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Fuel Gauge/sending unit

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Old Man Army View Drop Down
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    Posted: 18 July 2018 at 1:27am
 I have a confession to make, I'm a CJ 3B owner who spends a lot of time looking at this page.  My questions is, the fuel gauge reads 1/2 full when the tank is full.  It reads empty at about half a tank and never reads full.  I've never done electrical trouble shooting, where should I start?

Thanks,
Andrew
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stev Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 3:00am
Old Man Army,

I would think that your sending unit float wire / rod needs to be bent downward - so that the float with cause the sending unit to read full when it is full.

Also tell us what you like about the Cj2A Page.

Stev
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelyard Blues Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 4:34am
Make sure your gauge has a good ground. I replaced a sender and I still could not get it to read. I pulled the gage out and there was a lot of surface rust on the back of the dash where it mounted. I cleaned it up with sandpaper and reinstalled. Works fine now.
 
Micah
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Post Options Post Options   Thanks (0) Thanks(0)   Quote athawk11 Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 5:02am
Is your CJ3B a 12 volt setup or 6 volt setup?
1- 1946 CJ2A   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Old Man Army Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 12:22pm
Stev,

That sounds like a good idea, thanks.  I like the CJ 2A page because it has so much information and the owners are obviously passionate about their vehicles.

Best regards,

Andrew
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Old Man Army Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 12:27pm
Micah,

Thanks for your quick response. I wouldn't be surprised if that's not the problem. Now to figure out how to get in there to get the gauge out.

Best regards,

Andrew
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Old Man Army Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 12:30pm
I'm going to show how little I know, but I believe it's a 12 volt.  It's a 1961 and it uses a regular car battery.

Thanks,
Andrew
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rus Curtis Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 2:21pm
Andrew,
You're not the only 3B owner that spends time over here!  These guys have a lot of depth and have been at it a long time.  Plus, a handful are owners to more than one model of jeep.
 
Your 3B should have the cluster gauge and is most likely a 12V.  The battery will tell you by either counting the holes on the top or printed somewhere on the labeling.   A quick check with a voltmeter will also tell.
 
I try to always go simple first.   Your gauge reads something.  It responds to sending unit input.  The suggestion of checking your sending unit is where I'd start.  In the past, I'd always like to bench check the gauge and sending unit using a battery.  After connecting everything, I'd swing the float up and down to see if the gauge responds correctly.  There are some adjustments you can do to the sending unit to improve its readings.
 
After removing your seat you can pull the sending unit (cover the tank hole to keep debris out) and leave the wire connected and check the readings while moving the float.  That will tell you a lot.
 
There is some good info on the 3B page on this type gauge.  I also couldn't find your jeep in the owners list.  Consider contacting Derek to get your jeep added!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote athawk11 Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 4:30pm
A general rule is, you want to run a 12 volt gauge with a 12v tank sending unit…and a 6 volt gauge with a 6v sending unit.  Thus my question about the vehicle voltage.  For example, if your 3b has a 6v setup and a prior owner installed a 12v sending unit, it might cause some issues with how the gauge reads the information sent by the sending unit.

The sending unit is basically an adjustable resistor.   There are more Ohms passing through the sending unit while the tank is empty, than when the tank is full. In your case, your gauge can reach ‘E’, so the electricity needed is there and the connections and power supply are probably working okay. Another good sign is the gauge reading actually does vary with the amount of fuel.  So, you’re already in better shape many of us.

 The gauge or the sending unit could be failing, but I agree with the others.  A small float rod adjustment on the sending unit is worth the effort. You might also discover a fuel-logged float or the rod is getting caught on something that won’t let it move all the way to the top of its travel.  If all this looks good, this is what I would no next…  

While the sending unit is out, keep the wire connected at the center of the sending unit.  Since the sending unit is no longer attached to the tank, it is no longer grounded, so we need to secure an additional wire to the top plate of the sending unit.  Just wrap a piece of wire through one of the mounting holes.  Now, attach the other end to something metal on the 3B.  It doesn’t really matter what, but these connections on this wire need to be clean from rust or paint.  You now should be able to turn on the key and move the float up and down while watching the gauge. Perhaps have a tub of water close by that can simulate a full tank and an empty tank.  I can’t remember for sure, but a full tank holds around 5-6” of fuel when full.  Now, some trial and error.  Depending on the float, it might have some adjustability built into it.  If not, slight bends to the rod will adjust the float height when in contact with fluid.

One Caution:  DO NOT leave your key in the “on” position for long periods of time.  While in the trial and error phase, TURN THE KEY OFF.  Very important for the health of your ignition system.

Good Luck.

1- 1946 CJ2A   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Spinnas Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 6:55pm
Along with everything already mentioned(ground, check sender for full movement, etc), there should be a screw accessible from the back of the gauge to calibrate its reading.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote athawk11 Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 7:51pm
Originally posted by Spinnas Spinnas wrote:

Along with everything already mentioned(ground, check sender for full movement, etc), there should be a screw accessible from the back of the gauge to calibrate its reading.

Good point.  Some reproduction gauges do have an adjustment screw on the back.  Originals and some reproduction gauge do not an adjustment screw.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike S Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 10:13pm
SInce the sender is a variable resistance device be careful to clean all the connections from the sender to the gauge. Corrosion, dirt, and paint can affect the gauge reading.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote binthere Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2018 at 10:34pm
It could also be that the float is partly filled with gas and only reads half full. The new gages in the tank have a plastic float and it soon fills with gas. This has been discussed before, and I believe there is a brass float from a mustang that is the same as the plastic one. Just replace the float! Ask me how I know!!  LOL
chuck
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Old Man Army Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2018 at 4:59pm
Gentlemen,

Thanks for all of the advice.  I decided to start simple and pulled the old sending unit.  I cleaned the top and found a bare spot on the body to check if the unit worked.  It does!  I ended up cleaning the rest of the top of the sending unit, re-installing it, and now have a working fuel gauge!  No more wondering if I'm about to run out of fuel while driving.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ndnchf Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2018 at 9:33pm
Glad you got it fixed. 

FYI - Walck's sells a sending unit with a brass float, around $35-40. I put one in over the winter and it works great.
1948 CJ2A - It goes nowhere fast, but anywhere slow.
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