Fuel Gauge Wiring - Vintage Mustang Forums

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post #1 of 12 (permalink) Old 03-20-2017, 11:15 PM Thread Starter
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Fuel Gauge Wiring

I own a 1968 Mustang Coupe. When i bought it, the guy told me that the fuel gauge is off half a tank. So at half, its full, and it moves down to empty then back up to half a tank to indicate it being empty. You probably have no idea what i just described... Thats how confused i feel with this problem. so i took action. First thing i did, which i heard was the most common problem, was replace the in-tank sending unit. and when i did that, it stopped working all together. which was super frustrating. so after a couple of days scratching my head, not knowing where to go, i decided to follow the wire from the bottom of the tank and see whee it goes. So i started under the car and followed it to my trunk, and somehow, lo and behold, it ends there. it isnt connected to anything. and i have no idea where the other part of the wire is. and I've looked behind the dash to see if i could follow it that way but its a huge freakin wad im not messing with that. where do i start? is there a wiring kit i can get JUST for the fuel gauge? is there a diagram that would help me know exactly where this mire goes? anything will help really... Thank you
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post #2 of 12 (permalink) Old 03-20-2017, 11:40 PM
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The in tank pickup supplies ground to the gauge, or partial ground as it is. There is a resistance that causes the gauge needle not to go all the way, in most cases other than full.The proper wire coming from the gauge us run along side/through the rocker panel. You just need to find the cut wire and reattach. Just so you know, aftermarket pickups are known for not working well themselves . . so after finding the wire and fixing,m you still may not be happy.
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post #3 of 12 (permalink) Old 03-21-2017, 12:08 AM
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I can't find a wiring diagram for 1967 but in 1965 and 1966 it was a yellow wire with a white stripe that ran from the fuel gauge to the sending unit and Ford was pretty consistent with wire colors back then.
In my '65 Fastback the fuel wire splits off from the bundle containing the taillight and brake light wire bundle right at the left side wheelhouse, runs across the car to the right side, goes through the trunk floor and then back to the sending unit. Look for a single wire that is YEL/WHI loose in the trunk.
If you can't find the loose wire in the trunk then as Pete says, pull off the driver's door sill plate and look for a YEL/WHI wire in the bundle of wiring there.
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post #4 of 12 (permalink) Old 03-21-2017, 02:26 AM
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Try averagejoesrestorations.com 1968 convenience section. Good Luck
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post #5 of 12 (permalink) Old 03-21-2017, 08:00 AM
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Originally Posted by 2+2=GT350 View Post
Try averagejoesrestorations.com 1968 convenience section. Good Luck
I don't see any fuel gauge wiring in that schematic.
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post #6 of 12 (permalink) Old 03-21-2017, 08:53 AM
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Thats only half of the curcuit. http://averagejoerestoration.com/wp-...nstruments.jpg

On another note trying to get a read on the low fuel light relay is tough.

1969 Mach 1 S code...2nd owner
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post #7 of 12 (permalink) Old 03-21-2017, 07:32 PM
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Sorry here is both pages from Joes
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post #8 of 12 (permalink) Old 03-21-2017, 08:02 PM
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If it's like a '65-66, the wire comes up into the trunk on the right side and runs across the trunk floor to the left side, up the inner wheelhouse and down through the quarter panel void under the quarter glass, up the channel under the door sill plate, in to the kick panel and up to the instrument cluster.

The fuel gauge, itself, receives power (5V) directly from the IVR (instrument voltage regulator) and has an 18 ohm (approximate) thermal element, which is connected to ground through the sending unit, which is, itself, a variable resistor. When the fuel tank is empty, the sending unit's variable resistor produces the most resistance which limits current flow through the gauge and doesn't allow the thermal element to generate much heat. When the tank is full, the variable resistor in the sender produces almost no resistance, allowing a lot of current (relatively) to flow through the gauge's thermal element which produces more heat. The heat inside the gauge acts upon a bi-metal spring that bends, proportionally, to the amount of heat given off by the thermal element, thus moving the needle.

