69 XR7 low fuel light

one more problem encountered during my wiring checks today.
during testing of the low fuel light I encounter this ?
connect battery and low fuel light is on, ignition switch is off and fuel tank is dry and the low fuel fuse is okay
I change the relay with another that I have and no light on now, I put ignition switch to start to prove light but no light comes on.

I take the fuel sender out and change thermistor and try again with same results with both relays…
I check the fuel sender against the gauge reading and it seems to be fine…

Anyone else encountered anything like this ??
Any suggestions ?

I’ve beat up these LFL issues for many years on 67,68s. Don’t know if 69 has different functions.

“connect battery and low fuel light is on, ignition switch is off and fuel tank is dry and the low fuel fuse is okay”

I would say this relay is shorted to 12 volts so light is on.

"I change the relay with another that I have and no light on now, I put ignition switch to start to prove light but no light comes on."

Relay working so far. I never knew there was a prove out function. Looking at the 68 schematic I don’t see the wiring to accomplish that.
First thing to try out is unplug the sending unit and take a wire from the cable connector LFL receptacle to a good body ground. Turn key to ON and LFL should come on. If it does then your relay and wiring is good. Problem would be in the thermistor or grounding of the the sending unit or gas tank.

Get an ohmmeter and measure the thermistor from the sending unit pin to the sending unit body. In air should be between 10 to 40 ohms. If considerably higher or open then thermistor bad or unit not grounded to sending unit.

Sounds like ‘prove-out’ wire for the low fuel light is connected to the battery side of the starter solenoid instead of the starter side.
The ‘prove-out’ wire is there to illuminate the light when the key is in the start position.

I agree with Bob, and suspect the light went out with second relay as that relay is no good.

The low fuel system relies on four components.

  1. The low fuel relay
  2. Thermistor (Negative Temperature Coefficient, NTC type) in the fuel tank
  3. The low fuel light in the convenience panel
  4. The starter relay

The low fuel relay is a Single Pole Double throw relay with power being supplied to the NC post from the starter relay’s starter lug. The starter relay’s starter lug is only powered when the ignition switch is turned to the start mode. The NO pole is powered when the ignition switch is in the run/Acc position.

In the fuel tank, the fuel relay’s coil ground wire connects to the fuel tank sending unit into a thermistor within the tank. The thermistor is then grounded to the frame of the tank unit. There are two types of thermistors, Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). With an NTC, as the temperature increases, the resistance decreases. Conversely, when temperature decreases, resistance increases.

There are 2 situations when the low fuel light will turn on.

  1. When the Car is in the staring (crank) mode irregardless of the actual amount of fuel in the tank.
  2. When the fuel tank drops below a certain amount, this is dependent on the thermistor.

So examining the two conditions above, the relay is in its NC state and its coil is de-energized. When you turn the ignition key to start, the coil is still de-energized but power is passed to the NC contacts thus providing power to the low fuel light.

When you release the ignition switch to the run condition, the power at the starter solenoid is disconnected removing power from the NC contact of the relay. Power is now applied to the relay’s NO contacts from the ignition in the run position however assuming that there is enough fuel in the tank, the thermistor providing the ground is high in resistance thus the coil in the fuel relay does not switch thus stays in the NO position. When the thermistor is submerged in fuel, it stays cool and thus the resistance of the thermistor is high, effectively turning off the circuit at the fuel relay. As the fuel level drops and the thermistor is partially submerged in fuel, it heats up reducing its resistance eventually to the point that the coil in the relay energizes enough to pull the coil. The relay then switches the contacts so that the power provided by the ignition switch passes through to the output of the coil to the fuel relay light.

Coach Jack

Haven’t worked on my car last couple days …too hot around here.
I did try and move the Low fuel light lead from the battery side to the starter side of the solenoid but still have the light constantly on.
I did some testing and now see that when the black wire from the headlight harness is Hooked up to the battery side of the solenoid the Low fuel light is on and there is power at the red LFL lead.
I installed a new headlight harness and now wondering if there is a pin in wrong hole
Try and dig into it tomorrow

Worked on my car today and have solved the LFL on when connected to the battery. After tracing the red lead in the headlight harness I discovered that it should have been in a different hole, that another red wire was in. Reversed the 2 red wires and now the LFL prove it works when key is in start position.

Now the light still doesn’t light up with an empty so tank so I’ll have to dig more into the thermistor i’m thinking…enough wiring for today
thanks for all the helpful advise.