Making the tach work using the LS-series engines

The GM LS-series engines are very popular in SL-Cs, making up over 70% of all engine selections.  Most of these builds use the GMPP engine controller kits, which provide a tach output for the factory dash gauge.

However, it isn't quite a simple plug-and-play, because the output of the engine ECU in the controller kit (and in factory ECUs from GM generally) is a low-voltage square wave.  Many tachs have a problem reading this, so a solution was developed and documented in the controller documentation.

Tachometer output with the GMPP harness

While not strictly a part of building the kit itself, many people use the popular GM LS-series engines in their SL-Cs and some have had problems making the tachometer work correctly. This section details what is needed to make the standard Koso gauge work correctly. It is also applicable to other instruments and tachs as well.

Failure to get any tach signal at all, or an inaccurate one is almost always due to one or both of two things:

1. The tach signal as delivered from the GM ECU harness is a low-voltage square wave, and typically not adequate for most tachs. The solution, as documented in the LS controller documentation, is to build a pull-up resistor circuit. This is easy to do, and, assuming the rest of the wiring is done correctly, will always make the tach operate.

See the diagram below for details.

LS Series pull-up resistor network diagram


The bulkhead connector on the GM harness is used to source the tach output. Cavity "C" usually has a white wire, and is the Tach Out wire. The diagram above shows how you wire in a resistor to a source of switched ignition power to modify the normal output into a form that is compatible with the Koso and other tachometers. The wire marked "Pull High Tach Out" in the diagram goes to the Koso tach input. You can get the appropriate resistor from your local electronics supply store, or a Radio Shack. Known working resistor values include 4.7K ohms, and 5K ohms. 1/4 or 1/2 watt ratings are fine for this purpose. For those who aren't electronics geeks, resistors aren't polarized, so you can wire it in the circuit without regard to which end of the resistor points.

2. The second problem - usually manifested by a working but inaccurate tach - is that the GM ECU sends out a signal that is two pulses per rev, a signal that is often associated with 4-cylinder engines. The solution is to tell your tach that it is monitoring a 4-cylinder engine. Use the Koso documentation to set the gauge accordingly.