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On my '67 Panel Truck with a stock tach dash & 2001 LS1 with the 411 PCM I just hooked the tach wire from the PCM to the tach output post on the back of the tach and changed the PCM resolution from 6 to 3 in the PCM tach settings. (that changes it from 8 cyl to 4 cyl) Seems to work OK.... No resistor needed and no changes to the tach.
LockDoc
Here is what lsnova said in another post:
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Depends on what year your PCM is, but generally the 2003 and newer will need the pull up resistor. Also you'll have to change the tach resolution from 6 to 3 in the tune to make it read correctly since LS motors use a 4 cylinder signal, otherwise it will read half of what it's running (on a V8 tach). Other than that, just hook up the ignition wire like it was originally and hook the signal wire, the one that went to the coil, to the tach output out of the PCM, generally a white wire at C2 10, but that may depend on year and application.
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I can't remember who posted this.
If the tach is reading approximately half of what it should be, then you don't need the resistor. If the tach is barely or not moving at all then you need to put a 1K resistor from a switched power source into the tach feed wire. This will boost the signal.
Now, back to reading half of what it should. The signal from the PCM is similar to what a 4 cylinder would produce. This is why you can switch aftermarket tachs to 4 cylinder mode and they read correctly. If the tach you have isn't switchable, you can have a tuner change the tach output in the PCM to make the tach read correctly.
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