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Fuel gauge issues - 73 240z


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Hi Guys... my 73 240z had a fuel gauge working fine. Last month it went intermittent on me, reading very low, then finally stopped. 

 

So I checked the fuel level sending unit and yes, it was leaking from the top connection. I figured this was also why I smelled gas when the tank was about half full. So I bought an OEM replacement sending unit and figured a 30min job to swap it out. I drained the fuel tank, got the old one out without dropping the tank and hooked the new one up. 

 

Two days later, dozens of searches and hundreds of pageviews and I still can't get the gauge to read anything. 

  • I'm seeing the fuel sending unit reading a low fuel level with an Ohm meter, so that's good.
  • I've tried to ground the positive for the gauge (near the fuel pump) to read full. No movement.
  • I've checked the wire harness for the fuel pump and sending unit near the tail lights. It's intact.
  • I've reached behind the fuel gauge to see if anything is loose... it all seems tight.
  • I've checked the fuseable link to the starter and it's ugly, but working. 

Is there another fuse, ground or link that I'm not seeing?

 

All other gauges work (except clock)

Thanks all....

 

 

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Thanks for the quick reply as always Steve. I'm on the track at Road Atlanta this weekend in the Audi. Maybe next weekend?

 

It's not urgent, but it's driving me crazy that I can't figure it out. Seems simple and I'm reading that these are usually cases of faults and grounds rather than bad gauges. 

 

I just replaced the glass fuses that control the gauges too... 

 

Can a fuseable link that is frayed have an affect on one part of the car, but leave other parts working fine? That's my next replacement.

 

Another possibility is that my recent paint job may have had an affect on the ground in the rear hatch. Where is this exactly?

Edited by GeoJoe
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I was thinking if you put the two wires together it should peg on the empty side of the gauge? Could the clear lense be pushed up against the needle? Happened on one I bought, the guy kept finger tapping the actual gauge face trying to get it to work when the sending unit went bad.

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Thanks for the quick reply as always Steve. I'm on the track at Road Atlanta this weekend in the Audi. Maybe next weekend?

 

It's not urgent, but it's driving me crazy that I can't figure it out. Seems simple and I'm reading that these are usually cases of faults and grounds rather than bad gauges. 

 

I just replaced the glass fuses that control the gauges too... 

 

Can a fuseable link that is frayed have an affect on one part of the car, but leave other parts working fine? That's my next replacement.

 

Another possibility is that my recent paint job may have had an affect on the ground in the rear hatch. Where is this exactly?

The weekend after that is over at Jai's place near Chatsworth. 

 

I'm betting on the electrical connection. A multimeter is your bestest friend.

 

If you're in town over Labor Day, you could bring the car over on Monday, and we can put it on the lift.

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I was thinking if you put the two wires together it should peg on the empty side of the gauge? Could the clear lense be pushed up against the needle? Happened on one I bought, the guy kept finger tapping the actual gauge face trying to get it to work when the sending unit went bad.

 

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.

 

Also to note, but prob not related - I have an intermittent tach that will all of sudden start working, them stop, then start again with a few taps on the lens.  

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Here's the contrarian position. I had a fuel gauge that stopped registering. I got under the car and found the connections at the tank were loose. I got them tight against the sender, and I have a functioning gauge. I can test it with a meter and ensure there's a good connection, and I probably have a good fuel meter if it comes down to that.

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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.

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  • 5 years later...

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

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Find this connector between the dash and body wiring harnesses.

image.png

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.

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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

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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.

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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.

gaugetheory2.jpg

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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

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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.

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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

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1 hour ago, JDziark said:

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)?

I'm not really sure how helpful this would be. Yes, it would feed current back through the system as you described, but I'm not sure how much needle movement there would be. It depends on the resistances of the heating coils and I don't remember the values offhand. I know the sender unit is about 90 Ohms, but I don't remember what the coil resistances are.

It won't hurt anything to try that, but just not sure how much value there would be.

 

I think you're going to to have to pull the gauge and clean up the regulator contacts. Start gentle... Strip of printer paper pulled through the contacts a couple times just to burnish the surfaces.

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