Conquering Park/Neutral Signal and Gear Selector Switch (PNP)

This thread is intended to be informational (and not a question). We've done many engine swaps and have dealt with the park/neutral signal many times.

GM used two methods of receiving a park/neutral signal to the PCM
Going back to the 80s with TPI systems, there was a single (ORANGE/BLACK) wire that was grounded to indicate to the ECM that the vehicle was in P/N. This method was carried through 2002 for the Camaro/Firebird only. The 99-02 LS1 PCM receives a ground on pin 34 of the LS1 BLUE connector to indicate P/N.

The mid 90s introduced the gear selector switch on the side of the transmission. VERY convenient for not only P/N, but also backup lamps and neutral safety. For LS PCMs, this means a combination of 4 wires to the PCM to indicate the selected gear (but not necessarly the actual gear...that would come from the fluid pressure switch through the 4L60E/4L80E connector). If you've loaded a calibration into the PCM from a vehicle that has a gear selector switch, but your transmission does not have provision for the switch (like a 700R4), tie together pins 32 & 34 of the LS1 BLUE connector and connect them to your P/N switch.

Crank Learn requires a P/N signal. Without it, you can rev the engine to the moon and never complete the learn process.

Closed Loop may have something to do with the P/N signal for automatic transmission calibrations. Still trying to make sense of this. We recently had a conversion that did not receive a P/N signal and until we connected the P/N signal it would never exit Open Loop. However, as soon as we connected the P/N signal, with no calibration changes, Closed Loop operated as was set in the calibration. I am not suggesting that the vehicle must be in P/N for closed loop, but I am saying from experience that there's something about the P/N signal (upon startup?) that is important to Closed Loop operation.

Recommendation for Conversions
If you're running an automatic calibration the LS PCM, install a gear selector switch to the transmission. They're about $50 new and very convenient. In addition to the combination of 4 wires to indicate gear selection, they also have a separate wire that can function has a single ground source to indicate P/N for calibrations expecting only a ground on pin 34 of LS1 BLUE connector.

If all else fails...
No provision for a gear selector switch on the transmission?
No P/N switch on your shifter?
No problem.
By being crafty with the neutral safety switch signal, a relay, a ground, and a diode, you can provide a P/N ground signal to the PCM. We can build this circuit for you.


Sometimes these retrofit conversions are a challenge. I hope this information comes in handy to someone dealing with the P/N signal.