1964 Fuel sender W low fuel. | Ford Thunderbird forum club group 1955-2005 models

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1964 Fuel sender W low fuel.

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My390ci

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1964
I have a 64 that has a leaking float. I've pulled the sending unit out and it is in very good condition. Before I order a new float is there a way to test the Thermistor for the low fuel detection to make sure it's operating correctly? I'd hate to get it all back in, put gas in and find out the low fuel warning is not working and have to drain fuel and go through the removal process again.

Thanks in advance,

Steve...

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Get a piece of #14 that will.reach from sender at tank. Disconnect wire at sender Connect this wire to wire removed from sender.
Get someone to watch instruments.
Turn on Ign Sw to acc.
CONNECT free end of test wire momentary.
If you get light. The wiring is good. If not.This is where the TS gets more in deph. Get in touch with me and I'll try to get you through the Trouble Shooting.
 
Correction: free end to battery ground.
 
Thanks for the response. Which post on the sender do I connect the #14 wire to? Does it have to be #14?

Thanks in advance,
 
My390ci said:
I have a 64 that has a leaking float. I've pulled the sending unit out and it is in very good condition. Before I order a new float is there a way to test the Thermistor for the low fuel detection to make sure it's operating correctly? I'd hate to get it all back in, put gas in and find out the low fuel warning is not working and have to drain fuel and go through the removal process again.

Thanks in advance,

Steve...
Click to expand...
To check an original brass-cased thermistor, measure the resistance from the yellow wire to the case (ground).
It should be in the 500-700 ohm range. Note: The resistance will change slightly as you measure
because the voltage from the ohmmeter will heat up the thermistor slightly.

The thermistors used in the aftermarket fuel senders measure about 1000 ohms.
 
cougarsunlimited said:
To check an original brass-cased thermistor, measure the resistance from the yellow wire to the case (ground).
It should be in the 500-700 ohm range. Note: The resistance will change slightly as you measure
because the voltage from the ohmmeter will heat up the thermistor slightly.

The thermistors used in the aftermarket fuel senders measure about 1000 ohms.
Click to expand...
Hi Vic.

I will preface this with I'm not proficient in using a multimeter and look at U Tube videos to be sure I'm using it as closely to correct as I can.

When I put the red multi-meter lead to the yellow wire and the black multi-meter lead to the side of the Thermistor and the meter is set at 2000 ohms I get no reading at all on the meter nor at 200 ohms. If I put the meter on 20,000 I get a reading of 2.33. Does this mean the Thermister is not working?

This is an original NOS Ford-sending unit that I was able to purchase from a Ford dealership back in 1982 when I towed my bird home to restore.

Steve
 
You should not get a reading on the 200 ohm scale, because the thermistor should read higher than 200 ohms.

No reading on the 2000 ohm scale typically means that the thermistor is open (OL overload, or out-of-range), which is bad.
2.33 reading on the 20,000 (or 20K) scale means 2.33K ohms, is out-of-spec.
Double check this reading by reviewing the reading immediately after touching the probes to the case and yellow wire pin.
Oxidation on the pin or case ground may affect the reading.
However, the fact that the lower scale shows no reading, typically indicates the thermistor is bad.
I've only seen the thermistors fail open (no reading on any scale), not with slightly higher resistance.

The way this thermistor works is, in open air, it heats up and the resistance drops to a few ohms, thus grounding the low-fuel relay coil and triggering the light ON.
When the thermistor is covered in fuel, the heat is dissipated and the resistance stays high, keeping the low-fuel relay (and light) turned OFF.
2.33K ohms, which is effectively in series with the relay, is too high to trigger the relay.

In the mid '80s, I was able to obtain a quantity of the correct thermistors from the OEM manufacturer.
After using them to repair my cars and selling the rest, I discovered that the OEM no longer manufactured them. :-(
These new units lasted about 30 years before failing. It is a flawed design because the thermistor gets too hot and eventually burns out.
Always keep your tank at least 1/4 full, and they don't fail!

Several companies sell an aftermarket replacement thermistor for the 1965-66 Thunderbirds, that measures about 1K ohms.
These thermistors do not work in the Cougars or my test bench setup with the factory relay/wiring.
 
I seem to remember when these aftermarket senders first came out, maybe late '80s or early 90's reading about the poor quality of the "low fuel" model.

I'm thinking of just cutting a milk carton open, putting some gas in it, and putting the unit in the gas to see if it works. Foolproof test.

Thanks for your help.
 
Make sure to use a container that doesn't melt with gasoline and keep a fire extinguisher handy.
It might be safer to use distilled water.
 
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