Topic: Autopsy And Anatomy Of A Fuel Sender -

[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]

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?

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.

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

Randy. Got any ideas?
Jim

[Bob Gaines]

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

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.

[jwc66k]

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.

As I said, it's all steel, so grafting requires welding, probably braising. The "replacement" guts would be best obtained from a new unit. Then you are still faced with the mounting disc's finish no matter what. For this one, with a worn out wiper contact (it is copper or brass), soldering a new copper contact to the worn out one is intriguing. I'll have to visit one of the surplus electronics warehouses (it is Silicon Valley) and see what's available.
Jim

[Bob Gaines]

As I said, it's all steel, so grafting requires welding, probably braising. The "replacement" guts would be best obtained from a new unit. Then you are still faced with the mounting disc's finish no matter what. For this one, with a worn out wiper contact (it is copper or brass), soldering a new copper contact to the worn out one is intriguing. I'll have to visit one of the surplus electronics warehouses (it is Silicon Valley) and see what's available.
Jim

Just in case you misunderstood I was not talking about trying to repair the contact tonque or the windings  . Although they could be repaired in some fashion, the time and possibly the expense would exceed the much more simple task (relatively speaking ) of transferring the guts of another sender (new or used ) over to the old sender to keep the outward appearance of a assemblyline C4,C6,67 etc. sender. This certainly wouldn't be cost effective on a car where concern about assemblyline originality was not as much of a  concern.

[jwc66k]

Just in case you misunderstood I was not talking about trying to repair the contact tonque or the windings  . Although they could be repaired in some fashion, the time and possibly the expense would exceed the much more simple task (relatively speaking ) of transferring the guts of another sender (new or used ) over to the old sender to keep the outward appearance of a assemblyline C4,C6,67 etc. sender. This certainly wouldn't be cost effective on a car where concern about assemblyline originality was not as much of a  concern.

In order to do what you suggest, you would have to destroy the "donor" and the "patient" units. All the component pieces are spot welded together. The fuel outlet tube is flared for a rubber extension and will not slide thru the mounting disc. The tube appears to have a sealing ring as well. The tube inside has brackets and the resistor housing bracket welded to it.
A closer look at the wire wound resistor shows two wires wrapped around a tapered form terminated at the ground end. The wires' gauge appears to be 36AWG, that 0.005 inch. Have you ever worked with wires that small? I have, even smaller. (Look up "Litz Wire", 7/44ga.)
For this particular fuel sender, with it's wiper contact worn out, finding a suitable replacement contact is about all the can be done, or needed. Other failed senders will have to be considered on an individual basis.
Jim

[Bob Gaines]

In order to do what you suggest, you would have to destroy the "donor" and the "patient" units. All the component pieces are spot welded together. The fuel outlet tube is flared for a rubber extension and will not slide thru the mounting disc. The tube appears to have a sealing ring as well. The tube inside has brackets and the resistor housing bracket welded to it.
A closer look at the wire wound resistor shows two wires wrapped around a tapered form terminated at the ground end. The wires' gauge appears to be 36AWG, that 0.005 inch. Have you ever worked with wires that small? I have, even smaller. (Look up "Litz Wire", 7/44ga.)
For this particular fuel sender, with it's wiper contact worn out, finding a suitable replacement contact is about all the can be done, or needed. Other failed senders will have to be considered on an individual basis.
Jim

Jim,no need to get combative because I will gladly explain what works best for me.  Maybe you haven't done a transplant but I have on numerous occasions with success.  If I couldn't get the the sender working by removing corrosion /varnish from the contact or if the wire was broke internally then I did the trans plant.  I said before that the wire or contact could be repaired in some fashion . For me it is easier and faster to do the transplant. Of course you destroy the donor. That is why it is a donor. That is also why as I mentioned it is not cost effective unless you are trying to save a otherwise nice original sender because of the engineering number. I have done it two ways you can ether break it loose from the spot welds and re tack weld or solder the rheostat back in place or what I have found much easier is to cut the tube of the donor sender and the contact ribbon coming from the terminal on the tank side and splice the tube and ribbon to the sender you are trying to save. You have to do your measuring on cuts etc. so as to get close to the same orientation as the old sender once you are done but that is the basic description.  It takes two to make one good one and only makes sense in my case when I have a nice un pitted C4 etc. sender to save for concours use. Just what has worked for me.