You can test the fuel gauge either in or out of the car in a number of different ways. The easiest is to pull the instrument cluster then orient it so you can plug it back in, turn the key to the "on" position (don't leave it this way for more than a few minutes to avoid damage to the ignition coil) and using a jumper wire to ground the negative (sending unit) side of the gauge. The needle should immediately move to the "FULL" side. If the needle doesn't move then the gauge is suspect. You can also test across the gauges terminals with an ohmmeter, looking for the aforementioned approximately 18 ohms.

Testing the wire from the gauge to the sender is done along the same lines, although you don't have to remove the cluster to do so. Turn the key "on", again not leaving it in that position for more than a few minutes, and using a jumper to connect the sending unit lead (at the right angle plug) to a known good ground. Again, the gauge should immediately move to the "FULL" side.

Testing the sender (something which almost everyone who has installed a sending unit now knows to do prior to installation) is done with the sender out of the tank by connecting an ohmmeter to the sending unit post and to the body of the sending unit and moving the float arm through its travel which should result in a movement of the ohmmeter between approximately 73-0 ohms. I like using an analog meter here to see any fluctuations in the meter reading indicating a shorted or open winding in the sending unit's variable resistor.
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post #9 of 12 (permalink) Old 03-21-2017, 08:48 PM
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Question for Bart:

With these new aftermarket senders that don’t cause even a good dash gauge to read properly, is there any way one can tweak either the rheostat or the gauge (adding resistance, shorting rheostat coils, whatever?) to cause the system to show accurate indications. I just replaced a 1977 Ford van sender for a fellow and now all his readings are just at the bottom of the gauge (gauge itself tested good). Feel I made matters worse!
Just can’t imagine some vender hasn’t reproduced the same design rheostat as the original Ford senders. They’d make a zillion sales the first week!

Last edited by 70Machboy; 03-21-2017 at 08:48 PM. Reason: spelling
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post #10 of 12 (permalink) Old 03-21-2017, 08:58 PM
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Quote:
Originally Posted by 70Machboy View Post
Question for Bart:

With these new aftermarket senders that donít cause even a good dash gauge to read properly, is there any way one can tweak either the rheostat or the gauge (adding resistance, shorting rheostat coils, whatever?) to cause the system to show accurate indications. I just replaced a 1977 Ford van sender for a fellow and now all his readings are just at the bottom of the gauge (gauge itself tested good). Feel I made matters worse!
Just canít imagine some vender hasnít reproduced the same design rheostat as the original Ford senders. Theyíd make a zillion sales the first week!
There's some new solid-state "gadgets" out there to calibrate the senders to the gauges now to provide that "non-linear" effect that was built into the OEM sending units. Perhaps the biggest problem is that the same variable resistor is used in a whole TON of fuel sending units, and being exactly linear in their operation.... meaning that the resistance is directly proportional to the movement of the variable resistor's input where the OEM senders were non-linear and calibrated to take into account the various fuel tank shapes and sizes.

If the sender can't be "redneck calibrated" by bending the float arm to get better readings and is simply "off" in it's resistance values, a resistor could be added to the circuit if the gauge reading is too high, but if it's too low you're kind of "hosed".

Also, bear in mind that there is some level of adjustment that can be made to the gauge through the rear to set the high and low limits of the needle.

Bart

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post #11 of 12 (permalink) Old 03-22-2017, 03:01 AM
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Thanks for the info, Bart. I'll be trying a little of everything and if I can make something work half way decently I'll report it so others can glean from it.
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post #12 of 12 (permalink) Old Yesterday, 02:25 AM
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on my 69 Fastback, the fuel gauge wire as it runs under the door sill is thicker than all the rest. No idea if it was always that way or if a previous owner ran a new wire
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