in any forward gear. It shifts down to first when taking off from a stop. It works ok when shifting manually but still slips into 4th compared to a hard shift.

Would it be better to go with a trans with a bowden cable?

Any ideas or suggestions would be great. I feel I have tried everything to help the issue.

Thanks
 
  #2  
Old 07-05-2017, 01:06 PM
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Default 1980 300cd Trans shift problem

I for got to mention I also replaced the small white plastic vacuum switch on the top of the valve cover. I have also replaced the vacuum switch on on top of the injection pump with used ones. I have two of them and have replaced it twice with no change.

Thanks,
Warren
 
  #3  
Old 07-05-2017, 08:35 PM
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Default 1980 300cd trans. shifting problem

Thank you for the vacuum line routing. The vacuum line routing on my car is the same as your pictures. I got the routing diagram from the factory shop manuals.

Thanks,

Warren
 
  #4  
Old 07-05-2017, 09:06 PM
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Default 1980 trans shifting problem

I spoke to soon in my last reply. I double checked the routing of the vacuum lines and your way works way better than how I had it per the factory manual and the Mercedesource diagram. Now the trans kicks down. Seems to work great. I will know better when I drive to work tomorrow.

Thank you so much for your help.

Warren
 
  #5  
Old 07-06-2017, 11:07 PM
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Default 1980 Mercedes 300cd trans shifting problems

I drove the car to work and back today. 30 miles each way. The trans shifted better when it was cold. Once it wormed up, the 2 to 3 flaring was bad, to the point if I did not lift off the accelerator, the trans would just slip. It also would not kick down.
I am thinking of trying to adjust the modulator valve pressure up and see what that does. I get the feeling the trans pump pressure is low or the valve body is worn.

Any Ideas?

Thanks

Warren
 
  #6  
Old 07-07-2017, 08:31 PM
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Default 1980 300cd trans shifting problem

Ok, I'll Try that next Monday when I have a day off.

Again Thanks for the help.

Warren
 
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