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  #1  
Old April 24th, 2012, 21:01
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AnthonyZ28 AnthonyZ28 is offline
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Innaccurate fuel readings?

Ok, so my fuel gauge says I have a little under half of a tank, but the low fuel light is on. Im thinking the fuel gauge is the accurate one but what do you all think?
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Old April 25th, 2012, 07:04
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Re: Innaccurate fuel readings?

Only one way to find out. Throw a can of gas in the back seat and drive it until it quits.
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Old April 25th, 2012, 08:11
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Re: Innaccurate fuel readings?

Other way to figure it out, fill the gas tank. How many gallons to fill ?
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Old April 25th, 2012, 08:25
O-Gauge Steamer O-Gauge Steamer is offline
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Re: Innaccurate fuel readings?

I am shocked. An inaccurate Jeep fuel gauge...

Year would help here. The light is turned on by the cluster electronics. So, although it is pretty rare for the gauge itself to go south, anything can happen. Knowing what the gauge is reading and, knowing just how much fuel the Heep is capable of holding, go fill the tank.

If you are reading just under 1/2 tank and you put in 16+ gallons, there you go. Given that the average Heep holds 20.2 gallons, 15+ would indicate it to be 3/4 empty. The light, on my 98 at any rate, appears at the indicated 1/8 full level.

And, here is what my 98 FSM has to say on the matter:

"The PCM uses an input from the fuel gauge sending unit and internal programming to decide what fuel level message is required. The PCM then sends the proper message to the instrument cluster on the CCD data bus. If the PCM message indicates that the fuel level is below one-eigth of a tank for more than 10 seconds, the instrument cluster circuitry turns on the low fuel warning lamp and generates a single chime tone. Once the lamp is turned on, an increase in the fuel message of at least one full needle width for more than twenty secondes is required before the cluster will turn the lamp off."

So, what does all that jazz mean? I would start by confiming the fuel level by filling the tank and noting the actual level versus the indicated. If the two match, then the cluster electronics are suspect. Or the PCM...

If the Heep is OBDII, then the PCM can be read directly to see what it thinks. My FSM says to use the DRG tool but any quality reader will work. Provided it can actually access the sensors. If you wee closer, I'd say drop on by and we would fire up the laptop...

At any rate, post up the resolution when you find it.

I would reset the PCM and see if the issue changes. Just disconnect the battery and wait at least 10 seconds for the system capacitors to drain down. This returns the PCM to the factory settings. The PCM communicates with the Instrument Cluster over the Chysler Collision Data Bus. The messages it sends are read by the cluster electronics and all of the gauges/warning lamps are controlled. It is known as distributed intelligence in the computer world.

Last edited by o-gauge-steamer; April 25th, 2012 at 08:26. Reason: speeling dagnab it
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Old April 25th, 2012, 08:52
Talyn Talyn is offline
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Re: Innaccurate fuel readings?

Quote:
Originally Posted by o-gauge-steamer View Post
If the Heep is OBDII, then the PCM can be read directly to see what it thinks. My FSM says to use the DRG tool but any quality reader will work. Provided it can actually access the sensors. If you wee closer, I'd say drop on by and we would fire up the laptop...
The OBDII software you are using can access all those sensors? If so, what are you using as mine can't and I'm thinking of changing.
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Old April 25th, 2012, 11:41
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hubs97xj hubs97xj is offline
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Re: Innaccurate fuel readings?

Anthony's Jeep is a 93...

Obviously this is only relative to 96+, but I believe the PCM is just driving the gauge circuity based on the output of the resistor in the sending unit, and preventing misfire and evap codes if the reading is too low or high, respectively. I don't believe the PCM can calculate how much fuel is in the tank; if it could, there'd be no need for the sending unit. Fuel use estimates for the ScanGauge, and I believe the Torque app as well, are based on the other sensors and I think the injector controls as well. Those estimates are more accurate than the fuel gauge, in my experience. My low fuel light is on at 1/8th, which is between 5-7 gallons in the tank.

Last edited by hubs97xj; April 25th, 2012 at 11:46.
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Old April 25th, 2012, 21:39
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AnthonyZ28 AnthonyZ28 is offline
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Re: Innaccurate fuel readings?

Quote:
Originally Posted by o-gauge-steamer View Post
I would reset the PCM and see if the issue changes. Just disconnect the battery and wait at least 10 seconds for the system capacitors to drain down. This returns the PCM to the factory settings. The PCM communicates with the Instrument Cluster over the Chysler Collision Data Bus. The messages it sends are read by the cluster electronics and all of the gauges/warning lamps are controlled. It is known as distributed intelligence in the computer world.
I'll do that tomorrow. As of now the gauge seems to be accurately going down according to the driving im doing. It's reading at about a 1/4th of a tank now. Hopefully it's accurate and I dont run out of gas on my way to school!
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Originally Posted by hubs97xj View Post
Anthony's Jeep is a 93...
Thank you.
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