A REAL Schematic

[Midlife 814]

I'd love to see a true schematic for Ford's low fuel indicator/relay system.  Hint Hint, Nudge Nudge, You Know What I Mean?

[Mach1 Driver 560]

Mid, was it used on a 69 Cougar? That sounds like it might be one of their options. If so I have the Cougar wire diagram included with my Mustang info. and could take a look...lets see is there something we could barter for? ;)

[Midlife 814]

Oh I am very familiar with the wiring, but how the relay works for the dash cluster lamp is fairly straightforward, but it is the proof-out circuit that is somewhat baffling.  There's a resistor wire involved that makes it difficult to decipher. 

[Mach1 Driver 560]

OK Mid, but back to my original question- was it an option on a 69 Cougar? Your reply seems to indicate it was. There is a page of the diagram titled "1969 Cougar XR7 Ignition, Starting, Charging, Gauges, Convenience Prove-out and Tachometer". There is nothing in the body of the diagram that states "prove-out", but there is part of a circuit containing a low fuel relay. The remainder of the circuit is presumably on another page. Is this what you are interested in?? I do see a resistor (it could be a wire instead of a physical resistor) across the relay plug. This diagram is 37 pages long and the fuel relay is on several different pages so I'm not getting a complete picture of what this thing does yet.

Give me some clues- does this circuit have a fuel gauge and a low fuel light or something? If so I haven't found a light yet. This crazy diagram doesn't have everything on one page so it is difficult to find things if you don't already know they should be there.

[Midlife 814]

Low fuel indicators were found on higher-end Ford products: Thunderbirds, Cougars, etc.  What you see on the page you reference is all I have as well.  Why do I think it has a prove-out circuit?  I believe the resistor wire is there to provide a signal when power is first applied and allows the signal to degrade with time, allowing the light on the dash cluster to go out.  A line also comes from the starter side of the starter solenoid, providing a signal that the engine is starting.

There's a thermistor in the sending unit that senses liquid by cooling the thermistor.  The signal is carried to the relay, and then transferred to a low fuel indicator lamp on the dash cluster.  Power is provided by an ACC fuse.  The resistor wire has been measured to be about 45 ohms, a very short wire, perhaps 4" in length, that is very fragile and burns out or broken most of the time.  That's about all I know.

[Mach1 Driver 560]

Wire 366 goes from the relay to the firewall plug and then disappears on pg 2-E13. However, on page 2-E17 it shows 366 connecting to red 18 and says it goes to the starter motor relay and (power source) is indicated.  I don't see that connection to the starter motor relay on any other page. So where on the relay? That sounds like B+ to me- you know, the large lug on the left side.

[Mach1 Driver 560]

Thermistors come in two varieties. The one used here is a NTC, or Negative Temperature Coefficient. As the temperature goes up the resistance goes down, and conversely as the temperature goes down the resistance goes up. Power is applied to the thermistor through a parallel network of the 45 ohm resistor wire and the relay coil. When the thermistor is submerged in fuel it is cooled and its resistance goes up enough that the relay cannot pull-in. When the thermistor is no longer covered by fuel it will heat, and its resistance will go down, allowing the relay to pull-in.

It isn't clear on the wire diagram where the NC contact of the relay is connected, as all it says is the power source is the starter solenoid. It must be one of the two places indicated by the dashed line. When the starter is cranked the low fuel light will come on- just to show the light is operating.

The references I've found to this circuit say it was often disabled at the dealership (usually by removing the relay) because the light would annoying flash going around corners or when the fuel level was close to the thermistor.

Please see the attached. Edit, I noticed I left the 20A fuse out- it is positioned between ignition switch A and the relay.

edit, the picture fairy has deleted some stuff so I reposted the latest version

 

Cougar low fuel light.pdf

[Midlife 814]

Nice!  Power comes from the starter side of the solenoid, not the NSS line.  The red wire ends up with a ring connector that goes underneath the starter cable.  There's no line going to the output of the NSS. 

What I don't understand (at least on your diagram) is what actually moves at the NO and NC positions.  It looks like the lamp indicator switch moves, but that can't be correct, as that is entirely a passive line.  The coil/relay system must provide voltage to the circuit when the thermistor's resistance goes low enough, so there must be an internal switch within the relay itself.  BTW, the relay is mounted on a metal frame along with other circuit breakers, relays, etc. 

[Mach1 Driver 560]

OK good, you can disregard the dashed line running to the left. Although this would have worked, it just wasn't the way they did it.

Mid, you are looking at the internal switch of the relay: the NC, NO, and C contacts (although common was not labeled), but I guess I could have drawn it better. The symbol for the relay is what is normally depicted in a schematic for a SPDT relay, but again, I could have drawn it better. The Low Fuel Light is connected to the common which is NC (normally closed) to the top contact.  "Normal" is defined as the condition of no power to the relay coil. When there is sufficient  power to the coil, the relay coil magnetically pulls the common contact down, first opening the NC contact (normally closed), and then closing the NO (normally open) contact.

Instead of buying a more expensive relay with a custom coil, they used the 45 ohm wire to "tune" the circuit so that it has just the right amount of current flowing in the relay coil to make it operate when the thermistor is covered by fuel, or when it is not covered by fuel.

