Looking from under the car, there is a check plug on the transmission. Have to remove the plug, the oil should just drip out, if not, add to the level of the plug only.
For automatic trans, should be good with Dexron 3, but just in case check for type F. The dexron works in the standard also.
If it is the same engine, both either standard or auto, might have to check if the 1990 had over drive, cause the 88 or most of them did not.
Dexron 3 will do just fine.
On my 1993 Topaz, the transmission fluid dipstick is on the right side of the engine, near the front, just to the left of the battery. It looks a lot like the oil dipstick, but, when you pull it out, the writing on the dipstick clearly mentions that the fluid level should be checked when the engine is hot and idling, and that a certain type of Mercury automatic transmission fluid should be used to refill.
I assume that you have an Automatic., 2.3L. As you open the hood, on your right side, look down between the rad and rad hose, its down about 12 inches. Close to the rad. Should be there. If you have a standard, then you do not have one. When the car is up on jacks, there is a check the oil level plug, once the plug is out, the fluid should just drip out very slightly. Check you manual for correct fluid type and levels. Dexron III or better is good.
Low on fluid, dirty filter, worn clutches.
My transmission was low on fluid. My gears would slip, I'd be driving and then I'd push on the gas and I wouldn't go any direction. I was checking my transmission fluid while the car was off and it said there was enough fluid in it. I started checking my fluid while it was running and found out I was out of transmission fluid. Once I put fluid in it, my problem went away and has been running good since.
Check the master cylinder first.
and the question is?
Have to remove the transmission for that.
The switching is done within the transmission by internal pressure. YOu may just be low on fluid. I have had this happen to my 93 topaz three times (plus a fourth that happened before I got the car). Every time it happened the fluid level was full. The tranny used (ATX) on the topaz has a governor on it. If the governor is not set right it will not shift. I have had to have the governor gear replaced Dec. 2005 because of this. Once you have one problem with the transmission you may have problems the rest of the car's life. I am now on my 4th transmission.
Check the fuses, if fuses are okay, it is probably the VSS (Vehicle Speed Sensor) or wiring to it at fault.