Why won't an LS2 tach work in an E36 even after PCM reprogramming?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• LS Control System kit: The tach signal is a low-voltage square wave, and some tachometers or transmission controllers may need a pull-up resistor to read it. (Chevrolet Performance)
• LS Control System kit: GM suggests a pull-up resistor similar to 5,000 ohms, 1/4 watt. (Chevrolet Performance)
• Sources differ: Conflicts in value with PSI's 1k-ohm tach-circuit fix and MotorTrend's 5,000/10,000-ohm guidance; treat as device/harness-specific.
• The SGI-8 can recalibrate an ignition system tachometer signal. (Dakota Digital)
• On the SGI-8, switches 1 and 2 should always be on, and switches 3-10 adjust the calibration. (Dakota Digital)
• The HI VLT output is a higher-voltage square-wave output for some OEM tachometers. (Dakota Digital)
• OUT2 and OUT5 are open-collector outputs that require a reference pull-up voltage. (Dakota Digital)
• Engine cylinder count and tachometer cylinder count can each be set from 1 to 16 on the SGI-100BT. (Dakota Digital)
• LS controllers output a 5-volt tach signal while most aftermarket tachometers expect 12 volts, so a resistor must be wired into the tach signal wire. (Performance Systems Integration)
• Sporadic or incorrect tachometer readings call for a 1k-ohm resistor in the tach circuit. (Performance Systems Integration)
• Sources differ: Conflicts in value with GM's 5,000-ohm pull-up note and MotorTrend's 5,000/10,000-ohm guidance; harness/device-specific.

Confirm against the instructions for your exact parts before wiring. More: Tach and speedometer signals in our tuning guide.