Topic: 1967 convenience low fuel light remains on

[67gtasanjose]

I tried testing wire 366 at the relay by turning the ignition to crank and holding it.  I did not get any voltage. .

There is a connection in 366 where it comes through the cowl. It is a single bullet-style male connector. Try it at that location. CHeck continuity with an ohm meter perhaps.

... Looking at your latest diagram there is an internal bridge which should give power to 366 with just the key turned on but would this not give voltage to 366 back to starter solenoid?

NO. The relay isolates the two circuits. It can only make contact to one or the other points inside the relay.

  I will test wire 215 with wire 57 and see if there is power.  I checked the fuse and bulb and they are not blown.

215 is the OUTPUT voltage TO the low fuel lamp. It will only be hot (12V) in either crank position (relay installed) and/or if you ground 367 (relay installed) it will energize the relay and source 12V from the ignition source (7.5a fuse circuit 365A, across the bridge inside the relay).

[67gtasanjose]

Richard : In your second drawing, the way I am taking it, it is shown with the key in the "crank" position only. In the "ignition switch on" position, I would think that the contact would disconnect from 366, complete the circuit to the bulb. That way, when turning the switch to the "on" position, and being low on fuel, the bulb would light. I think I'll "mock" mine up a little sooner than I was going to, just to see what I can find (and hopefully not blow my Relay).

1.) When relay NOT energized, circuits are ready for bulb test only. (bulb lights when starter solenoid energized)
2.) When relay is energized (by grounding 367 -OR- if the Thermistor with resistor wire reaches the right combined resistance) relay coil pulls the switch closed, connecting 215 to 365A through the fuse and ignition switch (key on).

The dotted line across the middle of my relay I drew, is a "linkage" to the "arrow" or contact arm. It looks simple to me. I bench-tested mine a while ago and all worked just fine as I have pictured so I feel confident. I HAVE NOT TRIED TO DO A BENCH TEST though the thermistor & resistor wire yet though. I figured to wait till I begin assembling the car for that test.

[krelboyne]

I picked up a testing device for checking some of the Ford fuel stuff. I guess it is Rotunda? One of my Estate Sale finds.

[3stangs2fordtrucks]

Thanks guys,  I think I have a problem not related to the low fuel problem.  I tested for voltage going to the starter solenoid and there is no power to either the switched or ignition side.  Strange because earlier when I turned the ignition to crank the starter tried to start.  I disconnected the starter wire so this did not happen (the motor has been rebuilt but not started yet).  I think the problem may be related to the tilt/tiltaway system.  It tilts/away correctly when vacuum is applied and the door opened; however, there is a switch to prevent the car from being started while wheel is tilted.  I removed the fuse and the wire to the solenoid switch and that had no effect on getting power to starter solenoid.  So not sure this is a factor but I have checked fuses in fuse block, tilt wiring and convenience wiring and all are good.  It has a 4 speed so I moved shifter around in case problem with safety switch.  Strange thing was that the power was there when I started this process but now no power to solenoid. 

again I really appreciate you guys putting all this effort in this problem.  Hopefully, when I find out this problem low fuel problem will, with advice you guys have provided, be solved.  Another day, another fight. 

[midlife]

I think you are on the right track, suspecting the tilt-away system interfering with the NSS wiring.  Solve that problem, and you'll solve your no-power to the low fuel lamp.

[67gta289]

Wow a lot can happen after being at work 11 hours, then coming home to install a garbage disposal and stove now that the new kitchen countertops are installed.

Now that I have pulled the black feathers out of my mouth (eating crow) I did mess up the relay contact.  The note I had regarding connecting 366 to 365 not making sense is something I was going to go back and check, but you all did that for me.

Richard in particular gets the big gold star on his forehead.

As for the current status, the OP is correct in that the tilt wheel, when not locked and loaded, will prevent the starter relay from picking up (startus interruptus) and thus power being applied to 366.  If you look at that schematic (I'm not about to touch that now after what happened the last time ) the power used to energize the tilt solenoid to engage vacuum to trip the mechanism does not need to be in place.  All that you need is to have the tilt wheel locked in place.  Either that or gag the position plunger switch (looks suspiciously like a door switch used to turn the dome light on).  If the switch is not installed yet, find the two wires and jumper them together.  As Midlife inferred the tilt position switch interrupts the neutral safety switch circuit. 

Bob, as far as blowing out the relay, the only ways to do that are (1) hit the coil with a high potential, probably more than 24V for more than 15 minutes (the old design standard), or (2) use the contact to interrupt a large load, like a blower motor or starter motor.  Both of those are not likely.

