Hello All, New to the forum... My 1989 420SEL is having a strange trans issue. Driven softly shifts ok, if you drive it a bit hard, sometimes it won't upshift. Almost seems like the tranny is confused if that makes sense? If you stand on it and hit the kickdown, it will slip out of gear totally on the upshift. Putting the car back into park, then back to Drive it will shift ok. Backstory this did suddenly start a while back after being in a passing situation and hitting the kickdown, previous to that the transmission has always shifted perfectly. Any suggestions would be greatly appreciated... I am aware that it could be an issue with the actual kickdown switch itself on the floor... but any other things I should be aware of to check/diagnose?

Thanks so much!!
RG

Is the control pressurevcable attached and adjusted correctly?

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Prost!

Hmmm.. I did consider it could be a vacuum issue.. do you happen to know exactly where that is located? Thanks for the reply

Check the vacuum line and modulator as well, but the CP cable goes from the throttle linage to the tranny, and should be adjusted just to the point of no slack. It controls the shift points in combination with throttle position.

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Prost!

I remember my '86 had a flare in one of the upshifts, and it went away completely when I changed the fluid and filter. That was in 2004, and other than leaks, no more problems. Sometimes the simple things do help.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe

Thanks so much for the suggestions guys... Will update after checking vacuum, linkage, and a good service... !