Hi all, hoping to get some help with a fuel gauge that doesn't work. PO had some fun with the wiring so I wouldn't rule anything out. What I've tried so far:

• Installed new sender and measured resistance across the terminals, ~75 ohms with 1/3ish tank of fuel so that checks out.

• Checked resistance at the connectors by the taillights, passenger seat, junction box - all 75ish ohms so wiring/connections all good afaik.

• All fuses under the glovebox are good.

• Pulled the gauge out completely. On bench, I applied 12v to the yellow wire and grounded the black wire, gas needle moves, so gauge appears good.

Which brings me to the dash harness. I've attached a picture with the two pins that run the fuel gauge (the mates to the two pins I tested on the gauge harness), yellow wire is the signal from the sender and to my knowledge black is just ground. But when I use the voltmeter things don't seem to behave the way I expect.

These tell me the black wire should be good as a ground:

• Continuity between black wire and other grounds (cig lighter barrel and HVAC panel frame)

• +12V between cig lighter positive and black wire

But the yellow wire is where I'm confused

• Continuity between yellow and black wire. No resistance, multimeter just beeps

• 75 ohms between yellow wire and other grounds (cig lighter, hvac panel screws)

I'm just spinning in circles. Black wire is just as good a ground as the bare metal frame inside the dash. The yellow wire sends 75 ohm signal to that frame just fine, but when I try to send that signal to the black wire it just beeps with continuity. Any advice where to go from here?

Possibly relevant: I don't know if the ammeter works either. On bench, the needle jumps if I apply 12v to the two white wires with an LED inline, so the gauge works, but when the car was running I could never tell if the needle was moving or not...it was always right at or extremely close to the center/zero mark, I just don't know how its supposed to look under normal operation.

Also maybe relevant: Oil pressure gauge and tachometer both move sometimes, but not reliably. Very rarely they'll read the expected range, but more often just reads nothing. Water temp gauge has never read anything. Speedo does work. All gauge illumination works.

Edited January 4Jan 4 by wheezy

Here are a couple of videos on the fuel gauge:

Basically, you're not testing the gauge properly. To bench test the gauge, you need to connect 12VDC+ to the green wire, ground the black wire, and run the yellow wire through a resistor that is grounded on the other side.

You should have 12VDC+ to ground at the green wire coming into the gauge when the key is in ON. There is a voltage regulator in the gauge as discussed in the videos above, and the yellow wire comes off the regulator and goes to the fuel level sender.

You should measure voltage to ground at the yellow wire at the gas tank with the key in ON. When the key is first turned to the ON position, you should see about 12VDC to ground at the yellow wire. As the voltage regulator warms up, you will see the voltage fluctuating. You can also ground the yellow wire at the gas tank for a short period of time and have someone see if the needle moves on the gauge.

Ahh that makes a lot more sense. So if the voltage regulator is toast is that something that can be fixed or am I looking at a new gauge?

Edited January 5Jan 5 by wheezy

Aftermarket regulators are available. New gauges are unobtainium.

I have not tried this...yet, but I see a garage project coming.

Ratsun Forums

Bad gauge cluster voltage regulator? Easy $2 fix.

Problem You're temp/fuel gauges don't work and you've already checked the output from your fuel/temp sensors to make sure they are not the culprit. You checked continuity of the wires from the sensor

So tested the green wire for ignition, it’s only getting 10.4V - any chance this is enough to cause the gauge not to work? Any idea why its coming in low?

50 years. Stock electrical connectors and wire.

But, most likely, that shouldn't make the gauge not work at all - skew the reading maybe.

Their are adjusters (trim pots) on the back of the gauge to set and/or correct needle travel.

To me it sounds like your problem is in the wiring. Are you sure the sender has a good ground connection? The "signal" the yellow wiring is sending the gauge is a variable resistance from the fuel level sender and it's ground connection. It actually heats up a bimetal strip in the gauge that causes the needle to move. If you ground the yellow wire and the gauge needle goes to full, the problem is probably not in the gauge (if the gauge and it's trim pots have not been modified).

What I initially meant was: 50+ years and stock wiring means increased resistance in wiring and connectors. The gauge displays electrical resistance - so adding more resistance skews the reading (needle position). But doesn't normally kill the reading altogether - unless the Ohm value (resistance) provided is out of the range that the gauge will display .

