These starting safety switches or neutral safety switch whichever you want to call it are more commonly used with automatic transmissions but in this case of the 89 Z it was a manual transmission vehicle.
I am familiar with the Nissan ZX as I owned a 1988 model. In fact I have written quite a bit about my Nissan 300ZX problems. And although I had a lot of problems with my little sports car this was not one of them. I had to find the component location diagram and verify the color of the wires to properly by pass and test the switch.
Sometimes the location of the neutral safety switch is between the relay and ground. Same as in the first position the safety switch must be closed before current can flow from the relay to a grounding location. When the relay has a ground it lets current flow to the starter solenoid and therefore engages the starter into the teeth of the flywheel.
The starter safety switch can also be used with an automatic transmission and can be electrical or a mechanical device. Sometimes these switches are mounted near a shift selector or on the transmission housing near the linkage. Often the switch contacts are wired in series with the control circuit so that no current can flow throughout the starting system unless the transmission is in neutral or park.
Although I have never seen them I hear on some Asian and German vehicles mechanical safety switches can also be used. Mechanical safety switches for automatic transmissions are simply devices that physically block the movement of the ignition key when the transmission is in gear. The ignition key can only be moved if the shift selector isn't park or neutral.
I used my automotive meter to check for continuity and the test passed so I moved on to the starter safety switch mounted on the transmission. When I pulled the wires off the starter safety switch and verify that the vehicle parking brake was set in the wheels blocked and further verified that the vehicle was in the neutral position for gear selection. I jumped the safety switch for testing only and was able to start the vehicle.
At this point I knew that the neutral safety switch was bad but when I pulled it out physically unscrewed it from the transmission I could see that the mechanical part of the switch was damaged. For some reason the owner of the vehicle decided to go to the junkyard and purchased a used part.
I urged him to find out if a new one was available aftermarket through a parts supplier. On Amazon a new safety switch for old Nisan's is about 10 bucks. Note that on some models the replacement parts are a lot more. For more of the latest articles this next link take you back to the blog for auto repair. There you can browse through older stories related to car repairs.