Fun Wiring Challenge: 260z Seat belt interlock

[JAW]

260Z Seatbelt Interlock & Starter Circuit Troubleshooting

 

I’m working through a starter‑circuit issue on my early 260Z and wanted to document everything I’ve done so far to bypass the seatbelt interlock system. I’ve followed the factory service bulletin step‑by‑step, but I’m still running into problems and hoping the electrical experts here can help me figure out what I’m missing.

 

What I’ve Done So Far (Following the Nissan Service Bulletin)

Nissan

• Disconnected the seatbelt control module under the dash
Removed all three harness plugs from the module.
• Unplugged and removed the wiring distribution block on the firewall
This is the multi‑connector block tied into the interlock logic.
• Removed the red reset button on the passenger fender
Pulled both connectors and jumped the Black/Yellow (start signal) to the Yellow (starter output) per the bulletin.

Result:
Even with the entire interlock system removed and bypassed, I still hear a click under the passenger‑side dash when turning the key. Also, when trying to start, the starter solenoid spins but the starter doesn’t engage.

 

Second Approach: Installed a 4‑Prong Starter Relay

Since the jumper bypass didn’t solve it, I tried a more direct method:

• Removed the jumper at the reset switch.
• Installed a 4‑prong relay on the firewall.
• Wired it as follows:• Pin 30: Direct feed from the battery positive.
• Pin 85: Trigger wire tapped from the Black/Yellow ignition START wire just below the ignition switch plug. Clean start signal.
• Pin 87: Dedicated wire straight to the starter solenoid spade.
• Pin 86: Grounded to the firewall.

Result:
The car does crank with the direct battery feed.
But a new issue appeared:

When I turn the key back to ON after START, the starter continues to crank nonstop. The only way to stop it is to disconnect the battery cable.

I tried a relay with a resistor thinking the solenoid might be back‑feeding the relay coil, but the problem persists.

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Where I’m Stuck

I’m not an electronics expert on the 260Z, but I’ve done plenty of wiring work on 60’s cars. I’ve searched all over the web and found bits and pieces of info, but nothing that fully explains what’s happening here.

Autos& Vehicles

At this point I’ve:

• Removed the entire seatbelt interlock system
• Bypassed the START circuit per the bulletin
• Tried a modern relay setup
• Still have a mystery click under the dash
• And now have a starter backfeed issue where the relay won’t release

I’m clearly missing something in the early‑260Z wiring logic, possibly a hidden splice or secondary circuit feeding the START wire.

I’m reaching out to the experts here who have more experience with these cars. Any guidance, diagrams, or known trouble spots would be greatly appreciated. I’m stumped and would love to get this resolved.

Thanks in advance for any help.

 

[jhm]

Sounds like a sticky issue; but it also sounds like you're doing all the correct diagnostics.  I haven't had stock wiring on any of my last 3 or 4 cars; so I won't be any help there.  Replacing those old OEM relays with modern components is a major improvement, so job well-done there. 

 

One suggestion I can offer is to check your starter (and solenoid).  Really strange behavior, from your description.  Maybe the solenoid is failing, or the bendix gear, or???

 

It would be interesting to know what the problem really was, if you don't mind posting your results here once you've solved the issue(s).

Cheers, and welcome to the forum!!  👍

[Datsunpowers]

You didn't mention if an auto or manual so I will assume an auto. Here is the body electrical circuit diagram for a 260z. Take a look at the Starter Interlock System. Its the last section of the PDF. If it was me I would just jumper the start signal at the seat belt warning relay if its a auto car that way it will not start in gear.

 

BE91 is the circuit layout

BE94 is the Manual description

BE95 is the Auto description

 

Hope this helps.

BE Body electrical.pdf