This is for the 90 tpi iroc, it might work for others I dont know. I swapped from tpi to carb. I removed 95% of the wiring that went to the computer. I removed the computer also. Then my tach or oil pressure gauge would not work effectively, the tach would not work at all. All of the info I could find pointed to the white wire from the distributor, but it was hooked up, with voltage. Finally I got the repair manual out and found tach repair info. The way the 90 camaro is wired the ground for the instrument panel goes thru the computer. Its the blk/wht wire in the 6 wire plug for the computer. I jumped it out to the car body and the tach started working and all the other instruments worked too. So the tach repair got my complete instrument panel working.

 

blk/white is just chassis gnd, tach still gets spark pulse from white tach wire off coil.

 

Thats true, but the tach wont work without it. The one I am talking about is in the connector that was plugged into the computer under the dash passenger side.