SteveJ

Well, I searched more. They have 9 & 11 mm, too. https://www.authenticclassics.com/SearchResults.asp?Search=cloth+braided+type+fuel+hose

2? 4? What difference does it make?

And this place has 9 & 11 mm ID https://www.autohausaz.com/catalog?k=braided+&b=&page=1&sortby=r

Here's some, but at a much higher price. https://www.authenticclassics.com/Mercedes-Cloth-Braided-type-Fuel-Hose-8mm-I-D-p/auth-008783.htm

Many different sizes are available from Belmetric. https://belmetric.com/hose/gasoline/

I actually happened across some of your posts there when I was doing research on EDIS with Megajolt today.

I don't recall ever seeing security screws on the ignition switch module, and that includes 2 new Beck Arnley full switches that had Nissan part numbers on them. (I never knew Beck Arnley was a Nissan supplier until I bought them.) On the other hand while it's security via obscurity, it is not a trivial task to get to those screws. It would be a determined thief doing that, and at that point he's going to take your car pretty much any way he can. I have replaced the ignition switch module on a car just as I described. It helps to have a stubby screwdriver or a mini ratcheting bit set like pictured below.

Yes, the vertical is for the fuel pump, and the top of the T is for the gauge.

Yes, you can use battery negative for ground. (Also, if you look through my YouTube channel, you'll see a video where I show how to check the quality of your grounding. Bad grounding can cause some perplexing problems.) Yes, you can check the functionality of the sensor. You measure resistance to ground at the top of the T at the sending unit. Remove the connector from the sending unit. Get your baseline resistance reading between the top of the T and ground. It should show very high resistance. You will need to ground the pin on the connector that connects to the fuel pump relay so you get fuel to run the car. Have someone start the car while you continuously measure resistance to ground at the sending unit. Resistance should drop.

Yes, the oil pressure sending unit was changed for 78. In previous 280Zs, the flap in the AFM would close a switch to tell the fuel pump relay that the engine is running and kept power on the fuel pump. Nissan moves away from that Bosch fuel pump relay and used oil pressure as a signal that the engine is running. If you look at the 75-77 wiring diagrams, you see terminal numbers 36 and 39 on the AFM flap. This is how they documented the change on the 78 wiring diagram. So, yes, the oil pressure sender grounds when the engine is running on the vertical of the T. That circuit is for when the key is in ON.

You can actually test to see if the gauge is functioning properly. There is a T shaped 2 pin connector on the oil pressure sender. The top of the T is the wire that connects to your gauge. You want to measure voltage to ground on that wire with the key in ON. At first, you should see full voltage, then it should start to fluctuate. A while back @Captain Obvious gave a great description of how it worked, and I bought an analog ammeter to demonstrate it. Since voltage and current are proportional, you'll see the voltage reading start to dance after the gauge warms up. Keep in mind that the oil pressure gauge and temperature gauge use the same voltage regulator, so if you don't see the voltage start to dance when you are checking the wire at the oil pressure sender, you can do the same test at the wire for the temperature sender. If one works, and the other doesn't, my first guess would be that there could be a wiring issue or corrosion at the dash to engine harness connection. Anyway, do this test and report back, and we can figure out if more diagnostics are needed.

Not needed if you only get the back part. You can get to the screws on the back using the instructions I posted.

Could it be the module on the back of the switch? Yep. It could, and I know from helping other people that the symptoms can make it look like a bad ignition relay. The nice thing is that you can replace the back of the switch with an OEM part. The part number is 48750-E7705. You can try Courtesyparts.com or Nissanpartsdeal.com. Either site is reliable. To replace: Disconnect the negative battery cable. Remove the cover from the steering wheel. Unplug the connectors from the combo switch and ignition switch. You may want to label them for faster reassembly. Remove the 4 bolts under the steering column that holds it to the dash. Push down on the steering column so you can access the screws on the back of the ignition switch and remove the old switch. Install the new switch and reverse the disassembly process. Curse under your breath if you didn't follow my suggestion in step 3. Re-attach the negative cable and test.

The brake warning lamp is on the same fuse as the turn signals. You already established you have low voltage at the fuel gauge fuse. You probably have the same low voltage at the turn signal fuse. The voltage is enough for the light to come on faintly, but it's not enough to trigger the flashers to flash or get the gauges to operate.

