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I have a 1985 Jeep Laredo with the 20 gallon plastic tank. After a fill up, my dash gas gauge in the speedo reads WAY full and when driving around, it takes forever to move the needle--after I have driven around for the week, it starts to drop, but then faster than normal. When the gas gauge reaches 1/4 tank, the Jeep is empty and out of gas. For years, i have just filled it up before I get to a 1/4 tank, but my 15 year old boy is going to be driving it soon, so I want to fix it. Any ideas?

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1985LaredoCJ...There are several possibilities:  1) The gauge sender is inaccurate, 2) the voltage signal is inaccurate to the gauge, 3) the gauge or voltage at the dash instrument panel is inaccurate, or 4) there is a ground issue at either the sender to frame or instrument cluster to body, to engine, to frame.  

The gauge sender must be for a 20-gallon tank.  Otherwise, the depth of the tank will not reflect accurately.  Also, the sender arm must be able to operate over the depth/range of the 20-gallon tank.  It's possible that the sender is for a smaller tank.  It's also possible that the sender is defective.  Be sure you have a sufficient ground at the fuel gauge sender and dash instruments.  You can test this with your ohmmeter. 

There is a specific ohms resistance range for the OEM sender and gauge.  You could try reading this at the sender pole of the dash fuel gauge to avoid dropping the tank.  Fill the tank and see what the resistance reads when the tank is full.  Same at 1/2 full.  Same at your known near empty.  This may provide a clue about the in-tank sender's range and whether it matches your 20-gallon tank.  

Here are details from the 1985 AMC/Jeep® FSM.  I highly recommend that you find a copy of this manual if you do not have one.  In the same FSM Electrical Section, there is a flow chart diagnostics segment of several pages that covers the fuel gauge functions, diagnostics and defects testing.  Dealer techs dealt with these issues regularly and AMC/Jeep® addressed this trouble thoroughly in the FSM.

Tools needed:

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See the "Fuel Gauge Sending Unit Resistance (Ohms)" and "Fuel Gauge Resistance (Internal)".  Perform these tests at the gauge if you can.  Otherwise, the sender will need to be removed from the tank.  Wires disconnected, you can run an ohms test on the isolated fuel sender...Make sure your sender and float arm are the right depth/range for the 20-gallon tank...

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Make sure grounds are good at the dash, gauge, tank sender, body to frame, engine to frame, frame to body...Check for a proper ground between the sender and frame...Use an ohmmeter for this testing...

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Here's the wiring circuit for the Fuel Gauge...Note where grounds apply...See "S" test point at gauge.  "I" is ignition voltage with key ON.

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See brief section "Fuel Gauge Diagnosis"...The FSM also has the "DARS" section devoted to flow chart testing...

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This should get down to the root of the problem...If voltage, grounds and sender (isolated from tank) ohms check okay, the likely problem would be the wrong sender/arm/float assembly for this fuel tank.

Moses

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  • Moses Ludel changed the title to 1985 Jeep CJ7 Fuel Gauge Reads Inaccurately

Thanks, Moses. I was thinking the previous owner had put a standard 15 gal sending unit/arm/float in the tank years ago (even though this Jeep has a 20 gallon tank), but I did not want to influence your answer, so I appreciate the detailed response with different things to check before I just drop the tank and replace a sending unit that I might not have to. That said, after checking everything, if I do need one, I have not heard great things out there about any of the CJ-7 sending units, much less the 20 gallon type. Any suggestions on where I could get an accurate one that is made well? Thanks, Howard

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For quality and product awareness, Crown Automotive is reliable.  Omix-ADA would be an alternative.  Crown outsources a lot of Taiwanese product.  Others use PRC sources.  (Pick your Chinese preference?)  PRC quality has improved substantially over the years.  Taiwan is noted for quality machine tools and castings.

A 36-year-old CJ7 has a high likelihood of a fuel gauge sender change at some point...The checks will help narrow down the issue or at least qualify whether you should drop the tank or not.

Let us know what you discover...

Moses

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  • 2 weeks later...
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1985 LaredoCJ...Let us know how the new sender works and if this resolves your inaccurate gauge...Sounds promising!  If this works properly, your son will not be calling you or AAA for fuel...much safer.

Moses

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