I love my 560SEL. I just replaced the original 250K mile transmission with a used unit with 160K miles. It was fine at first, but now it shifts perfectly when cool, but loses gears when it warms up after about 20 miles highway driving.

I believe that this has something to do with heat, indicating a cooling system problem. My mechanic believes its a pressure issue with warmer fluid with lower viscosity causing the problem.

Can either of these issues be corrected wiutho9ut replacing the unit? Transmission shops seem reluctant to repair transmissions, pushing you to replace, which I'd rather not due twice in a year.

Any feedback would be appreciated!

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1991 560SEL (254K miles)
1980 Lincoln Mark VI - Givenchy
1979 Lincoln Mark V Collector's Series
1979 Lincoln Town Car Collector's Series
1979 Ford "Effie" Pickup (in gradual transformation to a show/street truck)
1969 Lincoln Mark III (waiting for "Overhaulin" to call)
1989 Lincoln Towncar
2001 Lexus GS300 (spousal unit's daily driver)