The problem turned out to be corroded terminals on the Neutral Inhibitor Switch (mounted on the front of the transmission). Here are a lot of notes, necesary because the Haynes Repair Manual for Nissan Altima 1993 thru 2006 was not correct for my 2001 model. In particular, it did not show the presence or the wiring for the Neutral Inhibitor relay. This relay was not defective, but examining the wiring to it helped trace the circuitry to the Neutral Inhibor Switch.

When the key switch is turned to Start, the Black/Yellow big wire is energized because it is connected to the White/Red 12V wire. Other circuitry causes the Brown wire to provide 12V to the Inhibit Relay at pin 1. If pin 2 is grounded by the Neutral Inhibit Switch on the transmission, then the Inhibit Relay will energize, connecting the Black/Yellow big wire to be connected to the Black/White big wire, which connects to the starter solenoid coil, energizing it.

Connecting 12V directly to the Black/White big wire at pin 1 on the Inhibit Relay will fire the starter.

The root cause of my problem which was causing the starter to not turn over when turning the key switch, was that pins 2 and 3 at the Neutral Inhibit Switch were corroded/dirty. Gently filing those restored operation.

8/18/2024

The Haynes book shows a diagram on page 7B-4, photos 4.2a with two connectors which supposedly go to the Neutral Inhibitor Switch. This appears to be for something else. Changing the transmission gear did not change the pins which had continuity. Leaving the connectors separated didn't affect the wiring from the Inhibitor Relay to the Inhibitor Switch, so assuming the Haynes book to be in error.

Shows which pins have continuity. Changing the transmission gear didn't matter.

Showing wire color coding

3 pin connector, also useless information. Didn't change with gear shifting.

These fuses and relays were working fine, but disassembled to get wire color coding.

Not very helpful information.

This is mounted to the front of the transmission. It is available under the battery box.

Wire coloring

Showing continuity for different transmission gear positions. This connector is on the neutral Inhibitor Switch, on the transmission, and accessible under the battery box.

The problem for me was that pins 2, 3 were corroded. Filed them, and normal operation restored.

This shows the 2001 Nissan Altima's relays. This is located on the passenger side firewall over the front passenger wheel.

This is a wiring diagram of the Inhibitor relay with color coded wires. Note that grounding pin 2 (Green/Orange) will cause the Neutral Safety switch to be bypassed, which might be helpful if there is urgent requirement to restore operation.

This connector goes directly to the key switch. Different pins are connected depending on key position.

Connector above plugs into this female plug. Note change of colors: