Here is an excerpt from a Viper install guide for post install testing of the Neutral Safety
circuit. While this test is specifically for use after install, it does show the intended
purpose.
3. Test the NEUTRAL SAFETY shutdown circuit.Important! Make sure there is adequate clearance to thefront and rear of the vehicle before attempting this test. 1. Make sure the hood is closed and no other shutdown circuits are active. 2. Set the emergency brake. 3. Turn the ignition key to the run position but do not start the engine. 4. Put the vehicle in Drive (D). 5. Put your foot over the brake pedal but do not press down on it. Be ready to step on the brake to shutdown the remote start system. 6. Activate the remote start system.
• If the starter engages, immediately step on the brake to shut down thesystem. If it does engage, recheck the neutral safety input connection. Thevehicle may use a mechanical neutral safety switch. (See H3/1 BLACK/ WHITE neutral safety switch input in Remote Start Harness Wire ConnectionGuide section of this guide.)
• If the starter does not engage, the test is complete. Once the system passesthe tests, the vehicle can be re-assembled and delivered. Do not the usethe remote start system or finalize the installation if it fails any of the safetycheck tests.
Basically, for a R/S install you want to verify that the car will not try to start the
engine if it is not in Park. Park is where you expect the transmission shifter is left when
the user exited the vehicle. Most newer cars have this safety feature buit-in. You can
easily test for it by trying to start the car in gear with the key. The car should also start
in the Neutral position but in Neutral, the key can't be removed from the ignition switch
and the assumption is that no one would leave a car in Neutral with the key still in the
ignition switch.
Some cars actually have a wire that shows a signal when the transmission is in Park
or Neutral, others don't. Sometimes this signal is a (+) and sometimes it is a (-). In
some vehicles it is just an open signal when in Park and Neutral and a (+) or (-) when in a
gear ( forward or reverse ).
Anyway, if your Prism tests properly and won't crank the engine when in gear, the
easiest route is to accept the Factory's built-in Neutral Safety circuit and connect
the R/S's Neutral Safety wire to Chassis Ground. Other options are to connect the
NSS wire to the E-Brake signal wire. This will force the vehicle operator to set / engage
the Parking Brake when exiting the vehicle to allow a remote start-up.
Connecting the NSS wire to Chassis Ground is perfectly acceptable if the above test
passes. Here is the NSS wire description from a Viper Install Guide :
PIN1 BLACK/ WHITE (-) NEUTRAL SAFETY SWITCH INPUTConnect this wire to a ground source if installing this unit in an automatic transmissionvehicle. If this unit is being installed in a manual transmission vehicle thenconnect it to the emergency brake wire. This input MUST rest at ground in orderfor the remote start system to operate.Important! Always perform the steps outlined in Safety Check section to verifythat the vehicle cannot be started in ANY drive gear and that the override switchis functioning properly.
Soldering is fun!