random downshifts on highway - PeachParts Mercedes-Benz Forum

angst · #1 06-29-2007, 06:28 PM

84300sd will kick down into third while at highway speed. I can coax it in to fourth by going into neutral a moment and then back into D. It will stay in 4th if I am real steady on the pedal but will downshift into third with little provication.
It seems to be getting progessivly less responsive to my neutral trick and is also now doing it enough to be near undrivable.
Fluid level seems fine, tried a couple different bowden cable adjustments. Where do I start?

angst · #2 06-30-2007, 02:57 PM

anyone?
Some more symptoms. It seems that it is light throttle that causes it. If I have to back off the pedal for traffic that is when it usually happens. It also seems to happen more after the car is warmed up and not as much when its cold.
The only thing I can think of at the moment to figure out what is going on is to perhaps run a vacuum gauge through the firewall and see if there is some abrupt change in vacuum that coincides with it downshifting while im driving.
Can anyone offer any ideas?

Knightrider966 · #3 06-30-2007, 03:14 PM

I would drop my trans oil pan and check the filter and change it first. Higher gears require a higher hydraulic pressure to make them operate correctly and something is interfering with this. Your drooping back into drive while on the highway is temporoarily making the rear servo build the hydraulic pressure because the driveshaft is making the rear servo take over by turning it while rolling down the road. It sounds like all this is affecting the 3 to 4 shift power piston in the valve body and something is making this somewhat sticky. After changing your fluid and filter, add about a quart of Lucas Tansmission fix to your hydraulic tranny fluid. This will help binding parts loosen up and also help your tranny to run a little cooler.