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Unread 09-22-2014, 06:45 AM   #1
Thunder_Chick3n
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Fuel gauge reading wrong

So when i got my 85 CJ7 the fuel gauge didn't work at all. I ordered a new fuel sending unit from Morris Offroad and after a few days I installed it and the gauge worked. The only issue was that it reads a quarter tank lower than what it really is. Does anyone out there have any ideas on what it might be?

Thanks

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Unread 09-22-2014, 07:00 AM   #2
Matt1981CJ7
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The float could be leaking, or just needs to be calibrated (bend the arm a little).

You can also test the ohms from the sender. I think it's the pink wire. Do a search for John Strenks CJ gauge testing page.

Good luck,

Matt
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Unread 09-22-2014, 10:52 AM   #3
80cj
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Many fuel gauge problems can be traced to a bad ground.
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Unread 09-22-2014, 11:06 AM   #4
hp_lovecraft
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Quote:
Originally Posted by Thunder_Chick3n View Post
The only issue was that it reads a quarter tank lower than what it really is. Does anyone out there have any ideas on what it might be?
I've replaced many senders over the years, and -every- one read wrong.
I complained to 4wd once and they told me that you are supposed to calibrate fuel senders.

But why? The gauge has a known range. Why wouldnt the sender be preset to that specific range? Anyway, I've used both Omix, Crown, and Spectra, and they were all way off.

You can calibrate by bending the tabs so it shows the correct reading, but since the tank is already put back in..... and its a pain to remove... I'm guessing you will probobly leave it like it is.
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Unread 09-22-2014, 11:27 AM   #5
Ken4444
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Fuel senders and gauges work based on the electrical resistance of the circuit. The changing float level changes the resistance of the circuit, and the gauge is an ohm meter that reads the resistance of the circuit. If the ground to your sender is poor, the circuit will have higher electrical resistance which will contribute to an incorrect gauge reading. As 80cj pointed out, you need to confirm the sender has a good ground. I replaced my sender and gauge before taking the 3 minutes it took to run a temporary ground wire to the sender to see if that improved my situation. That of course was a step I should have done on day 1.
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Unread 09-22-2014, 11:02 PM   #6
BagusJeep
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They are a PITA.

I replaced the rusted junk in my J20 last week. Now the good thing about a 1980 J20 is that when you slide under you can see the sender unit in the side of the tank, so any question about grounds or resistances etc can be answered straight away and you can get the multimeter directly on the contacts!!!!

When installed it was amazing. First fill up was bang on and it was all set. I left it and a week later it was reading zero. I put in a gallon and it crept up and an hour later was at zero.

Multimeter reads 67 Ohm, so it is definitely not the gauge...... got to pull the darn thing again and I know it has 10 gallons in it!!

Neither of my CJs have reliable gaiuges. Bagusjeep reads half full when it is empty but the needle moves. Puthijeep just sits there.

IF your sender unit and its plastic float are intact then you can adjust the reading by bending arms and tabs but I would not trust any of them. If you can get the empty mark about right you are doing well.
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Unread 09-23-2014, 09:34 AM   #7
Ken4444
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Quote:
Originally Posted by BagusJeep View Post
IF your sender unit and its plastic float are intact then you can adjust the reading by bending arms and tabs but I would not trust any of them. If you can get the empty mark about right you are doing well.
After I finally got my fuel gauge working well and giving consistent information (after replacing the tank, sender, gauge, and ground wire), it was obvious that when the needle was on "E" that I still had about 5 gallons left in the 15 gallon tank. At that point I was thankful for the consistency and decided not to spend any time tweaking it. I know I can drive for a few days as the needle drops lower than "E" and I still have some fuel left. When I buy gas, I will generally pump between 10 and 13 gallons, depending on how much past "E" the needle has gone. Like all of the other vehicles I've driven, you have to learn the gauge and what it's telling you.
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Unread 09-23-2014, 11:46 AM   #8
wm69
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Quote:
Originally Posted by Ken4444 View Post
Fuel senders and gauges work based on the electrical resistance of the circuit. The changing float level changes the resistance of the circuit, and the gauge is an ohm meter that reads the resistance of the circuit. If the ground to your sender is poor, the circuit will have higher electrical resistance which will contribute to an incorrect gauge reading. As 80cj pointed out, you need to confirm the sender has a good ground. I replaced my sender and gauge before taking the 3 minutes it took to run a temporary ground wire to the sender to see if that improved my situation. That of course was a step I should have done on day 1.
Where is the ground located? On my Green Jeep the gauge doesn't read properly. I have a new sending unit but haven't gotten around to installing it yet.

Quote:
Originally Posted by Ken4444 View Post
After I finally got my fuel gauge working well and giving consistent information (after replacing the tank, sender, gauge, and ground wire), it was obvious that when the needle was on "E" that I still had about 5 gallons left in the 15 gallon tank. At that point I was thankful for the consistency and decided not to spend any time tweaking it. I know I can drive for a few days as the needle drops lower than "E" and I still have some fuel left. When I buy gas, I will generally pump between 10 and 13 gallons, depending on how much past "E" the needle has gone. Like all of the other vehicles I've driven, you have to learn the gauge and what it's telling you.
Mine has the 20 gal poly tank on it. Reads full when full, and the gauge hits E when you have 10 gallons yet. I've done the same thing as you, driving it to E and knowing I still have a 1/2 tank. I guess after that, and rarely drive very long after it hits E just in case....would like to have it working properly though.

Interestingly enough, on the Scrambler when we drove it off the lot brand new the gas gauge would only go to 3/4 tank. When it's full, it reads 3/4 on the gauge. When it hits E you still have a little ways to go.
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