72 fuel gauge issue

I've had my 72 for 10 years now an the fuel gauge has never worked. Just moved into a new house with a 3 stall and I finally have room to work on my car.

So I have 12v down to the harness in the driver kick panel, ( red connector, yellow/white wire. Problem is under the driver door sill plate there are two yellow/white wires going to the trunk area where the wires go to the sending unit. By the way I don't have 12 volts to my sending unit. I'm losing my power somewhere between the kick panel and the sending unit.

Well what's confusing me is the two uyellow/white wires. Way up over the driver side rear wheel well, on the wiring harness I have one white/yellow wire coming out with a bullet style connector that has nothing plugged into it but it has 12v. Thanks for feedback everyone, good to see us 71-73 guys get our own site!!!

Someone a long time ago may have mixed up some wires. That 12v bullet connector going nowhere - I wonder if your fuel gauge would peg if you grounded it?

Wait, never mind - I forgot about the IVR, so there shouldn't be 12v there if it is working correctly. The '72 Shop Manuals never come out and tell you what it's supposed to be, but on earlier Mustangs it was 5v.

The rear wire harness is pretty simple, but you can't just pull it cause if the dome light. you can back trace, if you feel along the drivers side trunk drop down you will find the sender wires.

Run you hand and follow them until you get a good look at the wire insulator colors.

Then go to the kick panel and match the same colors at the connector.

When you tested the voltage did you pull the connector at the fuel sender? How corroded was the connector inside? Test the ground to the fuel tank also.

It's also possible that the gauge is dead, test resistance of the sender itself make sure it's not open.

You may need to pull the rear Seat and quarter panel trim, and the carpet holder/door sill plate, and back trace the wire into the trunk looking for a break.

Sending unit is new, and has 18 ohms of resistance. At the small red connector in the kick panel my y/w wire has 12v. I pulled the sill and checked at the latch side of the door and had 12 volts but there are two y/w wires. Ones is dead. There all in the factory loom and are unmodified. I also checked in the trunk and above the driver rear wheel well, I have 12v at the one y/w wire ( this one stops above the wheel well and has the bullet connector) the other y/w wire leads down to the connector to the sending unit and is dead.

Is the one with the bullit connector more green than yellow?

Green wire with green bullit connector is for the optional trunk light.

is the connector very rusty look at the rubber plug, inside there are the 2 connectors that fit over the poles on the sender. i had a lot of corrosion inside i took a toothpick and i cut a sliver of sand paper and glued it to the toothpick then cleaned out the connection, and took a small flat head jewelers screw driver and slightly bent in the connector so it really grabbed the sender connector poles. Lastly i coated the sender connector with dielectric grease to prevent more corrosion.

this may not solve your voltage loss but can help things later if you discover a broken wire up the chain.

If there is no power at the sender, it's possible your IVR may be dead. Do you have any other instruments that run off of it? The original owner of mine wasn't big on instruments, so my fuel gauge is the only thing powered by the IVR. (Everything else is idiot lights.) There could also be a problem with the printed circuit card.

that is a good idea, if you trace all the wires and can't find the problem then you'd have to pull the instrument cluster.

the IVR is attached to the back of the panel with little 9Volt battery buttons.

you could take the opportunity to replace both the circuit board(copper on mylar film) with a new one, and also replace the IVR with a new one. both are available as reproduction, if you still get nothing that gauge could be messed up.

I have my 12v at the red connector in the kick panel but i loose it somewhere between, if you look at my pic you can see the wire that has the bullet connector on it it has 12v but nothing goes to it.


here is the sender wiring, no voltage...

it may be a good idea to pull the rear harness and diagnose it with a ohm meter, a wire may have broken or shorted inside the harness covering.

I just had the rear harness out and it checked out fine. I pulled the gauge cluster out and looking at the back the p/o had cut the printed circuit so that there is really no ground for the fuel gauge its self. I have an aftermarket tach mounted in the factory tach location. Wondering if I sholuld get a new printed circuit or I think I could just run a ground wire since I dont have the idiot lights there anyway.


heres what its supposed to look like.... Now about this IVR.... its the snap looking thing on the top of the tach side, what exactly does it do because well I dont have one ha.

The fuel gauge "ground" stud on the back of the cluster is threaded so I could easily just add a ground wire, unless theres I need the IVR thing...

oh poop.

well the circuit board is make of copper filament glued to the Mylar, you should be able to solder a wire to the ground on the board or just wire directly off the fuel gauge studs to run a new ground or make a connection to the harness connector and tap in to get a fuel reading.

that is quite an expensive hack job there replacing the circuit board would be fruitless since you would end up hacking a new one up.

i would go with what you have and by pass it.

now you will need to run the fuel gauge through the IVR or hack something together i believe the gauge uses less then 12 volts to power it, i think 6 not sure.

You need an instrument voltage regulator (IVR). What it does is to pulse the 12v at such a rate that it is effectively 5v. If you try to run your fuel gauge straight off 12v, it will read past full for a little while until the gauge burns up. Personally, I would just spring for a new circuit card and IVR. Yeah, spending the money sucks, but it will be fixed right. They make electronic replacement IVR's now that provide a constant 5v, but I can't vouch for their reliability, as my original still works fine.

You need an instrument voltage regulator (IVR). What it does is to pulse the 12v at such a rate that it is effectively 5v. If you try to run your fuel gauge straight off 12v, it will read past full for a little while until the gauge burns up. Personally, I would just spring for a new circuit card and IVR. Yeah, spending the money sucks, but it will be fixed right. They make electronic replacement IVR's now that provide a constant 5v, but I can't vouch for their reliability, as my original still works fine.

Click to expand...

Well my next question is who sells em? I can't seem to find anyone who supplies them

Mustangs unlimited carries both the Regulator and circuit board.

So does NPD.

Im a bit stuck on what to do. let me get some advice from you guys.

I want to stay with the stock look, I dont want a 9'' tach mounted to my column but I do want accuracy so thats why I went with a after market autometer that looks "close" to stock mounted in the factory position.

I dont want a hack job behind my cluster. So here are my options:

1) replace the speedo/tach/fuel cluster with a "new" used one and get a factory tach (accuracy??)

2) order a printed circuit and IVR for mine keep the aftermarket tach

(but live with giant gaping hole in the rear of my tach housing, that bothers the crap out of me already)

I think that my mach had the idiot lights and not a tach (is there a way to tell for sure?) so if I order a tach cluster i probobly wont have the correct wiring under the dash anyway so i would run into problems there. Since the whole back side of my tach cluster has been "removed" the new printed circuit would have nothing to mount to..

Ive been on the fence for weeks about this and just want to get my fuel gauge workin no matter what the cost. what would you guys do?

Thanks!


as you can see in the picture at the top of the page, (i just noticed this) a guy can unbolt the rear of the tach housing. 3 bolts and I can just replace the rear of the tach housing. My only problem now is finding a new one.. any ideas? anyone have one??

Well I searched around and came across a complete used dash assembly, complete printed circuit and and IVR

I plugged it in for fun to see if and what worked and I have speedo lights and thats it, Still no fuel gauge. Didnt really care if it all worked cause I got it pretty cheap and I needed to replace the rear of my tach side that the P/O hacked up anyway.

Can I test the IVR??

I guess you can use a volt meter across the terminal