1969 Fuel Tank Sending Unit Access | Ford Thunderbird club group 1955-2005 T-Bird models

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1969 Fuel Tank Sending Unit Access

  • Thread starter Thread starter RichV13
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RichV13

RichV13

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Thunderbird Year
1969
My fuel gauge is pegged past "E". My research points to the wire from the unit to the gauge or the fuel sending unit as most likely issues. Is there any access to the fuel sending unit or its first connector either under/behind the rear seat or under the carpet in the trunk?

Thanks,

Rich

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On these older vehicles I think the only way to check of replace the float and sending unit is to drop the tank and do it as it's usually on the top of the tank. Make sure you're getting power to the sending unit wire before you do anything else. Might check the power in the wire going to the gauge also. Take your hand and hit on the gauge to make sure it's not stuck. Does the gauge flicker or move slightly when you turn the key on or off?
 
DEVIL DOG 62 said:
On these older vehicles I think the only way to check of replace the float and sending unit is to drop the tank and do it as it's usually on the top of the tank. Make sure you're getting power to the sending unit wire before you do anything else. Might check the power in the wire going to the gauge also. Take your hand and hit on the gauge to make sure it's not stuck. Does the gauge flicker or move slightly when you turn the key on or off?
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I've not done anything at the dash side. The gauge doesn't budge at all when the car is started/running or off. Right now the car is winter prepped for storage so I'm collecting suggestions for the sprig. It also has a full tank of gas (24 gallons).
 
Do your other gauges in the dash work?
The easiest thing I can tell you is to check for power at the dash gauge. I say this because the sending is a variable ground.
If you have power at the gauge ground the other side and it should read full. If not the gauge itself may need replacement.
If the gauge works the problem will be with the wire or sending unit.
 
You can access the sending unit and float without dropping the tank. I've done it a couple of times on my 68. It's located above and behind the rear axel. I is most likely a float issue as mine has been both times. Its a lock ring. You'll need a flat head and a hammer to get it loose.
 
I bought a sending unit, $86 on Mac's Auto with a Father's Day Discount. I'm assuming this is bad, but it's a reasonable cost for upgrade if nothing else. Any ideas on where to find the connector under the hood that connects the wiring from the fuel sending unit to the part of the wire harness that enters the firewall and ultimately lands on the printed circuit board? It appears on the electrical schematics that there are two connector blocks, one of which should be engine side of the firewall and the other which could behind the instrument cluster but prior to the 18-pin long connector that attaches to the printed circuit board.

I want to disconnect that part of the harness and test the wiring from that point plus the gauge. I should also be able to test the ground on the side that runs through the frame to the sending unit. If I had access to a lift, this could be over now. Just to add some fun, as I was looking around at vacuum lines, I came across a pair of loose wires that looked like they were pulled out of a connector. Are these my fuel gauge wires? Maybe. My head hurts from all the scratching.

Thanks,
 
Instead of doing all the searching for power to the sending unit under the hood just test it at the sending unit on the tank. Attach the wires that look pulled apart and see if that's the problem. I would check and see if your getting power at the sending unit on the tank first then trace it up along the frame rail if you don't. This is after you check if your fuse going to it is also good. Does the gauge flinch or move slightly when you turn the key on and off? It might also be a bad gauge
 
RichV13 said:
My fuel gauge is pegged past "E". My research points to the wire from the unit to the gauge or the fuel sending unit as most likely issues. Is there any access to the fuel sending unit or its first connector either under/behind the rear seat or under the carpet in the trunk?

Thanks,

Rich
Click to expand...
Rich, liftthe carpet in your trunk. The sending unit will have a wired mounted with a small nut. Ground this wire to the frame, turn the key on and your gauge should read full. If so, you probably have a hole in your float. If it is brass, solder it up after you have removed the gas.
If no reading, make sure you have voltage to the gauge, possible broken wire. Next, pull your clock and check the Instrument Voltage regulator. A small case with two wires on it. One is marked ignition, you should have voltage to that wire. With voltage, you should be getting around 8 volts on the second wire output. These stabilizers are notorious for failing. They reduce the input voltage and cause a pulsing meter reading. Good luck.
 
fasteddy said:
Rich, liftthe carpet in your trunk. The sending unit will have a wired mounted with a small nut. Ground this wire to the frame, turn the key on and your gauge should read full. If so, you probably have a hole in your float. If it is brass, solder it up after you have removed the gas.
If no reading, make sure you have voltage to the gauge, possible broken wire. Next, pull your clock and check the Instrument Voltage regulator. A small case with two wires on it. One is marked ignition, you should have voltage to that wire. With voltage, you should be getting around 8 volts on the second wire output. These stabilizers are notorious for failing. They reduce the input voltage and cause a pulsing meter reading. Good luck.
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Thank you for pointing me in the direction of the trunk carpet. My wiring was a bit different, but I found the wiring from the send unit enter into the trunk through a conduit of sorts. The ground wire was firmly attached to a frame member in the trunk via what looked like a spot weld. The yellow/white wire was visible, but I could not find the connector it was supposed to use to transition up to the passenger side dash. I was able to use a probe lead with a piercing probe on one end and a large alligator clip on the other. I connected the signal wire to ground as described and turned the key on. The gauge moved to full for what was probably the first time in a dozen years. I have a new send unit, now I just have to get the car lifted enough to change it. Much appreciated for the push in the right direction. Fortunately, I'm down to less than 1/4 tank.
 

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I have a new sending unit in place (along with a new tank - long story). I filled the tank and the gauge reads about 3/4 tank. Is there a way to calibrate the gauge, other than bending the needle? Would a different gauge read differently?
 
Apriliaalex said:
Following thread. My gauge reads about 1/2 when full. Any ideas what could be causing that?
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Increased resistance in either the signal or ground on the send unit.

Note that when you ground the signal wire of the send unit it should peg the gauge past full.

There is power from the instrument panel voltage regulator going to one side of the gauge. The other side of the gauge which I refer to as sending unit signal connects to the sending unit variable resistor, the other side of the variable resistor goes to ground.

As the level in the tank changes and the float an moves up or down, the resistance seen at the gauge decreases.

If additional resistance is seen by the gauge, the gauge will read a lower fuel level, half instead of full for example.

If the sending unit was replaced and is resistance range is greater than spec, you world see the same issue.

It is also possible to have either a bad gauge or voltage regulator. That voltage regulator isn't a solid state device, it's a mechanical bimetallic strip. You can find them online for about $100 shipped.

I hope that helps you with what you want to look for next. If you have the ability to lift the car, you can do some Adirondack diagnostics with the harness that plugs into the send unit.

There is also a chance you have a bad float and it has some gas in it. Not enough to sink the arm like an empty tank, but enough to prevent it from floating up to the full level.
 
Thanks for the tips. I was able to drive the rear of the car up on ramps to see if the sensor was orientated correctly. I noticed the other day it read 1/2 full (topped off tank so it was a full tank) and as I drive around it dropped down to almost empty. Once I turned the key off and back on it jumped right back to 1/2 tank. I do have the info center above the rear view mirror so the car does have the ability to have a low fuel light but when I replaced the unit I cheaped out and got one without a low fuel light sensor. Probably my problem.
 
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