To use the tester you unplug the TFI Plug going into the Ignition Harness and coil, computer and so on.
You then plug in the tester, hook it to the + post of the battery and crank the engine and watch the LED’s.
So there will be no large voltage spikes on the TIF (TACH) LED.
I do not have the TFI/PIP tester that Ford uses (the 105-00002) but I would think it would be wired about the same as the above diagram.
Here is a Photo of that tester stolen from ECO.
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You then plug in the tester, hook it to the + post of the battery and crank the engine and watch the LED’s.
So there will be no large voltage spikes on the TIF (TACH) LED.
I do not have the TFI/PIP tester that Ford uses (the 105-00002) but I would think it would be wired about the same as the above diagram.
Here is a Photo of that tester stolen from ECO.

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