What is the Neutral Position Switch?
The Neutral Position Switch or NPS is a switch on the transmission that goes to the Engine Control Unit (ECU). The NPS tells the ECU whether the car is in gear or in neutral. This is important because the ECU changes its mapping based on whether or not the car is in neutral or in gear.
NOTE: This is a separate switch from the starter interrupt which is pulled from the clutch switch and in most models has no connection to the ECU.
NPS and a Transmission Swap
When doing a transmission swap there are a lot of things to consider (DCCD, Auto to Manual Conversion, etc) but don't forget to make sure that the NPS matches the ECU in the car. The NPS default position changes based on the year of the car.
NPS defaults to open
2005-2006 Legacy GT/Outback XT
1998-2005 Forester (includes XT)
2002-2006 Baja (includes XT)
Part numbers for open switches
NPS defaults to closed
2007+ Legacy GT/Outback XT
Part numbers for closed switches
*We believe this change is due to remote start and push button start features beginning to be integrated into Subaru models that require a ground input when in neutral for the system to allow the car to start.
What is happening in practical terms?
For example, the car is a stock 2007 WRX and a 2004 STi transmission is installed. If the NPS is not matched, the ECU will think the car is in neutral when it is in gear and when it is in gear the ECU will think it's in neutral. With this condition, the ECU will not activate the AVCS and the cruise will no longer work. Once the NPS is changed out to match the 2007 ECU, then the ECU will recognize when the car is in and out of gear correctly and the AVCS and cruise will work again.
How do I change out the NPS on my transmission?
To change the NPS type, take the NPS from the old transmission (or one from a transmission that matches your ECU) and bolt it onto the new transmission. All NPSs have the same thread type so it's an easy swap mechanically, however, the NPS connectors will likely not match. If the connectors don't match, cut off the plug from the new transmission and wire it to the old NPS so it will match the new transmission sub-harness. It's a two-wire plug and it doesn't matter which order the wires are in.
IMPORTANT NOTE: Make sure to put the washer of the NPS back in when you switch them out. If you forget this step, the transmission will not go into gear. How do we know this? We plead the 5th. ;D