Transmission knocks at 20-30 mph and 40 mph, like it can't shift.

SOURCE: 1989 Mercury Topaz Transmission problem !!!

overdrive should be activated by a solenoid ..internal and perhaps an external one also.. in 89 the computer was primitive and i dont think controlled shift points..also if engine health is bad or missing or vacuum to any cacuum modulator at trans is damaged or compromised.. these issues could occur...good luck

SOURCE: will not accelerate past 40 mph

simple tune up could do it, or a bad MAF or MAP sensor could give similar problems

SOURCE: Transmission hesitates shifting into second and perhaps third gear when driving slow. When acceleration is positive transmission shifts smoothly. Reference "99 Mercury Transmission" as Subject when

Several things could be causing your problem:

1. Auto transmission fluid is low
2. Auto transmission fluid needs changing; this is an area that gets neglected by a lot of people;check the fluid and if its brown or gives off a burning smell, it needs changing. Very old transmission fluid will cause slipping in the transmission; if this is the case, it should be changed along with the filter

SOURCE: Transmission. I hit the gas and the car goes nowhere.

Possible tranny problem. If your linkage from shifter to tranny is adjusted properly and your shifter gear needle is right on the PRND21 then it is likely your transmision

SOURCE: 1997 Sable SW runs great but Check Engine Light

Maybe bad sensor, find and replace.