Fuel gauge max all of a sudden

[vintageman]

For those who have not seen my dashboard swap, I successfully swapped my spare dash but for some reason my fuel gauge now peges our all the wY . I checked my grounds to body and they are good. This was not an issue 2 or. 3 weeks ago before the swap.

Thanks
Tom

 

[Vicus]

Tom,
Maybe the wire got damaged and now touches the body of the car somewhere.

 

[TheRktmn]

Make sure that the fuel gauge is mounted correctly.

There are small cutouts on each side of the stud girdle that align with tabs on the cluster back panel.



If the gauge is skewed so that the stud touches the back panel your gauge will go full range. This applies to the Oil, Temperature and Fuel gauges. A shorted Amp meter will be very obvious by the smoke and stink.

 

[midlife]

Make sure that the fuel gauge is mounted correctly.

There are small cutouts on each side of the stud girdle that align with tabs on the cluster back panel.

View attachment 87472

If the gauge is skewed so that the stud touches the back panel your gauge will go full range. This applies to the Oil, Temperature and Fuel gauges. A shorted Amp meter will be very obvious by the smoke and stink.

The 7123 dash clusters are plastic and if a post touches the housing, there is no short. The problem of gauge shorting is most prominent on 69/70 dash cluster housings with the circuit cards.

 

[TheRktmn]

The 7123 dash clusters are plastic and if a post touches the housing, there is no short. The problem of gauge shorting is most prominent on 69/70 dash cluster housings with the circuit cards.

Ah, crap - I forgot which forum I was in. Mine applies to 65-69 models with metal back panels. +1 for bad printed circuit card.
(insert embarrassed and kicking rock guy here)

 

[mrgmhale]

If you check for pulsating voltage to the fuel sending unit at the the fuel tank sending unit (Circuit #29, Yellow wire with White Stripe) you will see if voltage from the Instrument Voltage regulator - through the fuel gauge - is getting back there or not. If I recall correctly you ought to be getting a pulsating voltage of around 5 to 5.8 volts or so when the connector for the sending unit is unplugged. If you are getting no voltage, the wiring for Circuit #29 likely got pinched and is shorted to ground "somewhere" between the fuel gauge and the sending unit. If that is the case I would begin to check for voltage on Circuit #29 beginning near the instrument panel where the Circuit 29 Yellow White Stripe wire leaves the instrument cluster outlet then trace it back to the area it is shorted. Below are some links to some documentation on my Google Drive that ought to help you track this down (especially page 26 of the "ChristopherNeal_71-73Mustang_DashLightsFuelGaugeInoperative_20201005.pdf" file). I hope all this helps you out:

https://drive.google.com/file/d/163_8kBiDCr278JmlLTt_fZNjkSmWuT8O/view?usp=drive_link

https://drive.google.com/file/d/11mmcL0D1ScqUTiHIatW25sKN5-y6LYAa/view?usp=drive_link

 

[vintageman]

Thank you Gil, I really appreciate the files and will definitely be using these for troubleshooting.

Tom

 

[mrgmhale]

Thank you Gil, I really appreciate the files and will definitely be using these for troubleshooting.

Tom

You've got this, although you may not yet realize it. The fuel gauge circuitry is one of the easier electrical systems despite current coming from the Instrument Voltage Regulator, and then traveling through the sending unit.

 

[vintageman]

Got it working tonight, when I set the dash back in place, the main harness that goes behind the cluster got pinched. There is supposed to have a plastic routing sheath but that's long gone that the harness goes through and is protected. I loosened everything up and was able to move the harness and tested the gauge and it works properly now. If I ever pull the dash again, I doubt that I will reinstall with everything connected again, if I would have had the cluster out, then I would have seen the issue during the installation. Over the next couple of days, I will remove the tape that wrapped around the harness and check for damage, fix and re-tape. Thanks everyone for the advice and I should be back on the road soon.

Tom

 

[mrgmhale]

Got it working tonight, when I set the dash back in place, the main harness that goes behind the cluster got pinched. There is supposed to have a plastic routing sheath but that's long gone that the harness goes through and is protected. I loosened everything up and was able to move the harness and tested the gauge and it works properly now. If I ever pull the dash again, I doubt that I will reinstall with everything connected again, if I would have had the cluster out, then I would have seen the issue during the installation. Over the next couple of days, I will remove the tape that wrapped around the harness and check for damage, fix and re-tape. Thanks everyone for the advice and I should be back on the road soon.

Tom

I had no doubt you wouls resolve that issue. Many thanks for reporting back what you found. I will be extra caqutious with my future dash removals - which I hope will be very few indeed. I've paid my dues time and again. I am hoping to be able to enjoy our pony cars with minimal grief for my remaining time on this planet (which I hope is a very long time)...

 

[vintageman]

We hope so to Gil, hang in there brother.

Tom