Edited January 5Jan 5 by cgsheen1

Interesting topic. I've wondered about the "regulator" in the past. I dug around in the FSM, and also read that Ratsun link and think that there might be a problem using that solution with this specific problem. The FSM only shows a regulator being used for the temperature and oil pressure gauge. Not for the fuel and ammeter.

Pulled a few things out to illustrate. Still worth exploring, just not for this problem.

From Ratsun. He's talking about temp/fuel, but that's two different gauges in the Z's.

"You're temp/fuel gauges don't work and you've already checked the output from your fuel/temp sensors to make sure they are not the culprit."

From the 1974 FSM. Looks like three triangles represents the regulator.

The fuel gauge has a heat coil. Seems like that could fail and cause problems. No heat, no needle movement.

Edited January 5Jan 5 by Zed Head
The forum software screwed up my image pasting.

2 hours ago, Zed Head said:

Interesting topic. I've wondered about the "regulator" in the past. I dug around in the FSM, and also read that Ratsun link and think that there might be a problem using that solution with this specific problem. The FSM only shows a regulator being used for the temperature and oil pressure gauge. Not for the fuel and ammeter.

Pulled a few things out to illustrate. Still worth exploring, just not for this problem.

From Ratsun. He's talking about temp/fuel, but that's two different gauges in the Z's.

"You're temp/fuel gauges don't work and you've already checked the output from your fuel/temp sensors to make sure they are not the culprit."

From the 1974 FSM. Looks like three triangles represents the regulator.

The fuel gauge has a heat coil. Seems like that could fail and cause problems. No heat, no needle movement.

Look again. There is a voltage regulator in the fuel gauge. That's what those videos show.

I didn't watch the videos. I went to the Nissan documents.

You're saying that there is a mistake in the Factory Service Manual? The videos are for the OP's 1974 260Z fuel gauge? Not clear why I should disbelieve the FSM.

I checked 1978 to see if things changed with a voltmeter. The picture is different but the description is the same. It would be odd to edit the FSM and leave an error. But it is possible, of course.

The diagram you posted from the FSM shows a points style voltage regulator aka the bimetal strip. The strip opens the points in a manner similar to the voltage regulator in the charging circuit. Where is the error?

The voltage regulator is on the right side of the fuel gauge.

And it's on the left side of the oil pressure gauge where it is also wired to the temp gauge.

By the way, it looks like the Triumph guys have been replacing the bimetal strip regulator with a solid state regulator.

Voltage stabilizer for temp and fuel, anyone going solid...

I'm trying to calibrate my fuel and temp gauges, but find myself fighting the bi metal voltage stabilizer.. I plan on going for a 10v solid state unit. Has anybody else done this?

This link shows the internals.

https://forum.triumphdolomite.co.uk/viewtopic.php?f=5&t=4550

Edited January 6Jan 6 by SteveJ

43 minutes ago, SteveJ said:

The diagram you posted from the FSM shows a points style voltage regulator aka the bimetal strip. The strip opens the points in a manner similar to the voltage regulator in the charging circuit. Where is the error?

So you're saying that Nissan used the wrong words. When they saif voltage regulator they meant bimetal strip? The air regulator has a voltage regulator in it? A bimetal strip used aa a switch is not the same as a voltage regulator, unless regulating means on-off.

Just looking for a clear description. The modern voltage regulator linked in the Ratsun post takes 12 volts or more and regulates it down to a steady 9. I don't see how a bimetal strip, that bends due to a temperature change equates to that.

Seriously, I'm not sure what you're saying. The words are wrong, or the description is wrong, or Nissan left some words out.

The modern VR's are really interesting devices. I learned some new stuff.

https://www.allelcoelec.com/blog/Everything-About-the-LM7809-Voltage-Regulator.html

"The LM7809 is a reliable positive voltage regulator IC, renowned for providing a steady 9V output in various applications. With its ability to handle currents between 1A and 1.5A, it finds widespread use in fields like robotics and embedded systems, where consistent voltage supply is serious. To achieve optimal performance, the LM7809 requires an input voltage of at least 11V, maintaining a minimum 2V headroom to deliver its regulated output. Designing a proper power supply with adequate input reserve is required to ensure efficiency and prevent interruptions in power flow."

Edited January 6Jan 6 by Zed Head