why did you use the obd2 harness, you only had to remove the crank angle sensor from the obd2 motor and an intake manifold sensor from the obd2 intake manifold(or just not even plug it in, or you could have swapped obd1 manifold onto the motor) for the obd1 harness to mount to everything, but i guess it doesnt really matter if your saying everything is working, but i used my 94 harness and just removed those two sensors, alot easier and no complications to deal with, with using an obd2 harness, you said everything else is working fine so i dont see how the tach would be affected unless the plugs have different resistances and output from having the extra 2 sensors but the face that its an obd2 harness in and obd1 car just doesnt seem like you should be using it, just double check, and if it still doesnt work, swap the obd1 harness back into the car, thats what i would do anyway........