2003 Fuel gauge will not go below half level

Joined
Sep 8, 2021
Thunderbird Year
2004
We’ve changed both fuel pumps with new and still having the same problem; any suggestions?

Thanks!!!!
I am having the same problem with my wife’s 2004, gage stays at half all the time even when we fill the tank. Replaced fuel pumps and the fuel gage worked as it should. Now it’s stuck at half again. Car runs fine but this problem just won’t go away.
 
Joined
Sep 22, 2018
Thunderbird Year
2005
We’ve changed both fuel pumps with new and still having the same problem; any suggestions?

Thanks!!!!
This may help. It describes how the fuel tank & its two pumps function.
The Fuel Tank & How It Works



Thunderbird Fuel Tank System

The fuel tank is a dual-container (saddle type) which straddles the drive shaft of the vehicle. During fueling, the tank fills up the primary (fill side or passenger) side, then fuel spills across the saddle to fill the secondary side (driver’s side).
The Fuel Pump Module (FP module 9A309), within the fill side tank, filters and is the pump which provides fuel to the fuel system lines forward to the FG-1083 or FG-1011 (workshop manual) inline fuel filter (behind the Fwd Left Wheel splash shield), then to the fuel injector supply manifold. The FP module also supplies a small motive flow of fuel to the secondary side jet transfer pump (9A307) using the fuel transfer tube. As the vehicle is run, fuel is "jet pumped" from the secondary side of tank first via the connecting tube back to the FP module. When secondary side of tank is empty, the FP module draws fuel from the primary side of the tank.

Fuel Level Sending Units

The saddle fuel tanks are totalized by the PCM to register fuel quantity on the gauge. Both the electrical FP module and the mechanical jet pump have fuel level sending units, or, "floats". In the fill side, the Fuel Pump (FP) module contains the primary float that reads: half to empty. The secondary side float adds to the fuel totalizer to give: half to full tank. The PCM sums the readings from the primary and the secondary and sends that quantity to the gauge.

The secondary side of the tank is the last to be filled and the first to be emptied. If there is fuel left in the secondary side and fuel level in the primary side of the tank begins to drop, the PCM (powertrain control module) will conclude there is a fault with the jet transfer pump.

Float level faults
A fault at the secondary side of tank will cause the totalizer to display the fuel level of the primary side sending unit. This results in maximum reading of half tank. This reading will stay the same until fuel level on secondary side is empty and fuel on primary side of tank begins to drop below its half of the tank quantity.

A fault with the primary side sender will cause the totalizer to principally display the secondary side fuel level. With full fuel, the tank would appear as half on the gauge. As fuel is used out of the secondary, the fuel gauge will display the secondary fuel quantity and display zero as the secondary tank empty's to empty. At this point, the primary side is still full of fuel and pumping fuel, but the level totalized= zero from the secondary + the float level zero from the primary, therefore zero on the gauge. (this assumes that the frozen primary float level is at zero.)

If the primary float had frozen in any level other than zero, the fuel registered would display the failed and frozen primary float level totalized with the secondary. As the secondary is used dry, and the display would continue to display this failed primary float position without change until the primary fuel is used to dry. The car would then be out of fuel, yet still displaying a fuel state, the "failed" primary fuel position. The thought here is, if you continually "top off" when you reach 1/2 on the gauge, this failure may be dormant for some time before it is noticed.



Precautions and Observations

At the 1/4 mark, I refuel and do not run the primary pump below a 1/4 tank because of the lack of fuel immersion, heating and premature failure of this electrical pump in the primary side. At the 1/4 mark, this at least shows me that I may not have much of a failure with the primary sender.


As a parameter within our OBD, I am typically running at a slightly oscillatory 55.2 psi fuel pressure with 33-55psi considered normal on page 310-00-1 of our work manual.

Just for grins, I ran tanks dry to "the light". According to the OBD, the fuel totalizer measured 12.8%. I interpret this as the remaining percentage of the 9 gallons within the primary side and this would approximate 23 miles to empty.
 

doug7740

Active Member
Joined
Dec 29, 2014
Thunderbird Year
1955
We’ve changed both fuel pumps with new and still having the same problem; any suggestions?

Thanks!!!!

Since you have changed both the fuel delivery module and the jet pump module, before I would go any further I would check the resistance of both modules while they are installed in the fuel tank. If one of the modules are not in specifications that could indicate a defective module or the float is coming in contact and sticking inside the fuel tank.

The fuel delivery module is located in the right side of the fuel tank and the jet pump module is located in the left side of the fuel tank. Fuel level status is sent directly to the REM, which is hardwired to the fuel senders.
  • The normal operating range of the fuel delivery module is from 16 +/- 2 ohms at empty (E) to 155 +/- 4 ohms at full (F).
  • The normal operating range of the jet pump module is from 19 +/- 2 ohms at empty (E) to 160 -/+ 2 ohms at full (F).
doug7740
1955 Thunderbird Blue

Fuel Gauge Testing.jpg

Jet Pump Resistance Reading.jpg

Sending Unit Wiring Diagram.jpg
 
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