[KevinK]

Have you looked into the suppliers who refurbish these? Or do they not work on them?

I know on the mid 80s ones before the repro came out, the wire wrapper resistor,  was a cerimic board which could be swapped out and was common to other models.

[jwc66k]

It takes two to make one good one and only makes sense in my case when I have a nice un pitted C4 etc. sender to save for concours use. Just what has worked for me.

Bob,
I think your method has merits. I would like to see one done. Why don't you do your "two to one" technique, take some pictures, write it up and post your procedure. There are more than one bad sender out there.
Jim

[Bob Gaines]

Bob,
I think your method has merits. I would like to see one done. Why don't you do your "two to one" technique, take some pictures, write it up and post your procedure. There are more than one bad sender out there.
Jim

I will add it to my to do list.

[jwc66k]

I will add it to my to do list.

We all wait breathlessly - 
Jim

[jwc66k]

A try at resurrection -
I got ambitious and got to work on trying to fix the worn out fuel sender contact. It can be done. Here’s what I did. The worn out contact was copper. I determined this with a magnet. It also appeared to be a hard copper alloy, not easily bent, but flexible, springy and thin. There may have been something else attached to the spring part that was actually the contact. As it is copper, solder should work.
For a replacement wiper contact I first considered the contact used in large wire wound resistors.
After the third electronics surplus store (this is Silicon Valley), I found absolutely no wire wound resistors with a large enough electrical capacity. Almost everything is surface mount technology now.
Other things to consider: use 600 grit sandpaper and a fine watch file to clean the area to be soldered; use 60-40 solder (do not use acid core); pre-tin all copper surfaces to be soldered; use a soldering fixture.
Sit down, grab a beer and think. Where do I get hard copper without spending a fortune – ah – off to my junk box (I have a couple dozen).
Bingo!
I found a ground contact strap, C6MA-18870-A, that was used on 69 and 70 Mustangs to ground the hood to suppress radio interference. Pic 1 - It was stiff and small enough to work if I snipped off the tip. It was also slightly curved so it would not score the resistance wire in the fuel sender – Pic 2 and 3. The arrows in Pic 3 point out the wire winding, the rusty steel strap from the wire winding to the connector pin in the mounting plate and a small spring inside the wire resistance housing to provide tension on the contact arm and additional electrical contact. The rust is causing most of the electrical problem as it does not conduct electricity.
The sanded spring contact – Pic 4 – 600 grit and watch files to rough up the surface for soldering. I used the same 600 grit sandpaper to remove “crud” from the resisting wire winding.
Pic 5 is the soldering iron I used. It is rated at 130W, and dates to World War II when it was used in radio manufacturing at a company I worked for in the mid 60’s. The holding fixture is a copy of the ones used to assemble components.
Pic 6 is my work area. I use my table saw as a workbench which is located in a shed alongside my garage. The strange light is from corrugated clear plastic sheets that make up part of the roof.
Pic 7 is the contact soldered in place. I “tinned” both pieces then soldered them together. The contact was subsequently moved back about 1/8 inch as it was too long. It was sanded with 600 grit to give maximum electrical conductivity.
Pic 8 is the finished project, the spring on the shaft, the new contact (before it was moved back and re-soldered) the spring portion and a rough sketch of the contact – in black, the vertical shaft, the hard copper spring, its bends and its staked on attachment, and in yellow, the added on new contact. 
Now for the bad news. The contact does not move freely with movement by the arm with the float can. It does move a lot better now, but still is sticky. The “stickiness” is from rust, the shaft into the wire resistance housing (the one with the small spring). The final tests utilized an ohmmeter to check resistance as the float arm was moved. It read 25 ohms at full and 75 ohms at empty, functional but not acceptable.
Conclusion. This type of repair can be done. What caused the failure was the contact material selected and the amount of rust on almost every steel section, and dried “varnish” from gas on the resistance wire.
The transplant method may be a better method (hint Bob).
Jim

[Bob Gaines]

Interesting detailed post . It sounds like you got close. It makes me think of the Edison quote "I have not failed. I've just found 10,000 ways that won't work." I for one appreciate the effort.