Auto Trans won't shift under WOT

[Travis71280]

Hey everybody, I was just wondering if this is normal for an automatic transmission in a 1971 280sl. The car seems to shift well, but upon hard acceleration the transmission will not shift from 3rd to 4th gear without letting off the gas a little, sometimes reaching 8000rpm without shifting . I'm not sure where to start at since its shifts fine in all the other gears. Is it the kick-down switch, low transmission fluid level, or something inside the valve body of the transmission? Any help is appreciated.

[hands_aus]

My understanding is that these transmissions need the power to drop off for a blink before they change gear.
I think it is around the modulator vacuum pressure.
Similarly they don't change down a gear unless conditions are right eg low rpms, low vacuum
My 250sl hangs on and will pull away from very low speed.
If I kick down, it will pick up speed more quickly.


BTW it is helpful if you have your location on your profile.

[graphic66]

You might want to keep it under 6500 RPM [redline] until you figure out what is wrong. It should most definitly shift way before 8000 RPM. One possible cause is contamination. When was the last fluid/ filter change? Shifting while the throttle is full should take place at 62MPH from third to 4th gear.

[ja17]

Hello,

In 4th gear, the transmission will shift at redline with the accelerator pedan fully depressed and the kickdown switch engaged.

[Travis71280]

Thanks everybody, I meant 6000, don't know why I was thinking 8000 . All I know that pointer was real close to the read line so I knew I had to get out of the gas. Its probably a little of everything. The whole car has set for 15 years in a pole barn and I dont know what happens to tranny fluid when it sets that long (I learned shortly what gas and antifreeze does ) so the transmission might still be contaminated. Also I think the kickdown under the gas pedal froze up since the car doesnt want to downshift on hard acceleration. Think that may be it? I did however change the transmission fluid and filter a while ago, but have been fighting a transmission fluid leak where the dipstick comes into the pan. Probably just need to change that gasket but don't have time to work on it now due to summer classes. Well at least I have some direction to work in, going to try to top off the transmission fluid first though and see what happens, as my grandfather used to say "start with the inexpensive stuff and go from there." Thanks again.

[graphic66]

Around 6000RPM is fine, the Mercedes Transmission service manual claims it should shift at 62MPH, kind of strange that they would specify a speed as cars with different axle ratios would be at different speeds at full RPM. That is where I got the 62MPH info from and I tested my car and it shifts promptly at 62MPH with a 3:92 Diff gear.
  Your kick down not working very well could be the switch under the pedal. Until you fix it you can shift down to third at any time under 62 MPH safely I would assume for manual kickdown. I also now like that I know I can downshift without needing to floor the accellerator if I use the engine for braking or just want to downshift. Not a great idea, brakes are cheaper than trannies, but it can stop faster in a panic stop possibly. Not sure of the facts here but it seems like you would stop quicker than just with the brakes alone.
   I also always heard you should downshift a standard tranny car to save the brakes until the resident genious [may he rest in peace] over on the Unimog site explained that wearing your expensive tranny to save some cheap brakes doesn't really make sense. On a long downhill it can help with not overheating brakes but mostly now I just put a manual tranny in nuetral, stop and shift to first when taking off. Everything seems very happy that way.

[Travis71280]

Thanks graphic66, that just saved me some headache. That does make sense now since it finally shifted just above 60. I have always been a believer of downshifting to save brakes but i dont get too crazy with it in an automatic especially with how expensive these MB transmissions are. I'm almost positive that downshifting causes you to brake quicker but ive also found it also helps you to be in the right gear when you get back into the gas when your not comin to a complete stop.