Waddams, I looked at wiring diagram for the starter and if your problem is that the starter won't crank over the engine, I don't see the problem being in the solenoid wiring inside the trans.
Am wondering about that myself. I've got the FSM's for the truck. I put a snip from the FSM power circuit diagram showing everything connected to that circuit. I think the "AT-MAIN" is what the mechanic diagnosed is shorting. That component, from what I can tell in the auto transmission and starter/charging system FSM also has the Transmission Control Module. Said TCM used to be called the Automatic Transmission Control Unit, or AT-CONT for short. Circuit 12 is labeled "AT-CONT" on the nameplate I pull off to get the J/B to replace the fuse that blew.
On several other Nissan forums, there are a bunch of stories of guys that the "AT-CONT" fuse keep blowing, and other issues, such as the relay that burned out on me before. Many times, they traced it back to a bad TCM shorting and pulling too many amps, blowing the fuse and/or burning out some element in the relay.
The fix was a trip to the dealer to replace the TCM and have the new one reprogrammed as it's a proprietary thing only Nissan can do.
Before you go the transmission route I suggest you find a qualified mechanic to pin point the fault instead of jumping to conclusions.
I understand the sentiment - the one that gave me this diagnosis performed an "advanced" electrical diagnosis test. Exact comments on the report are "found short in the transmission control harness located in the valve body of the transmission". I'm pretty sure the TCM is also wired in through the same wiring (but I'm not 100% certain).
I do appreciate that this mechanic basically told me that his fix was replace the transmission, but he thought I might be able to find someone that could fix it cheaper, so rather than try to hit me for that cost, he just replaced the fuse that was blown and advised to go check around to see who else I could find that might be able to fix it cheaper than full replacement. I thought the guy was being honest and reputable.
I do know that anti-theft security systems are a pain on older vehicles. I wish my 2001 4Runner didn't have one.
I don't believe it's equipped with a security or anti-theft system. The FSM's show where the relays, switches, dash lights, etc. should be for models that have it. In mine, the fuse box relay spaces are empty, the door switches are not there, the dash light spaces are just plastic covers with no lights.
If your truck has a security system there is a 10 amp fuse for the security relay that provides the ground for the park/neutral relay that provides power to that small wire on starter, if you do not have security then the park/neutral relay has a direct ground and the relay provides power to the small starter wire from 40 amp fuse for ignition switch. The neutral safety switch is powered from a 7.5 fuse. I don't mean to discredit the mechanic that diagnosed the problem but I don't see any connection between starter circuit and transmission solenoids. If you have a short blowing a fuse, then you know what circuit the problem is on and with some patience should be able to pin point the fault.
No security system per above. I agree with your point about the no connection above. If there is a connection, I think it's likely in the TCM going bad, through the AT-MAIN that runs off circuit 12 per the snip included.
As for the neutral safety system - mechanic said his tech checked the whole thing and did not find a short or any faults in the neutral safety circuits/switches/system.
There's a ton more diagrams in the FSM's for the starter/charging, transmission, and engine control system that all show portions. I haven't seen a diagram that puts it all together, and while I'm able to read electrical diagrams with a little knowledge, my skill level isn't up to this at the moment.
Given the N+P interlock relay blew, then fuse 12, both are tied to the NS system, but that system is also apparently connected to the TCM from what I can see, and yesterday's report, it seems most likely that the TCM is going bad, shorting, overdrawing, and the overdraw damaged the relay, then blew the fuse. That's my best guess at the moment.
I went ahead and snipped the AT-MAIN wiring diagram, showing the TCM connected through the AT-MAIN, which is what I think the tech yesterday called the transmission control harness. Also snipped the diagram from the AT FSM showing the PNP switch feeding to the TCM, via circuit 12, so I know the TCM is connected on circuit 12 to the neutral safety system.
I'm thinking my best bet might be a dealer who can replace and reprogram a new TCM, assuming dealer can confirm the diagnosis.