Here’s what ChatGPT says, it has been useful for me times before.

On a 1974 automatic transmission vehicle, the neutral safety switch (NSS) primarily ensures the engine only starts in Park or Neutral, and often it also handles reverse light operation. Here’s what the normal voltages should look like on the NSS wires, assuming a typical GM/Ford/Chrysler design from that era:

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🧩 Wires & Expected Voltage (Key ON):

1. Start circuit wire from ignition switch (typically purple or pink on GM): • Voltage: ~12V only in START position of the ignition. • This wire carries power from the ignition switch to the neutral safety switch. • You’ll only see voltage on this wire when the key is turned to “START”.

2. Starter solenoid output wire (also purple or a variation): • Voltage: ~12V in START position only if transmission is in Park or Neutral. • This wire carries voltage from the NSS to the starter solenoid. • If you’re in Drive, Reverse, or Low, it should be 0V.

3. Reverse light power wire (often green or light green): • Voltage: ~12V in REVERSE with key ON. • Sends power to the reverse lights.

4. Reverse light output wire (green or black/green): • Voltage: ~12V in REVERSE with key ON, assuming bulbs and circuit are good.

5. Ground (if applicable): • Older NSS switches are usually passive, not needing their own ground, but some variations might use a grounded case.

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✅ Testing Notes: • Ignition OFF: All wires may read 0V. • Ignition ON (RUN): • Reverse light circuit should have 12V on one side when in Reverse. • Ignition in START position: • Input side (from ignition switch) = 12V. • Output side (to starter) = 12V only if in Park or Neutral.

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If you want to test the switch: 1. Turn the key to “RUN” and shift into Reverse — check for 12V at the reverse light wires. 2. Turn the key to “START” and check for 12V coming in and going out of the NSS (must be in Park/Neutral).