I was fixing the engine on 92 mercury topaz now the gears only shift when hot.

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: '00 Mercury Cougar Wont get out of Park. gear shifter is stuck

sounds like your brake/shift interlock solenoid is not releasing.
Your owner manual will tell you how to bypass in emergency.
Check your fuse first. fuse panel on instrument panel.

Please rate as fixed if this helps.

Fuse 23 - 1 5 amp
Fuse 34 - 7 1/2 amp

check enigne light not related.
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The selector lever lock can be moved mechanically out of position ”P" when for example the battery is disconnected or completely discharged.
Using a ball point pen or similar item as an unlocking lever, push forward and hold in position.

Move the selector lever out of position "P".

NOTE: If the selector lever is moved back to ”P" it is re-locked.

Selector Lever Lock

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: Mercury Topaz serpentine belt diagram

I have a 93 topaz, the diagram label is under
the hood itself.