The sensor for the tach doesn`t connect into the transmission. It is attached to the adaptor plate on the drivers side. You might be able to see it looking down behind the oil filter housing. It held in with one small hex bolt.
It some how reads the rotation of the flywheel.
I think there are 2 connectors on the firewall. one for the speed sinsor, the other plug is for the Rack Position Sensor, the box on the rear of the Injection Pump which has nothing to do with the Tach. both connect into the black box behind the passenger foot area side panel.
Charlie