Tach wiring question

[mcstein]

88 ranger 2.9l 5sp

I just fixed a no spark problem by replacing the wire that goes from the tfi-iv module to the coil. It was the dark green/yellow. I did a lazy mans fix and just strung the new wire around the cabin instead of through the bundle of wires.

As a result the truck starts and runs but I lost my tach. I'm reading conflicting articles that the tach was either wired to the coil or to the computer. I believe mine is supposed to be wired to the computer. my haynes onky shows 1 green/yellow wire going to my coil from tfi. It also shows a wire going from yhe eca to inst cluster.

So why would cutting out that green/yellow wire lose the tach. Is there supposed to be another wire that splices into the tfi to coil wire for the tach to get signal from? I was figuring my tach got its signal from thr computer(eca).


Here's a copy of the haynes diagram im using. I copy and pasted from the internet.

 

[RonD]

On engines with single coils all tachometers get their signal from the "-" terminal on the coil.
This terminal is Grounded and unGrounded by the TFI module, that's how spark system works, each time coil is unGrounded it sparks, those "pulses" are divided by 6(V6 engine) and that equals 2 RPM(not 1).

Computer also gets that signal, but in '89 your tach wire will be spliced to coil "-" wire

Diagram here: http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Ignitionsystem_1989_2_9.JPG
Bottom of diagram.

When engines went distributorless, they got a CKP(crank position) sensor, in these systems the tach is often wired to computer