Yet another fuel sending unit/gauge problem! Argh!

[Captain Obvious]

All of the gauges will move around a little bit by design (that's the compensation feature I mentioned above), but they should not move around much. If everything is working properly, you can see the needles wiggling around a little if you're really looking for it, but if you're not really watching for it you shouldn't notice it. It certainly shouldn't move around by an eighth of full scale.

And when you ground the yellow wire back at the sending unit, the gauge is supposed to shoot up to "F" and stay there. Solid.

Have you put your meter on the Yellow/Red feed wire from the fuse box right at the gauge? Is the voltage solid steady there. And to answer your question above, I do not know where or how they make that transition from the green wire at the fuse box to the Y/R by the time it gets to the gauges. I suspect it's done inside the harness.

Skipping to the end of the story... It sounds like there is an internal problem with your fuel gauge. You could pick up a spare and give it a try?

To answer your question about how much current goes through the gauge for the different readings, here's something I whipped up for a later year. No guarantee that the numbers are the same for your 72, but I suspect they are. This is sender unit Ohms vs needle position:

[Mark S M]

Thanks for the feedback and info. I measured the voltage at the meter and it was steady. I even jumpered right from the fuse box to the meter and there was no change. At this point I believe it is the meter as well. I will pull it and test on the bench again, that was why i was asking for the current levels so I could check the performance.

[Captain Obvious]

Well the gauge design inside is deceptively simple. Two heating elements wrapped around two bi-metal strips, and a switch between the two. The circuit is simple, but the physics is complicated.

First thing you be to clean the switch contacts. Carefully open them just enough to get a piece of paper between them and pull the paper around a little to scrub the contacts. Other than that, the only other thing I can think of that might be happening is a questionable connection to one of the heating element wire ends. There are four of them (two ends on each wire) and IIRC, they are soldered at the ends. Maybe one of those solder joints is failing, or maybe one of the wires is failing right at the joint.

[Mark S M]

Thanks for all your info. It does seem to be just dirty contacts in the gauge. When I first bench tested the gauge I only attached to the yellow red and yellow leads. I had the old sending unit to work with and when doing it this way I had steady readings. This morning I attached the ground as well and low and behold I had a mis-reading, bouncing gauge. I opened the meter and saw the contact points you mentioned and cleaned them with some fine emory cloth. I also made note of the adjustable contact position and turned it back and forth a couple times then returned it to its original position. It seems to be working properly now. Once I get the dash back together I will try it on the road and report back. The only question I have now is do I replace the 48 year old instrument lights that are still working or do I leave them as it seems they have proven themselves.

[Captain Obvious]

Excellent. Glad you fixed that one.

As for the old bulbs... If I had the gauge out in my hand, I'd go ahead and just replace them all.

[matt90gt]

Old topic, but just had issues with my gauge.  Wanted to share.  Pulled the gauge and, and heard a rattle.  opened it up and one of the mounting tabs for the front plastic lens broke off and was wedged in mechanical assembly.  Removed the piece and gauge is working like a champ again. 

Edited June 4, 2023 by matt90gt

[Captain Obvious]

Haha!! Easier fix than the other issues mentioned above. Good work!

One of the cross beams has gone out of skew on the treadle.

[AK260]

To keep adding to this excellent thread…

 

I was playing under the car today and stink of fuel! 


problem: The gauge drops to below half within say 50 miles of filling up, then stays close to E forever, until all of a sudden it’s below it and you’re driving around with your sphincter pulsating, wondering if you will run out before the petrol station arrives.

 

So I started by draining the tank completely and adjusted the sender bend for fully empty to not be way below the mark but “just” below.

 

Then filled up and observed the needle in 5L increments.

9L is c. 2 imperial gallons (5.54L / gal). Knowing my typical 15-17mpg average (with a heavy right foot), that gives me a safety margin when on E, but an uncomfortable one that I won’t be visiting unless it’s an emergency - as it would suck all the potential crud at the bottom of the tank.

The edge of the white tape aligns with needle position at totally empty.

I really wanted to know the 15L position as that is roughly 1/4 of a tank and I tend to fill up before that. But I now know where a true 1/4 tank is.

 

Finally at 27L with all the fuel I had to hand - I believe a full tank for a European 260z is 65L (tested by shoe horning 63litres at the pump) so another 5L and it’ll be roughly at half a tank. 

This is progress as I will now trust the gauge more. With another 5litres, the needle will be pretty close to half. But I suspect it will be shy of it.

 

I managed to semi-slice a decent chunk of flesh off my finger using a wheel chock, but currently held on with tissue and electric tape - I’ll spare you the graphic photos! That ended play for today and I’m currently waiting for my wife to get home so that I may go to A&E to have it checked out! So I didn’t get to go buy that extra 5L in the can to try (maybe I’ll get it on the way back from hospital 😛 )! I will report back when at 32 litres.

 

What I’m wondering though is, has anyone used the trim adjustments on the back of the fuel gauge to fine tune rather than constantly pulling and guess bending the sender arm? 

 

 

Edited January 10 by AK260
Typo haven!

[Patcon]

@SteveJ

[SteveJ]

I'm not sure what year your Z is. Up through 76, I believe the specs are 60 L for fuel (15.85 US Gallons) for the North American model. I would assume a 77 or 78 260Z (if they exist in the UK) would have the larger tank like North America. The problem is that I don't think the sender does anything to account for the shape of the tank. I'm not sure if the adjustment screws would help, either. The top 2/3 has only about 1/2 the width of the bottom third. 

To simplify things due to the fact that I have a simple mind, we can model the tank somewhat like this:

So the darker grey top half is about equal in volume to the light grey bottom half. Since the tank is 60 L, that means each portion is 30 L. That means 15L would be 1/6 of the way to the top of the tank, and 27 L would come up to a little under 1/3 way. It looks like the gauge readings are somewhat consistent with that.

[AK260]

Thanks @SteveJ, that’s really great information.

Yes mine is a ‘77 RHD 260Z; I believe we got the 260 2 seater from 75-78 in the UK (happy to be corrected). It has a slightly different looking fuel tank to the US 240/260 cars.

It’s very similar to some 280Z tanks that I’ve seen advertised on eBay. Here’s a pic from when I was refreshing it during lockdown.

That top breather hose with the crazy 180° loop was crushed on mine and instead of buying a reproduction one for a small fortune (which likely will go again over time), I opted for IIRC 17mm ID hose and inserted a 15mm pipe bending spring into it. Or was it larger? I can’t be sure.  Regardless, the stainless steel pipe bending spring works a charm and will never let that pipe crush again. Before this fix, the top part of the tank wouldn’t fully fill and the needle only went to just over 1/2. I suspect it was because it got to the level of the smaller breather pipe just below it. Made filling up a lot cheaper 😛

Now that I know more precisely than ever before how much I have in it, it would be interesting to see what my next full fill up ends up as (60 or 65L).

 

Given your explanation, I will likely leave it there for now and see what it does when I start putting miles on in Spring / Summer. Thanks again, much appreciated.

Edited January 12 by AK260

[AK260]

And here it is with 32L in - as suspected, not quite at half way but the best it’s been ever.  I’d rather leave the adjustment where it is so that I’m certain about the 15L and the 9L positions.

 

edit: one of the guys in the UK club who’s had Zs for 40 years was saying that apparently they read differently when warm! I suspect it’s something to do with the voltage regulator electronics inside the gauge! 

Edited January 11 by AK260