there may be more than one sensor that will make the alarm go off.
first locate the sensor for temp reading to guage, on your thermostat housing you will have to two sensors, one on each side, One with the Tan wire should be the temp sender to the dash gauge, the other should be the over temp switch which sends the alarm when over temp is achieved.
There is also a low oil pressure sensor (switch) and a low gear oil sensor (switch) in resevoir on top of motor,
Any one of the three could cause the alarm.
The over temp switch wire if removed and touched to ground (with the key in the on position) will make the alarm go off.
The oil resevoir's low oil sensor will go off if the bottle is held upside down. If that is suspect unplug the wires going to it. the alarm should shut off.
The low oil pressure switch/sensor works the same as the over temp and I beleive it is located down by the oil filter if not it would be at the back of the motor behind the distributor..........not 100% sure
If i am not mistaken the motor has to be running when testing these on the motor. Otherwise when the key is simply turned to the on positon the alarm will go off as it normally does......as no oil pressure is present.