Restoring - General discussions that span across many different groups of years and models > Misc Items

Autopsy And Anatomy Of A Fuel Sender -

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jwc66k:
My gas gauge don’t work. How many times have you heard that one? This dialogue is about 64 to 66 Mustangs. It may apply to other years. The low fuel circuit on other years is totally different.

There’s a couple of tests you can do to figure out what the problem is – the gauge, the sender or the wiring. Jack up the car so you can get access to the front of the gas tank. Disconnect the sender lead at the tank and ground it to the car’s body. You can do this with a number 10-32 X 3/4 (or longer) screw. Touch the head of the screw to a scraped off part of the body. With the ignition switch on, or to accessory, the fuel gauge should read full. Put 12vdc from the car battery to the lead and the gauge should read empty. That test is a bit more difficult to do as you need a 12 to 15 foot wire with a clips. Use the same screw. Don’t put 12vdc to the sender, it won’t tell you anything. A wiring fault electrical check will show nothing. That may include the gauge. It’s best to do a wiring check with an assistant.
If you have the fuel sender out of the car, you can do a resistance check using a good old ohmmeter. First you got to figure out where the float arm is when the tank is empty and where it is when full. Hold the sender as it would be when installed in a tank, the fuel pickup will be down. Slide the float arm up for full, then down for empty. The float arm should slid relatively free, with maybe a slight rubbing noise from the housing. With one lead of the ohmmeter on the screw terminal on the outside of the sealing plate, and the other on ground – the plate itself is a good source for ground – take some readings. Empty should read about 75 to 100 ohms, full about 10 ohms.
There’s one more “test” you need to do with the sender. It requires at least two sets of hands, or ohmmeter leads with alligator clips, one for ground, one for the terminal outside. Read the resistance when the float would indicate a half tank. That would be about halfway. Moving the float arm will change the resistance reading on the meter as it moves.

If you get no resistance reading, check to see if the meter is on and the meter settings. Touch both leads together. The meter should read zero, or very low like 1 or 2 ohms. Now check where you put the leads on the sender. Trained technicians do this all the time. If you are satisfied the problem is not meter or operator related, a no resistance reading on the meter indicates that the resistance wire inside is busted. In technical terms – open.
There’s another situation where your readings are 75 to 100 ohms all the time, full, empty or half, the tests are almost always the same. That’s a sign there’s internal problems. The wiper arm is not making contact with the resistance wire.
Now into theory of operation.

The fuel sender is a simple variable wire wound resistor, one of the oldest designs of variable resistors. (There is a resistance wire from the ignition switch to the coil but it is not a variable type.) One end is usually attached to a fixed voltage, the opposite end “floating”, and the wiper is adjusted to a specific required resistance which gives a lower required voltage at the wiper arm. More resistance, less voltage – less resistance, more voltage.

There are variations in hooking up the resistor.
The variation used in the Ford fuel sender has the wiper arm attached to the float that moves in accordance to the fuel level in the tank. To reduce the possibility of a spark in a gas and gas vapor filled tank, and it would not take much of a spark, the “floating” end is grounded. The wiper arm is also grounded. As the wiper moves up the resistance wire, the resistance drops. That’s how you get a full reading. As the wiper moves down the resistance wire, the resistance increases. That’s how you get an empty reading.

See the rough schematic in picture “FuelSender08Schematic”.

Now to other pictures:

FuelSender01 – A genuine non-operating Ford Mustang Fuel Sender C4ZF-9275-B. As you can see there is a lot of rust. Almost everything inside is made from steel. In a tank with no fuel, or very little, condensation builds up, and that causes rust. In a functional car, keeping the tank full all the time prevents rust, even in winter.

FuelSender02Tabs – These three tabs hold the cover on the resistor housing. Remove them to see the insides. I used a medium pair of diagonal wire cutters to get under the tab’s crimped edge and bend the tabs out of the way. Remember, they are steel.

FuelSender03Open – From left to right, the arrow points to the strap, that is the internal electrical connection from the external terminal to the resistor wire in the housing. It is bent out of the way. It’s a heavy duty electrical connection. The next arrow points to the resistor wire housing. The last arrow points at the wiper arm contact.

FuelSender04Wiper – This is the part that slides on the resistor wire. As you can see, it is worn out. Note the staked on arm, the “X”. I assume it’s a copper alloy. It is non-magnetic. Difficult to repair.

FuelSender05Winding – The resistance wire. It’s wrapped around an insulator. Note the changing width. The white at the left is the insulator for the heavy duty strap (pic 3). The rivet on the right end, partially hidden behind the tab, is the resistance wire ground.

FuelSender06Schematic – This portion of the fuel circuit schematic is from the 66 Mustang Electrical Assembly Manual. Note that the lower end of the resistor symbol (red arrow) is NOT grounded. This is a bad practice on the part of Ford. If the sender were wired like this, it would explode. The same drawing flaw is in the 67 Electric Manual. The 64 and 65 Electric Manuals just show a “box” labeled fuel sender.

FuelSender07Schematic – This portion of the fuel circuit schematic is from the 66 Ford Mustang (plus others) Service Manual. It shows a very tiny electrical connection symbol at the bottom of the resistor. If you weren’t looking for it, you wouldn’t notice it.

FuelSender08Schematic – A basic schematic of a variable resistor with reference to the text above. As you can see, moving the wiper to the left shorts out the resistance wire to the right which gives a low resistance signal to the gauge. Move it to the right and a full resistance signal goes to the gauge.

Jim

midlife:
That's how I understand it as well.  Good job!

DM_1964:
excellent detailed write up and a really interesting read there Jim.

I'm interested in Pic 4, you mentioned it's difficult to repair, so there is a way to replace the worn out metal piece either by building it up by welding or welding a new piece on?

jwc66k:

--- Quote from: DM_1964 on June 26, 2017, 10:09:54 PM ---excellent detailed write up and a really interesting read there Jim.

I'm interested in Pic 4, you mentioned it's difficult to repair, so there is a way to replace the worn out metal piece either by building it up by welding or welding a new piece on?

--- End quote ---
Thanks. I've always wanted to take one apart - actually, I did decades ago, pre-digital camera era.
As long as the wiper contact is copper (probably an alloy but what) a repair contact can be soldered to its "remains". The "remains" have some spring action remaining so it might be possible. The question is what an original looks like.

--- Quote from: midlife on June 26, 2017, 09:59:48 PM ---That's how I understand it as well.  Good job!

--- End quote ---
Randy. Got any ideas?
Jim

Bob Gaines:

--- Quote from: jwc66k on June 27, 2017, 12:06:08 AM ---Thanks. I've always wanted to take one apart - actually, I did decades ago, pre-digital camera era.
As long as the wiper contact is copper (probably an alloy but what) a repair contact can be soldered to its "remains". The "remains" have some spring action remaining so it might be possible. The question is what an original looks like. Randy. Got any ideas?
Jim

--- End quote ---
If the metal tongue is wore out or there is a break inside the windings somewhere it is simpler to graft in replacement guts from a donor sender to save the original assemblyline one IMO.

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