Fuel gauge issues - 73 240z

[GeoJoe]

Which fuse?

[SteveJ]

Which fuse?

The same fuse as for your other instruments. It's the middle fuse on the left.

[siteunseen]

I've read the opposite. If you put the two wires that send the signal to the gauge together, it should top out at max fuel level. Doesn't work for me. For now it's just resting on the empty side. 

You are right, it should read FULL when shorted together.

[JDziark]

The fuel gauge on my 72 240z is not working.  I've searched this site and have a question.  Thanks in advance for any help you can offer.

1.  It appears that the voltage at the yellow wire by the fuel tank should cycle between about 9 and 3 volts.  I read less than one volt on that wire (it is not zero).  I traced the wire back to the harness under the dash and at that point it is also <1 volt.  Can someone verify that the voltage of the yellow wire by the tank does indeed cycle between about 9 and 3 volts?  The yellow wire can be probed from behind the cover in the rear of the car that is used to change tail light bulbs.  Any ideas on why I'm getting such low voltage and what could be wrong and fixed?  It was working before I tore the car apart.  I have reinstalled the dash a while ago and am not looking forward to messing with the gauge but I have a feeling that will be necessary at some point.

Thanks again,

John

[SteveJ]

Find this connector between the dash and body wiring harnesses.

Check the yellow wire there. If you have a signal there, look for continuity on the yellow wire through the body harness.

 

So it ended up that we did find out what was wrong with @GeoJoe's fuel gauge. I checked continuity on the yellow wire on the body harness. It read open line. The wire was replaced, and the gauge works.

[JDziark]

Steve, thanks for getting back to me so quickly.

I did check for continuity as well as voltage at the dash.  When I first encountered the problem, I thought I might have a broken/frayed wire from the dash to the tank but continuity was good.  Also, the voltage of the yellow wire at the dash is also < 1 volt.  It would seem that for some reason, the source of the yellow wire (the gauge?) is putting out only a small amount of voltage.  Any other ideas?  Thanks again,

John

[SteveJ]

The positive side of the fuel gauge is the yellow/red wire. Since that wire also powers the oil pressure and water temperature gauges, you can eliminate that as the source of your problem if those gauges are working.

The likely culprits would be a poor connection between the fuel gauge and the dash harness or the fuel gauge itself.

[Captain Obvious]

Here's a thread with a bunch of info about how those gauges work:
https://www.classiczcars.com/forums/topic/64427-no-oil-pressure-after-rebuild/

I'm thinking you might be having a problem with the little regulator switch contacts inside the gauge. Source impedance too high, load drags the signal down, blah, blah, blah... That sort of thing.

If the contacts get dirty, the voltage seen back at the sender unit will be lower than desired.

[JDziark]

Fantastic, great help and your explanation makes sense to me.  Looks like I have to remove the gauge - something I'm not really looking forward to!

I'm thinking of trying something.  See if this makes sense to you.  How about if I back feed a 12V source at the yellow wire by the Sender Unit?  That should send current through the 'Second Heating Coil' and the 'First Heating Coil' then to ground.  Shouldn't that make the needle move?  Not sure what I'd do after that even if the needle does move but if it doesn't, wouldn't that mean that that one of the coils is bad (since that would not allow current to go to ground)?

Thanks again for you description.  Super helpful and if you know any tricks about removing the gauge, please let me know.  As I mentioned, I reinstalled the dash and don't want to take it out again.  My temporary fix will be to keep the tank reasonably full and watch how many miles I drive between fills.

John

[SteveJ]

You don't need to pull the dash to remove the gauges if the car is stock. IIRC the FSM has instructions on removing the gauge. You definitely need to detach the heater control cables.

As for feeding back the signal. Don't. Get the gauge out first. Then you can set up the test. You will need to set it up like the diagram the Captain posted to test it properly.

[JDziark]

Steve, thanks.  To answer your previous question, the other gauges work fine so it looks like it is a fuel gauge issue.  I did review the FSM on gauge removal but it still doesn't look like a lot of fun!

Any particular reason why I shouldn't back feed the gauge?

Thanks for you comments.  John

[SteveJ]

It doesn't work that way.