SteveJ

Good job tracking it down. I can only point you in the right direction. You have to put the effort into it to find the root cause.

Is the horn relay still connected? If the wiring is stock, the only way for there to be a complete circuit is for the green/black wire in the steering column to be connected to ground. When you press the horn button, the button connects the tab on the horn wire to the steering column. You may want to take the cover off the steering column and make sure the combo switch wasn't installed too close to the steering wheel. Is it a stock or aftermarket steering wheel?

That would tell me there's a problem with the alternator.

No. If there isn't a load, such as a light on, a hot fuse won't cause a drain. An internal voltage regulator is still a voltage regulator. A failure of that component could cause a battery drain. For some reason I missed your question. ONLY the circuits in red require the key to be in the ON position to be energized. You can turn on the headlights and parking lights with the key any position. So, going back to my original questions, do you know how to use an ammeter? If not, do you have a test light? One or the other is likely to be necessary to find a cause for draining the battery. Have you checked all of the other items I listed for potential drains? Again, how long does it take the battery to drain?

That's okay. There is the switch between the fuse and the lights. As for battery drains, check the list I supplied.

It sounds like the voltage regulator. I ditched the externally regulated alternator on the 240Z so long ago, I don't remember if there was a clicking sound when I plugged it in (last replaced in 1995). On the 260Z I only unplugged it once and never plugged it back in. They shouldn't be getting warm, though. The 0.5A draw is pretty strong. I bet it goes away with the regulator unplugged. It looks like you are running a points style VR. Have you opened it up and tried to adjust it per the FSM?

Got it. Try spraying a little graphite lubricant into the keyway and moving the key around. Otherwise, consider getting a new ignition switch. https://zcardepot.com/products/ignition-switch-with-keys-240z-260z-280z-510?_pos=5&_sid=1b328e775&_ss=r It's not an exact fit, but it should work.

What do you consider special about 24 inch diameter tires? Something along the lines of 185 70/R14 would be around 24.2 inches. Also 205 50/R16 would be around 24.1 inches.

That's a loaded question. Let me phrase it another way. What circuits can only be powered when the ignition switch is on? The fuses with red only are powered with the key in ON. The fuses in green use the combo switch (headlight/parking light switch). The common fuse is for the cigarette lighter. The clock is on the same fuse as the dome lamp and map light. (I think the wiper park function is on that fuse, too.) So what could drain the battery? Bad voltage regulator Bad alternator Clock (even if it's not running) Dome light, map light, and glove box light (door not closed all of the way - Note: This only applies to the 240Z. The parking lights have to be on for the 260Z or 280Z glove box light to come on.) Aftermarket stereo Aftermarket alarm Aftermarket accessory like remote locks (The previous owner had that on my 260Z. I removed the remote lock and fixed the door lock properly.) Unknown cross-connection Short in the starting motor (unlikely but here for completeness) To help you with the diagnosis When did it start? How long does it take to drain the battery? Do you know how to use a voltmeter and ammeter? Do you own a test light?

Are you seeing the temp gauge creeping up at highway speeds? Could you be running too lean?

No, it doesn't make sense for the fan to run all of the time at highway speeds. You usually have enough air flowing through the radiator then. Are you sure it's running at highway speeds?

Hence why my diagnostics changed to fit the data.

Have you opened the float bowls lid to see if you have fuel in the bowls? Do you think your floats are too high or too low?

I double checked the testing with an 1156 bulb using my parking lights and headlights (all LED). The improvised test light only lit up when I turned on the headlights. My more conventional test light (similar to this - https://www.harborfreight.com/612v-circuit-tester-with-5-ft-lead-63603.html?_br_psugg_q=circuit+tester) lit up with only the parking lights. That means I might have steered you toward an inconclusive test (false negative). You could re-test with a purpose built test light like I linked or with an ammeter. Just because the clock isn't running doesn't mean it's not drawing current. It might be drawing more current if the motor is trying to overcome increased friction.

