have you checked the fusable links?

 

Your voltage check on the injector plugs may be flawed. Here is how this works. When you turn your key to run or start the EFI relay (located in the fender behind the battery) supplies B+ (battery positive) power to the injectors. The ECU grounds the injectors to make them fire. To do a voltage check you need to unplug all 6 injectors. Put your black probe on a ground or on the neg battery post. The red probe will then be inserted into each side of each injector connector. You should have B+ on ONE side of each connector. If you leave all the injectors connected and pull one connector at a time and check for voltage you will have B+ at BOTH sides of the connector. This is normal. it sounds like you inserted the black probe into one side of the connector and the red into the other side of the connector? Yes? IF so then your initial readings are flawed but your second (black probe on the battery) are correct.

The ECU is just a on off switch. It determines how long to keep the lights on (injectors). When the switch is off there is no ground to complete the circuit so you will show battery voltage all the way to the 35 pin connector. The FSM instructions tell you to test for B+ at the 35 pin ECU connector because then not only are you testing that the injectors are getting power, you are also testing the power wire to the injectors and the wire from the injectors to the ECU (the ground path).
All of the test are done from the 35 pin connector FIRST. If the reading are not within factory specs then the individual item being tested is to be isolated and tested.

The important grounds for the ECU are the ring terminals on the intake manifold. One of those ring terminals is actually up to 5 grounds for the 5 items on the engine. The wires are soldered together inside the wire harness. There is also a ground wire on the injector blower mount bracket that is important.

So that one wire that was chewed thru that came off the B+ terminal probably goes to the fusible link box. Like Bon said, check the links. You may have to remove the link box to check the integrity of the wires under it.

Please please please keep in mind that reverse polarity for even a 10th of a second will kill your ECU. Dont mix up your battery terminal wires and dont accidently ground the positive while disconnecting the battery. ALWAYS disconnect the negative battery post FIRST. AND always reconnect the positive cable FIRST when hooking your battery back up. This is important!

AND always disconnect the battery before disconnecting the ECU. This is also important.

If you think you have a bad ECU and you dont have another known good running ZX available to test it, I do ECU testing for forum members.

The only way to test your ECU is to put it in a compatible know good running car. I have a 1981 and a 1983 so I can test all years regardless of FED or Cali, Auto or manual. You pay shipping both ways.
Here is a link to how I do this. I post the videos on YouTube for review.
ECU TESTING

 

hey thanks for writing all that out, it does make a lot more sense now. Also recently replaced the fuseable links and I didn’t think to check them again because I had left the battery disconnected (from both terminals) while I wasn’t driving it.

also, I should clarify that the voltages are tested at both the injector site as well as the 35 pin connector, and don’t suggest a bad connection between the 2. I’ll re-check the voltages keeping what you said in mind though.

Regarding switching the terminals, I am very confident that this is not the case, as my + terminal wire does not easily reach to the B- terminal, even if the battery is placed in the car backward. I hear a lot about people somehow doing this on 280 forums, and cannot fathom why this is so common for people to do.
Heck, even PO did this to the car before he sold it to me (hence why fuseable links + ECU are “new”)

My starter also just quit when I went to test if the voltage would drop while cranking, and so I am wondering if there is just more rat damage for me to discover, or it’s also possible that I just need to replace the starter too. Either way I got a lot to do. 🥲

 

You can send me a message to text me if you are interested in getting the ECU(s) tested.
seven zero one - 4 7 1 - fifteen thirtyfour.
chaug@bis.midco.net

 

Regarding testing the ECU I’ll keep u in mind- there’s another guy in my area with a NA 280zx that I wanna check with first. I’m not sure what year his is since he has aftermarket exterior B-pillar things, but next time I run into him I’m gonna ask. I’ll let you know though if I do need to get it tested elsewhere though

 

Keep in mind the compatible years..1979 to 1981 and 1982 to 1983. What Nissan did in 1982 is they beefed up the ECU's so there was no need for dropping resistors. You will not have them in your 1983. And the fuel pump is run by the ECU. That means that you only have one fuel pump relay instead of 3!