Month: December 2016

Gen V LT1/LT4 VSS signal information

” The LT1/LT4 instructions states the VSS input is optional unless running a Connect and Cruise transmission, this is incorrect. The LT1/LT4 control system MUST see VSS, if it does not see VSS the ECU will limit throttle to 30%. It does this to protect the engine from over speed.”  The above information comes directly from GM Performance and Powertrain dated October 2016.

There needs to be an updated and improved I-sheet for the T56 install kit because nowhere in the I-sheet is there anything about the electrical connections that are needed or where they are located.  Now that we know that the VSS is needed, it’s critically important that installers get the right info or deal with an engine running in “limp mode”. The VSS signal connector on the end of the LT1/LT4 harness looks like this:
It’s a 2 pin clip-on connector that will only work in the Solenoid lock-out connection, not the VSS connection.  The T56 magnum VSS connection is on the passenger side, opposite the lock-out connection as shown below:
The connector shown below is generally referred to as a Ford T56 VSS connector, this should be part of the T56 install kit, but it isn’t. To use this connector you’ll have to cut off the connector that’s there and wire in the new one. Since this is a reluctor (magnetic) generated signal, either side of the connector will work.

In my previous LS installs, I did not connect the VSS signal into the ECM because I ran the signal directly into the electronic speedo.  The LS3 engines, from what I can tell are all set up for automatics rather than manual transmissions (even though I thought I ordered the manual version), so the signal the ECM is looking for is double what you get from the T56, so it didn’t work.  On the LT4 ECM, it’s looking for 40 pulses which is what the T56 produces, so for this installation it will work correctly.  Some T56 transmissions produce a 17 pulse signal.

To drive both the ECM and a digital speedometer from the same signal follow this schematic from Autometer.  Circuit 821 is the positive wire for the VSS in the GM controller wiring harness and it’s purple/white, use this wire to drive the sender signal on the speedo if you have problems with the VSS out signal from the bulkhead connector.
The other issue in the LT1/LT4 I-sheet is that signals from the bulkhead connector will drive a speedo and tach, but the oil pressure will require it’s own sender. The only way you can use the oil pressure signal from the bulkhead connector is to use a factory style gauge. There is no provision to add a water temperature sender on the LT1/LT4 engines, but you can add one in the radiator hose or the radiator itself, or you can modify your water pump as shown here. For detailed installation instructions for Autometer gauges click here.

Dakota Digital VHX gauges offer an BIM -01-02 module that takes the signals from the OBD-2 connection, rather than the bulkhead connection and converts the signals correctly to run their gauge packages. For more information see my Dakota Digital post.