Back to the tilt, when I was much younger, and (gasp) would drive my car, park it, and walk away, I would often reach down and pull one of the two red (if I recall the color correctly) wires of the tilt switch.  No cranking after that.  A poor man's theft deterrent system.

[preaction]

I picked up a testing device for checking some of the Ford fuel stuff. I guess it is Rotunda? One of my Estate Sale finds.

Scott, that was a great find and a real time saver.

[midlife]

As an aside, I've refurbished about 10 harnesses with the low fuel relay wiring.  Of them, only 2 had good resistor wires; the rest were broken.  This wire is covered in an asbestos-type material and is extremely fine.  How it is crimped to the standard spade connector is beyond my ken.  I suspect it cannot be soldered (except silver-solder? which I do not have the skills or tools to do so).  I suspect when that happens that the low fuel relay will not work correctly (either lamp is always on or lamp never comes on)...

[3stangs2fordtrucks]

Sorry to take so long to get back but I am slow.  I jumpered the two wires from the starter safety switch located on the steering column but this did not solve the problem of no power to the starter.  I think it was not in the tilt system but the adjustment of the neutral safety switch.  I messed (my way of proceeding) with the shifter and switch and was able to get power to the starter solenoid so the starter bumped with I turned key to crank. I then disconnected starter wire to keep from bumping starter.

I was able to complete the list recommended by 67gta289.  I am extremely grateful for your help.  From your list:  (1) With connectors disconnected, I measured from wire 365 to 29 with key on and between wire 366 to 57 with key in crank position.  Both measured around 12v (11.20-12-20).  (2) I plugged in connectors and turned key to crank and light came on.  (3) Disconnected plug in trunk to thermistor (this sending unit is a new one recently purchased from WCCC) and jumped wire 367 to a ground and low fuel light came on.  (4) I disconnected the sending unit from the harness and measured the resistance from the thermistor.  I measured wire 367 (at the end of the harness and also on the plug of the sending unit) and wire 29 (back side ground on sending unit) and the resistence was around 1200 ohm.  I have about 10 additional low fuel option sending units and with the exception of one they all measured 0 ohm.  The one other one measured 90 ohm.  (5) with everything connected I wrapped a small wire around the thermistor plug on the back of the sending unit and tested with a ground on the chassis with the sending unit in a bucket of water located just so thermistor was submerged but electrical plug (including underneath of sending unit plate) was dry.  I also tested with thermistor out of the water.  I did this several times.  All readings were around 11.20 volts either in or out of water.  The low fuel sending unit light did not come on.  Maybe I am not measuring correctly but I took care to make sure the wire wrapped around the plug from the thermistor did not touch the sending unit or any other wire.  Please let me know if this measurement method seems ok.  If not I will try the needle method you suggested.  Thanks again for your help. 

[67gtasanjose]

Good that all works as you described. There has to be an issue with the thermistor. Perhaps the specific gravity of water may be too heavy to operate the thermistor. It may be a huge fire risk to try gasoline on a bench-test so I won't suggest that. Perhaps if the sender was mounted inside a fuel tank and you could just plug everything in and add a jumper wire, to ground the fuel tank itself (to ensure a good ground). Put about 3-4 gallons of gas in the tank and see what you get. It is nice there is a drain plug to empty the gas back out again too

[67gta289]

I don't think specific gravity matters.

Consider the drawing.  When the ignition switch is in the run position, if nothing is broken, there is always a current flow through the fuse, then the relay coil in parallel with the resistor wire, then through the thermistor, and then to ground.

When the thermistor is submerged in fluid, whether water or gasoline, enough heat is transferred away from the thermistor to keep the thermistor resistance low enough, resulting in the relay being energized.  There is enough voltage drop across the relay coil to pull it in and turn out the indicator light.

This is why the resistor wire is important, it is sized so that the "voltage divider" circuit works properly.

When the thermistor is not submerged in fluid, there is less heat transfer, so the thermistor heats up.  It is this repeatable property that is fundamental to success in this design.  When the thermistor heats up, the resistance increases.  As more voltage drop shifts from the relay coil to the thermistor, eventually the relay does not have enough potential and it drops out, and the indicator light comes on.

There is a time component to this, and as stated previously I would guess it is in the 10's of seconds, not minutes.  This allows for variability in relay coil, resistor wire, and thermistor manufacturing.  It also prevents the light from flipping on and off when the level is close to the low level and you hit the brakes causing sloshing.