Without an opportunity to poke around the car, it's hard for me to pinpoint where the stray voltage is coming from. One other test to consider doing is to remove the connector from the relay and test resistance from the W/B wire at the connector to the fuel gauge fuse. If you see 0.3 ohms again, you know the wire is intact. I've seen a lot of questionable repairs on wiring done by people with good intentions and bad knowledge. It's part of what makes distance diagnosis of electrical problems challenging. I can tell you exactly how it should work according to the factory manuals, but Bubba may have come up with a cheap fix at some time or other. Then again, it could just be a bad ignition relay with corrosion building internal resistance, dropping the voltage.

Your testing shows a potential issue with the relay. Let's verify your testing. Unplug the ignition relay. Measure the voltage across the battery terminal. (This is battery voltage.) Measure voltage to ground on each W/R wire. You should have battery voltage at each wire. I understand you had the same voltage on each wire as the battery. Put the key in ACC and measure voltage on the L/R wire. You should have battery voltage. I understand you had the same voltage here as the battery. Put the key in ON and measure voltage on the L/R wire and the B/W wire. Both should have battery voltage. I understand you had the same voltage on each wire as the battery. If all those tests pass, turn the key to OFF and plug in the relay. Put the key in ON and measure voltage at the RADIO fuse and the FUEL GAUGE fuse. Both should have battery voltage. I understand that you had voltage at the RADIO fuse but not the FUEL GAUGE fuse. Furthermore, the turn signals did not work. The fuel gauge and turn signals are powered by the same side of the relay. If you don't have voltage for the gauges at the fuse box (with a good fuse), I would expect the turn signals to be non-functional, too. I'm also betting that your reverse lights aren't coming on, either. Let's look at what all powered by the W/B wire. W/B Wire Floor Sensor Cal only Fuel Pump Not during cranking Inhibitor Sw AT only Fuse Box (IGN) G Turn Signal Brake Warn Lamp Turn Signal Power Seat Belt Warn Tmr Fuel Lvl Warn Lamp Reverse Lights L Gauges Cooling Fan Relay Fuel Gauge Water Temp Gauge Oil Temp Gauge Tachometer Speedometer Unk Func Kickdown Sw AT only Among other things if that relay is faulty, you could lose power to your fuel pump when the key is in ON. If the coil or contacts are breaking down, it would not surprise me if it was worse when the car is hot. Your engine could fire with the key in start and die shortly after when the key goes to ON due to the pump losing power. As far as connecting the relay in the wrong direction, yes it's possible but difficult. That edge should hit the piece of phenolic that is near the one row of pins on the relay. Also if the relay is attached to the relay panel, I would expect that you would have to twist the connector significantly to get it to be in the wrong orientation.

If the turn signal (flasher) fuse has voltage but the fuel gauge does not, the first question is "Did you check on both sides of the fuse?" If it has voltage on one side but not the other, I'm thinking it's a bad fuse. Otherwise, I would have to wonder if the fuse box is damaged. Here's the back side of the fuse box. The inner columns are the supply. The outer columns are the wires that go out to the loads.

If you're looking for a switched source that isn't going through a relay, that would be the B/W (On and Start) or L/R (Acc and On). Neither has terminations near the center console. You could get a set of male and female 6 pin 6.3 mm connectors from Vintage Connections and get the power from the back of the switch. I would advise fusing it as close to that connector as possible.

Clock power (as I mentioned in my first response) and cigarette lighter power. R/L & G/W - lighting (gauge lights, etc.) The T connector is probably for the HVAC panel. The bullets correspond to the indicator lamp for AT models

Also, have you taken the time to download the 78 factory service manual? It's a free download on this site. There is a link in my signature.

Test port for the BCDD. Look on page EC-6 of the 76 FSM

I would say those links are good. Did you ever do the tests I suggested earlier? They could go a long way toward figuring out your problems.

I saw that on the AC controls that I have sitting around.

Looks good.

The green fusible links are green fusible links. https://zcardepot.com/collections/electrical/products/fusible-link-wire-green-280z-1974-78?_pos=9&_sid=4a0072d4b&_ss=r Now you might need different terminals, but those are available through a myriad of sources, including Vintage Connections. That reminds me. I need to generate more content for YouTube. Wire crimping would be a good topic.