Fuel gauge not accurate
08-31-2015, 08:10 AM
#1
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Year: 2004
Model: Grand Cherokee
Engine: 4.0
Fuel gauge not accurate
My fuel gauge in my 04 wj 4.0 is off by 1/4 tank or more now about 2 months ago I switched out the instrument cluster out for one that came out of a 2000 or 2001 wj would that cause this issue? Or is it the sending unit itself? Thank you.
08-31-2015, 10:26 AM
#2
Old fart with a wrench
Do you have the white gauges or the black back-lit ones? I'm thinking you can try swapping your old fuel gauge into your new cluster to see if that works or it's the tank unit. IDK about you, but I wouldn't want to pull out the tank unit only to find out it's good.
08-31-2015, 11:13 AM
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08-31-2015, 01:15 PM
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Yea it was fine after the swap I remember I topped off the fuel about a week and a half ago so it was alittle past the full mark, drove it for 2 days and it was still marking full the third day it shot down to 3/4. Since I was concerned I filled it up again and it's still marking full after 3-4 days
08-31-2015, 04:36 PM
#6
Old fart with a wrench
Okay, here's how mine acts. On fill-up, it stays at the full mark for up to 70 miles, then drops quickly to 3/4. after that, it seems to follow a slowly dropping level reading down to 1/4, then the bottom falls out quickly. I usually don't let it get lower than 1/4. The reason for the slow start is you have to burn off enough fuel so the float starts floating instead of being pinned to the top of the tank. On some cars, the shape of the tank or the float's location has a lot to do with how it reacts. In your case, the float might be sticking at the top of it's travel.
All fuel level electrical gauges work as a triangular resistor bridge, that is the dash unit has a coil with a needle over it and a fixed resistor as well. The tank unit is a variable resistor with a brush attached to the float arm. The actual voltage in the tank is very small, like about 0.5 to 0.75 volts. Most of the voltage is bled off to ground thru the fixed resistor. I had an old 37 Dodge where the fixed resistor came loose on it's mounting studs and got very erratic, giving me all different reading. I finally took the gauge apart and found the loose connection. After all, where was I going to find a fuel gauge for a 37? The ground connection at the tank is critical also.
All fuel level electrical gauges work as a triangular resistor bridge, that is the dash unit has a coil with a needle over it and a fixed resistor as well. The tank unit is a variable resistor with a brush attached to the float arm. The actual voltage in the tank is very small, like about 0.5 to 0.75 volts. Most of the voltage is bled off to ground thru the fixed resistor. I had an old 37 Dodge where the fixed resistor came loose on it's mounting studs and got very erratic, giving me all different reading. I finally took the gauge apart and found the loose connection. After all, where was I going to find a fuel gauge for a 37? The ground connection at the tank is critical also.
08-31-2015, 06:15 PM
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Okay, here's how mine acts. On fill-up, it stays at the full mark for up to 70 miles, then drops quickly to 3/4. after that, it seems to follow a slowly dropping level reading down to 1/4, then the bottom falls out quickly. I usually don't let it get lower than 1/4. The reason for the slow start is you have to burn off enough fuel so the float starts floating instead of being pinned to the top of the tank. On some cars, the shape of the tank or the float's location has a lot to do with how it reacts. In your case, the float might be sticking at the top of it's travel. All fuel level electrical gauges work as a triangular resistor bridge, that is the dash unit has a coil with a needle over it and a fixed resistor as well. The tank unit is a variable resistor with a brush attached to the float arm. The actual voltage in the tank is very small, like about 0.5 to 0.75 volts. Most of the voltage is bled off to ground thru the fixed resistor. I had an old 37 Dodge where the fixed resistor came loose on it's mounting studs and got very erratic, giving me all different reading. I finally took the gauge apart and found the loose connection. After all, where was I going to find a fuel gauge for a 37? The ground connection at the tank is critical also.
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09-03-2015, 11:02 PM
#8
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I had fuel gauge issues on my '03 WJ which turned out to be a failing float sensor, I replaced fuel pump and float unit, problem solved. Dave's description of WJ gauge characteristics relating to fuel tank level is spot on, exactly how my gauge reads. Hopefully you don't have to drop the tank, not too difficult, but not fun either.
09-03-2015, 11:13 PM
#9
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If you ever do have to drop the tank, opportune time to replace the external filter.
Last edited by Bearstream; 09-03-2015 at 11:19 PM.
09-04-2015, 12:16 PM
#10
Old fart with a wrench
Remember the ground on the tank is a wire in the harness connector that goes to the pump. The tank is plastic so grounding it doesn't do anything. It's the same ground as the pump, so if it's not good, the pump doesn't run either. Just saying.
09-04-2015, 05:08 PM
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09-30-2015, 08:40 AM
#14
Old fart with a wrench
Wait a minute! Jeremy, where did you come from? It sounds like you have a different problem.
09-30-2015, 10:26 AM
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Haha dave1123 you are correct! The mobile version of the site had me thinking I was replying to my own thread but I was replying to a "related thread" . Sorry
Last edited by Jeremy Millner; 09-30-2015 at 10:29 AM.