For the OP, I would do these things next:

1. Figure out how to ground the wire 29 with the thermistor plugged in.  I understand that you would want to do this in a non-destructive manner.   Based on what you wrote, if you ground 29 the light will go on, and if you lift the ground (open circuit) the light will go out.  Is that the case?  If so everything is in place, but now we need to determine if the problem is in the thermistor or with a bad quality ground.

2. With the same setup, a temporary known good ground on 29, get the circuit hot again, with the thermistor in the air.  No need for a bucket until we can get the light on.  If we do, submerging in water should turn it off after an unknown (yet) time delay.  I would really be tempted to use thermography to measure the surface temperature of the thermistor.  If you have a quality meter, you could try to get in series and measure amperage.    If this works (I would probably wait at least 5 minutes just because I don't know how long it takes to heat up), then submerge and wait again to see it clear.   It this all works, then your wire 29 is bad and you need to chase that.  If it does not work, the thermistor is bad.

I'll try to get out and check mine this weekend.

[3stangs2fordtrucks]

I think wire 29 is where I am confused.  It is a virtual wire.  It does not exist as a wire but is the ground on the sending unit face.  There are three prongs coming off the sending unit but only two wires in the harness.  One is for the low fuel guage in the instrument cluster and the other is the low fuel light in the convenience package.  The third prong on the sending unit is a dead end.  There is no wire in the pigtail coming from the sending unit for that prong.  Instead, I think the thermistor grounds to the sending unit itself which is grounded to the gas tank. 

In my tests I never got a light to go on from testing wire 367 to wire 29.  Per item 3 in my check list I jumpered wire 367 to a clean ground (on car frame) and the low fuel light came on.  If I connect wire 367 (one coming from low fuel thermistor) and the ground on the back of the sending unit (virtual wire 29) I do not get a light (maybe because sending unit is not grounded since it is not in the fuel tank but laying in the trunk or bad thermistor).  I will try to ground sending unit and repeat measurement to see if get light to come on. 

I tried to test wire 367 by tying a wire to the plug where it meets the sending unit while ensuring it did not touch face of sending unit.  I then ground  this to frame, ground on back of sending unit, etc; all without success.  I tried sticking needle inside pigtail harness and where plugs from harness meets pigtail.  All readings were around 12 v whether in the water or dry. 

I have two multimeters.  I have a Fluke 23 and Fluke 25.  Both exceed my capabilities but am willing to try to measure amperage.  I have seen probes that fit on multimeters that measures heat but dont
't believe I have one.  Always up for new toys.  U-tube maybe will be helpful to get started.  I think I have a grounding problem but not sure why light does not come on when thermistor is dry. 

 

[3stangs2fordtrucks]

Hi,  I finally got the light to come on by attaching a small wire around the ground post on the sending unit and grounding it to the frame.  When I submerged the thermistor in water the light remained on.  Therefore, I guess the end of this road is that the thermistor is faulty.  I am not sure what the resistance reading indicated? 

I contacted WCCC to see about getting a replacement and they said no problem but they are out of stock.  I am on their notify list and hopefully when new ones come in I will be in business. 

Again, I want to thank the many people who participated in this thread.  I can't believe there are a group of you guys who are willing to take your time to help people like me.  Very impressive. Hopefully, this thread will be helpful for many others who have similar problems for years to come. 

When I get the replacement from WCCC and install it, I will send a final post. 

[67gta289]

Good news.  If I'm not at work tomorrow I will measure the resistance of the thermistors that I have - hopefully at least one is good.

Last week I installed an electric heating mat under a ceramic tile floor, and the sensor is...a thermistor.

[67gta289]

Took longer than expected.  I have two complete systems, one mostly installed in my car, the other in a box.  Turns out that I had two out of two bad relays, but I was able to repair both   I also have two out of two bad thermistors .

Curiosity got the best of me, so I did quite a bit of testing.  It turns out that both relays pick up at around 7.35V, and drop out at around 6.95V.  The relay coils are 42.6 ohms, and the resistor wires are around 47 ohms, which has a Thevenin equivalent of 22.3 ohms. 

At the pick up voltage drop of 7.35V, and 22.3 ohms, the current requirement is 0.33 amps.

At a nominal 12V, and 0.33 amps, the thermistor voltage drop is 4.65, which would make it around 14 ohms when hot.  Of course it could be less than 14 ohms, and by design, given tolerances, I would probably go for the 10 ohm range.

The one from WCCC was backordered, so I'll look around at other options, then test and report back.

Anyone care to double check my calcs?