SteveJ

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.

And to add on to @Yarb's post, fusible links are designed to burn up internally and leave the insulation intact. Touch the leads together on your meter. The reading may not be quite zero, and that's the reading you should get when you measure across the links.

Keep in mind that the tests I listed in my long post will work with the car not running. Do those first. Also, post pictures of your fusible links, including clear photos of the bases that hold the links.

I actually keep one of those handy for when I go over to help someone with his car.

I like Auxito LEDs from Amazon. Those are polarity independent. I have never had a problem with them, and neither has anyone else I have recommended them to. https://www.amazon.com/gp/product/B07TQLK6SH

First let's go over the ignition relay and what it does. That way we can trace down the problems. Note: The copy of the 78 diagram I have still shows the relay drawn incorrectly. I think I made the proper adjustments. Color Code L - Blue B - Black W - White Y - Yellow R - Red G - Green When there are two letters, the first is the main wire color, and the second is the stripe. B/W is black with white stripe. So there are two coils in the ignition relay. One coil is energized by the key in ACC or ON. That comes in on the L/R wire. The set of contacts is connected to a W/R wire that traces back to the black fusible link. It goes out as the L/R wire The other coil is energized by the key in ON, and that comes in on the B/W wire. The set of contacts associated with that coil is connected to a W/R wire from a different fusible link. It goes out as the W/B wire. I traced out those wires coming off the relay. L/W Wire Fuse Box (ACC) L/Y Air Con Blower Motor AC Relay Mag Valve Vac L/R Wiper Wiper Power Washer Pump L Radio Radio Power Antenna Sw Rear Defogger 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 So many of your issues revolve around the ignition relay. Is it bad? Test with a voltmeter. Note: All of these measurements will require the black probe of the meter on a good ground. 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. Put the key in ACC and measure voltage on the L/R wire. You should have battery voltage. Put the key in ON and measure voltage on the L/R wire and the B/W wire. Both should have battery voltage. 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. Let us know the results. Considering you said the wipers work, the ignition relay may not be the issue. The dash lights are another issue. The red wire is for the headlights. The G/W and G/L wires are for the dash lights (and running lights including tail lights). Describe the non-start when warm in more detail. Does the car crank? It might be a vapor lock issue. Do you have an AC car? Have you checked the blower resistor? Is the turn signal flasher plugged in? It should be hiding under the dash on the driver's side. The hazard flasher is there, too. The turn signal flasher should have G/Y and W wires going to the connector.

Actually, there wasn't a wire added to the 78. The ignition relays are the same.

RHD Japan has it. https://www.rhdjapan.com/search/category/fuel-system_fuel-pressure-regulators/brands/sard/

You could get a tee like what @Patcon linked and buy a schraeder valve to go into it. https://www.summitracing.com/parts/ear-9616sverl or https://www.amazon.com/Earls-9616SVERL-Male-Schrader-Valve/dp/B01EKUFV90

Instead of cutting into an existing wire... Use what's already in the dash. On the clock, there is constant power at the LW (blue/white) wire. There is switched power at the L (blue) wire on the water temp/oil pressure gauge. You would need to get two male shells (https://www.hi-1000ec.com/product/1133) and two female shells (https://www.hi-1000ec.com/product/1135) along with male pins (https://www.hi-1000ec.com/product/1662) and female pins (https://www.hi-1000ec.com/product/1663) to go in those shells. If the meter doesn't require constant 12VDC, you only need one male shell and one female shell, and you can take power from the blue wire. You can then add a small harness between the dash harness and the gauge. Most of the wires will be straight through, but the wires where you need voltage would also split off to go to your new gauges. The voltage regulator doesn't have any special properties that would give you a better signal than the two wires I pointed out. As long as the wiring isn't corroded or hacked up, what I pointed out should be fine.

You mean like this?

You talk about old gas in the front carb. How old? Has this car been sitting for a long time? In addition to the float level, the path between the float bowl and nozzle could be clogged. I saw that with my 240Z that sat for a long time. How old are the hoses that go between the float bowl and nozzles? (I thought I posted about cleaning out the float bowl, but I don't see the post here or in my history.)

I seem to recall that not 240Zs had the wells for the seatbelt retractors. I do not know if those cars only had non-retracting seatbelts. Of course, with that you show, it supports my suggestion above.

@grannyknot makes some great points. I used a seatbelt system similar to the one MSA sells for many years in my 240Z, but when I changed out the old seat belts in my 260Z, I went with the Wesco Performance Roadster belts (https://www.wescoperformance.com/noname.html). @jfa.series1 found them independently of my posts on them. You mount the retractor on the upper mounting point. While I understand you have already purchased the MSA belts, sometimes you have to absorb the sunk costs and move on.

Repeat as per @Patcon's instructions. Also keep in mind that not many of these gauges are calibrated. You may want to borrow a friend's gauge if you can to compare readings. Also make sure your battery is fully charged when you do a compression test. Cranking the engine can take a lot of power out of the battery. As for the air flow meters, I prefer the second one for the reason you stated earlier. The Unisyn type blocks the air flow, so you have to work fast. I do better working methodically. The "snail shell" style lets you see the effects of adjusting the screws as you turn your screwdriver.