Engine miss

'88 has a bad miss that also has the tach showing 1500 RPM more than actual. It is a stick. When it is firing on all 4, the tach reads normal RPM's. What would be causing these two things? Where do I find the computer terminal to read the codes? I found something that looks like it betwen the firewall and wheel well on the drivers side, but it does not look exactly like my code book shows, and it only has three wires leading into it. Thanks