I have redrawn the relay to show both the powered and unpowered states in the new attachment

edit, the picture fairy has deleted some stuff so I reposted the latest version

Cougar low fuel light.pdf

[Mach1 Driver 560]

this shows the three basic states in the Cougar low fuel light system

edit, the picture fairy has deleted some stuff so I reposted the latest version

Cougar low fuel light.pdf

[Midlife 814]

Very good, and interesting!  So the line going to the starter side of the solenoid has only two functions: applying 12V when cranking as a prove-out circuit, and when not cranking providing no voltage to the lamp.  Not a great design, as that red wire is not fused.  I suppose one could have tapped into the red/blue starter/NSS line, but that is also unfused.  I was under the impression that the resistor wire performed a timer function for the prove-out circuit: once energized, it would stay on for say 10-15 seconds.  That resistor wire is incredibly delicate and most are broken or have no continuity.  I cannot repair them, unfortunately, and they are not reproduced.

As a final suggestion, it might be nice to place a box around the low fuel relay components itself, which would clarify the innies and the outies to that relay.

[Mach1 Driver 560]

13 hours ago, Midlife said:

That resistor wire is incredibly delicate and most are broken or have no continuity.  I cannot repair them, unfortunately, and they are not reproduced.

Wire a resistor in to replace the wire, go to mouser.com: 71-RH02545R00FE02. They are $4.07 each, 45 ohm +/-1% 25w. You only need around 10w but that was the only one my limited search revealed that would absolutely work, and you can buy only one if you like. Some have a 500 minimum. More wattage is good, but not enough will just burn-up. For it to properly dissipate the heat it should be screwed down to the dash sheet metal somewhere, so just make the wires long enough to do so. Heat shrink the ends but don't cover the cooling fins. The body looks to be about .75" long and the base is .8" across, with total length around 1.375" across the leads; if I'm reading their charts correctly. That's a big baby! Using a resistor wire only cost Ford a few pennies.

[Midlife 814]

5 hours ago, Mach1 Driver said:

Wire a resistor in to replace the wire, go to mouser.com: 71-RH02545R00FE02. They are $4.07 each, 45 ohm +/-1% 25w. You only need around 10w but that was the only one my limited search revealed that would absolutely work, and you can buy only one if you like. Some have a 500 minimum. More wattage is good, but not enough will just burn-up. For it to properly dissipate the heat it should be screwed down to the dash sheet metal somewhere, so just make the wires long enough to do so. Heat shrink the ends but don't cover the cooling fins. The body looks to be about .75" long and the base is .8" across, with total length around 1.375" across the leads; if I'm reading their charts correctly. That's a big baby! Using a resistor wire only cost Ford a few pennies.

Aww...can't do that because I don't have access to the sheet metal---only the bare harness.  I do have resistor wires, but it is 7.5" per 15 ohms, and to make 45 ohms, that's a lot of wire to wrap!  Plus, I don't know how much current is drawn and the resistor wire resistance vs. current curve.  *sigh*

[aslanefe 333]

While we are on the low fuel warning circuit, how can I check the sensor on my project 70 XR7's fuel sending unit to see if it works? Looks like just checking continuity between the sensor connection and case of the sending unit with and without submerged in liquid is not the right way. As far as remember, the sensor is soldered to the sending unit and only one wire coming out of it.

[bigmal 225]

I know this is getting off topic but I have a question re the fuel sender wire that runs to the instument panel. I replace the wire from the point under the door latch forward as it was damaged. I am now getting a little nervous that this may be a resister wire. Is that corect or  is it a conventional wire?

[Mach1 Driver 560]

You could connect an one ohmmeter lead to the mounting flange, and the other lead to the wire going to the thermistor. Take a reading, then put an ice cube on the thermistor and since it has a negative temperature coefficient, the resistance should go up. The thermistor is in the 30-50 ohm range. That's a spooky circuit to me- you are putting 12v and almost half an amp into the gas tank. That makes me wonder just how safe it is (as in go boom). There is a guy that makes a circuit that works off the fuel sensor that he says won't give you the annoying blinking light when the sloshing fuel approaches the thermistor, as is so common with the Ford circuit. He is on one of the Cougar forums- Google: "Cougar low fuel light" and you should find it.

[Mach1 Driver 560]

3 minutes ago, bigmal said:

I know this is getting off topic but I have a question re the fuel sender wire that runs to the instument panel. I replace the wire from the point under the door latch forward as it was damaged. I am now getting a little nervous that this may be a resister wire. Is that corect or  is it a conventional wire?

There is a 10 ohm resistor wire between ignition switch terminal A and the constant voltage regulator, but nothing (no resistor wire) between the fuel gauge and the sender. It is a conventional wire.

[Midlife 814]

The resistor wire I was referring to is a short, 4-5" affair, that crosses two pins on the relay plug.  It is covered in asbestos cloth, and is hard to miss.  Bigmal: you're A-OK.

[bigmal 225]

Thanks guys. I feel a lot better now

[Midlife 814]

Bigmal: have an adult beverage on us!  That'll make you feel a whole lot better!

[RPM 1,190]

Heck, it'll make me feel better after reading all that!

[bigmal 225]

3 hours ago, Midlife said:

Bigmal: have an adult beverage on us!  That'll make you feel a whole lot better!

Consider it done. Starting with Jack and ending with Daniels. Maybe more than one.

[BuckeyeDemon 211]

really nice job.

[Mach1 Driver 560]

I see many people asking how to connect to switched power for the latest do-hickey they want to add, so I added Page 5 up above in the first post. It shows the fuse panel and the wires associated with it. There are three fuses currently connected to the ignition switch that are switched on in "Accy" and "On". They are 1, 2 and 3. On my Mustang the 20A #2 fuse is totally unused and goes to an accessory plug that is just hanging there waiting to be used.

[mackseal 10]

Thanks for re-posting this. It will help greatly. Since I have none of the options listed for fuse 2, I guess that will be the best and easiest location to wire the EFT power source to. Is there a method or devise recommended to plug a pig tail directly into the fuse well (instead of tapping into or splicing the wire coming off it)?

Thanks gain for you help.