You are correct. Mid-grade is typically a blend of premium and regular. I am fortunate enough to live near a boating area so I can get 90 ethanol free or 87 ethanol free. I can even get 110 at a pump about 10 miles from my house.

The running lights/dash lights circuit is problematic in the S30s. I had to replace the fusebox in my 73 about 27 years ago after it melted down at that circuit. The front side markers tend to get corroded. The rear side markers could, too. The corrosion can bridge the positive and negative wires to bypass the bulb. It isn't a dead short, but it is very low resistance. The wiring to the running lights should be green with a blue stripe. You photo seems to show a green/white wire, but it's difficult to repair. You may want to consider using LED bulbs for the running lights and using a lower rated fuse. Also inspect/clean the side markers. Here is something I wrote up a few years back on the subject:

The test light is a poor man's ammeter. The process I described to him was designed for him to come back to us for more advice. That way we could evaluate the circuits and see if we could find the offending component.

Less than 1 mV is the same as noise. If was an analog meter set on the lowest voltage scale, you probably wouldn't see any meter deflection. Things that can drain the battery and you might not notice: A bad door switch that leaves the light on (mentioned) The clock, even if it's not running. (That's on the cigarette lighter fuse.) Voltage regulator going bad. Aftermarket (or previous owner) modification such as stereo or car alarm. How do you detect what's causing a drain? Well, probably one of the most elegant ways is to construct a test light to replace a fusible link. Parts: 1156 bulb socket (https://www.amazon.com/Socket-Harness-Sockets-Adapter-Connector/dp/B09BCMWFP5) 1156 bulb (https://www.amazon.com/Bonlux-10-Pack-Dimmable-Landscape-Replacement/dp/B0928RWLG3) Clip leads (https://www.harborfreight.com/28-piece-electrical-clip-set-67589.html) You want an incandescent bulb because you want low resistance in your test light. Attach a clip to each wire of the socket and install the bulb in the socket. With the doors closed and key out of the ignition, pull a fusible link. Clip the test light to the fusible link block. If it doesn't light up, put the fusible link back and move to the next fusible link. If the test light turns on, note the color of the fusible link and take a picture of the fusible link block so that we can tell which link it was. Once we know which link, we can let you know how to drill down better to the guilty circuit.

Georgia Z Club meeting tonight. There was a good showing of cars. Here are most of them. The two black/gold AEs were both purchased relatively recently. There was another couple of S30s that showed up later, and a couple later models parked on the other side of lot, so I couldn't squeeze them in. When I came out to take this photo, a gentleman named Tom came over from the gas station because he saw the Z cars. He owned a 71 in high school and college. I told him to look up our club on Facebook. Maybe he'll visit.

Yes, they are. I linked @26th-Zup with them when he was looking for some parts. I've also helped them with some customer's cars.

Where in GA? Resurrected Classics? Max is a member here, but he rarely posts.

Make sure the dome light isn't staying on. That can cause a "mystery battery drain". The incandescent bulbs aren't that bright, and you might not notice it in the day. I have seen that as a current draw in someone's car before. It was caused by a wonky door switch that the plunger wasn't breaking the circuit like it should.

No, it is not. Did you take a measurement of voltage to ground at the solenoid terminal on the starter with the black/yellow wire removed?

This may help you: https://forums.hybridz.org/topic/124734-understanding-the-260z-and-280z-wiper-circuit-reverse-polarity-park/

The black/yellow wire should only be energized if the key is in the start position. If you have a voltmeter, see if there is voltage to ground at the black/yellow wire. Also check for voltage to ground at the terminal on the starter where the black/yellow wire lands. One of two things may be happening: The switch on the back of the ignition switch is going bad and supplying voltage (but with a very small amount of current) to the black/yellow wire. The starter is going bad and somehow is backfeeding the solenoid, again with a very small amount of current. I didn't see any obvious cross-connection possibilities in looking at the wiring diagram. I hope no one ever decided to re-wire things to "make them better".