gas gauge not reading
Thread Starter
CF Veteran
Joined: Aug 2009
Posts: 1,126
Likes: 30
From: Detroit, Michigan
Year: 1998
Model: Cherokee
Engine: 4.0
I'm thinking about making a video for youtube, even if it's onlt putting this all back together, it might help out some folks, especially since the only one's I've seen were Grand Cherokees.
Member
Joined: Oct 2013
Posts: 211
Likes: 1
From: Chicago
Year: 1998
Model: Cherokee
Engine: 4.0
I had a fuel guage that started to act erratically and eventually died ... engine runability started to degrade, there were problems with starting, etc. ... once the fuel gauge sending unit was replaced the problems disappeared.
The metal contacts on the fgsu eventually wears out and sends erratic and/or no signal to the PCM.
The fuel gauge sending unit data is used with other PCM functions that prevent misfires, starting, LPD - evap, o2 sensors, spot errors to set trouble codes, etc.
The PCM sends 5 volts to the fuel guage sending unit which returns resistance values back to the PCM.
fuel pump module connector
Pinouts
====
6 5 4
3 2 1
====
pin 1 fuel pump feed
pin 3 fuel level sensor (DB/LG) <<<< PCM C26 (5 volts)
pin 4 fuel return sensor (BR/YL) >>>> PCM A4 (resistance value)
pin 6 ground
(resistance values for sending unit's variable resister)
Full Up - 20 Ohms +/- 5% = 21.0 to 19.0 Ohms
Full Down - 270 Ohms +/- 5% = 283.5 to 256.5 Ohms.
The metal contacts on the fgsu eventually wears out and sends erratic and/or no signal to the PCM.
The fuel gauge sending unit data is used with other PCM functions that prevent misfires, starting, LPD - evap, o2 sensors, spot errors to set trouble codes, etc.
The PCM sends 5 volts to the fuel guage sending unit which returns resistance values back to the PCM.
fuel pump module connector
Pinouts
====
6 5 4
3 2 1
====
pin 1 fuel pump feed
pin 3 fuel level sensor (DB/LG) <<<< PCM C26 (5 volts)
pin 4 fuel return sensor (BR/YL) >>>> PCM A4 (resistance value)
pin 6 ground
(resistance values for sending unit's variable resister)
Full Up - 20 Ohms +/- 5% = 21.0 to 19.0 Ohms
Full Down - 270 Ohms +/- 5% = 283.5 to 256.5 Ohms.
CF Veteran
Joined: Dec 2012
Posts: 4,169
Likes: 6
From: York, PA
Year: 1998
Model: Cherokee
Engine: 4.0
I really hope you change your mind with the Airtex pump, because you'll be replacing it in a year or so probably, and a good chance it'll leave you sit. Also, you can get new straps AND J-bolts for that price from Napa, all in one kit.
CF Veteran
Joined: Aug 2010
Posts: 8,357
Likes: 103
From: Canton, MI
Year: 1999
Model: Cherokee
Engine: 4.0
It was several years ago I had the problem with the fuel ga.
After reviewing it ... Pin 4 can be a voltage measurement .
The FSM calls it a signal ...
It would depend on how the PCM handles inputs ... more likely its a voltage measurement from Pin 4.
From what I can determine the PCM monitors a signal from Pin 4 then controls the fuel gauge on the instr. cluster via B+/B- twisted pair. (or the PCM may periodically send 5 volts to the fuel sending unit then read the results.)
I am not sure how the PCM works, there is no documentation freely available. I would guess it temporarily stores the results from inputs as data then uses the data for other functions such as for emissions and engine operations.
The fuel sending unit can be tested using resistance values (or voltage values while connected to power).
Those resistance values are from testing the fuel sending unit, not from specifications and may vary ..
After reviewing it ... Pin 4 can be a voltage measurement .
The FSM calls it a signal ...
It would depend on how the PCM handles inputs ... more likely its a voltage measurement from Pin 4.
From what I can determine the PCM monitors a signal from Pin 4 then controls the fuel gauge on the instr. cluster via B+/B- twisted pair. (or the PCM may periodically send 5 volts to the fuel sending unit then read the results.)
I am not sure how the PCM works, there is no documentation freely available. I would guess it temporarily stores the results from inputs as data then uses the data for other functions such as for emissions and engine operations.
The fuel sending unit can be tested using resistance values (or voltage values while connected to power).
Those resistance values are from testing the fuel sending unit, not from specifications and may vary ..
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