Question about 1994 Lexus LS 400

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FUEL GAGE DOES NOT GO UP ALL THE WAY

Greetings. I have a 1994 lexus ls 400 the fuel gage goes up very slowly when put fuel in the tank than it comes back down and stay down.

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  • coolnajmi91 Jun 18, 2009

    I have to replace the fuel sending unit Lexus Ls 400 sending unit is inside the trunk right behind the back seat. Gas tank does not have to be lowered unit is aces sable from inside the trunk.

    Thank You you have been a great help.

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HI, this is a malfunctioning sending unit that is located inside he fuel tank. this sending unit, transfers a Resistance signal to the gauge, in return, allowing you to see how much fuel is in the tank. if this mechanism becomes corroded or, damaged, this will cause the symptoms you are experiencing.

This damage is usually caused by fuel deposits left behind overtime. the modern day fuel we use can destroy sending units because of all the detergents and, nitrogen cleaning agents put in the fuel at the refinery. this is goo for cleaning out the rails and fuel injectors but, it usually can hurt the sending unit mech at the same time.

The fuel gauge sending unit is attached to the fuel pump Assembly. you will have to remove this assembly from the tank to replace it. the tank has to be lowered as well.

Posted on Jun 18, 2009

  • Michael Masters
    Michael Masters Jun 18, 2009

    Ok, good deal. i did some further research on the accessible regions and, it says that you can access it this way. this is way more efficient than lowering the entire tank.

    Thanks for the update.


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Fuel Gage
The IPC displays the fuel level as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the fuel level percent. The fuel gage defaults to empty if:
• The PCM detects a malfunction in the fuel level sensor circuit.
• The IPC detects a loss of class 2 communications with the PCM.
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Hooking up a professional or factory scanner an checking input data to the PCM from the sending unit inside the fuel tank ,viewing the information sent to the IPC , Checking for diagnostic trouble code's .
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Circuit Description
The fuel level sender changes resistance based on fuel level. The PCM monitors the signal circuit of the fuel level sender in order to determine fuel level. When the fuel tank is full, the sender resistance is high and the PCM senses a high signal voltage. When the fuel tank is empty, the sender resistance is low and the PCM senses a low signal voltage. The PCM uses the signal circuit of the fuel level sender in order to calculate the total remaining fuel percentage in the tank. The PCM sends the fuel level percent via the class 2 serial data circuit to the instrument cluster in order to control the fuel gage. The fuel level information is also used for misfire and EVAP diagnostics.

Install a scan tool.
Turn the ignition ON, with the engine OFF.
With a scan tool, observe the Fuel Tank Level Remaining parameter in the PCM data list.
Does the scan tool indicate that the Fuel Tank Level Remaining parameter is less than the specified value?
1%

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Test Description
The numbers below refer to the step numbers on the diagnostic table.
This step tests for a high resistance in the signal circuit and the low reference circuit of the fuel level sensor.
DTCs will be set in the PCM when you perform this diagnostic table.
Step
Action
Value(s)
Yes
No
Schematic Reference: Instrument Cluster Schematics
1
Did you perform the Instrument Cluster Diagnostic System Check?
--
Go to Step 2
Go to Diagnostic System Check - Instrument Cluster
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Disconnect C104.
Connect the J 33431-C Signal Generator and Instrument Panel Tester between the signal circuit of the fuel level sensor and the low reference circuit of the fuel level sensor on the male terminal side.
Turn the ignition ON, with the engine OFF.
Important: Verify the J 33431-C resistance settings with a DMM.
Vary the resistance on the J 33431-C from 40-250 ohms.
Refer to Fuel Level Specifications in order to convert from resistance to fuel gage display.
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