If you mean will crank, but no fire, then check for spark and fuel. If you mean will not crank, then check the wiring and the starter.
Get the alternator checked before your battery goes dead.
i have an 87 topaz where the alternator goes bad also. have replaced it twice. it is just a common problem in the topaz's
The battery light is for low alternator output voltage. Try testing the alternator output using a miltimeter and if its less than 13 volts your alternator isn't putting out what it should. If it has low output check the belt tension and all wire connections before replacing alternator
Battery + cable goes to one of the large posts Starter cable goes to the other Large post Trigger wire goes to the small post (if there are 2 disregard 2nd)
THE FUEL FILTER IS UNDER THE HOOD ON THE WHEEL WELL, LEFT OF THE ALTERNATOR.
Mercury Topaz was created in 1983.
The starter solenoid is on the left side fenderwell next to the battery.
the drain nipple is on the bottom of the radiator on battery`s side.
Try looking in a Ford Tempo or Mercury Topaz.
the belt is worn and loose change the belt and tension properly
Clean them and see.
alternator-,ac pump-,steering pump-,crank pully-, idler pulley-,water pump