Can you tell me what wires you have coming off your modified tach?
You will need 3 connections to hook up a tach. A power wire that gets 12 volts when the key is on, a ground and a wire to the coils positive terminal.
However without knowing what the wires on the back of your tach are I cant tell you exactly what to hook up where. Especially since I thik you are missing a wire
In the original wiring harness you would have had red for power, black for ground and green for the coil.
If you can send me a picture of the back of the tach I should be able to tell you what goes where
Im sorry, I thought you were still talking about the cluster wiring.
The larger guage wires are the coil input wires and the smaller guage is the output to the distributor. You should want to deal with the smaller guage white wire to drive the tach
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Before I start getting confused with wire location and color...how did you choose what color wires to put where?
Unfortunately your picture wasnt very clear for me to get a good look. Before you do anything let me work on clearing up the pictures so I can zoom in. You mentioned before that the board was labeled. What was the labeling?
Also, take an ohm meter and check the resistance through all the different wire combinations and tell me what the readings are.
When you hook up the bat+ lead make sure that its only has power when the key is on, not constant power.
Also when you hooked up all the wires did you try to run the truck? Or did you do it with just the key on?
Thats very odd. Does the needle peg when there your brown wire is not hooked up at all, so it is getting no signal from the coil?
I have seen tachs peg but its always been when the signal from the coil was the wrong voltage or when it was hooked up to the wrong coil wire....never had it happen when the coil wasnt hooked up at all.
Try hooking up just the positive battery and ground with the key off and on and tell me what happens