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Old 09-16-2022, 09:42 PM   #1
vince1
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Fuel gauge reads too high.

Jeep cj5 tank, Equus gauge.

Before removing the gauge some months ago it was going all wonky flickering all over the place.

I put some grounds all over the truck, cab to frame, engine to frame, tank frame to truck frame.

Re-installed radio and fuel gauge. Radio works, fuel gauge reads high. With ignition off the gauge reads about 1/4. Full tank with ignition on it pegs out over full. Do you think it could be internally damaged or is there something else to check? Thanks.
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Old 09-16-2022, 10:55 PM   #2
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Re: Fuel gauge reads too high.

If the Jeep fuel setup is similar to the GM gauge, a pegged out gauge usually means a disconnected wire between gauge and sender or a bad ground from sender to frame.
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Old 09-17-2022, 11:38 AM   #3
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Re: Fuel gauge reads too high.

I should have also written that before filling it up with the key turned to on the reading would jump from 1/4 to 1/2. The amount of fuel I put in was more than half leading me to believe that it always reads about 1/4 too high.

The 1/4 reading with key off or gauge out of the vehicle leads me to believe that the gauge is defective. Should it not zero itself at the empty position? A picture of a new gauge shows the needle on E.

I guess my options now are to run the tank dry and take it down to have a go at the connections (not going to happen unless absolutely necessary), or undo the crimp on the bezel and reset the needle (something to play with), or buy a new gauge.
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Old 09-17-2022, 01:42 PM   #4
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Re: Fuel gauge reads too high.

What the gauge reads and/or where it "settles" with the ignition switch turned to OFF is of no consequences. No voltage is applied to the circuit anywhere, so ignore the OFF reading.. Responding to fuel levels, no matter the accuracy indicates all wiring and components are intact.. The symptoms you have are classic gauge resistor failure. The resistor is a rectangular item attached across 2 terminals on the back of the gauge.. It can't be checked in place, so remove it before testing.. The resistance should be in the 85 - 90 ohm range.. Any other value indicates the resistor has changed value and must be replaced..
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Old 09-20-2022, 05:36 PM   #5
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Re: Fuel gauge reads too high.

What is the fuel sender ohm reading empty
What is the sender ohm reading full

What gauge part number do you have
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Old 09-21-2022, 08:36 PM   #6
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Re: Fuel gauge reads too high.

Thanks guys,

I took the bezel ring and glass off, after having no joy in getting the plastic stuff apart. Then the indicator plate came off easily and basically the needle just fell off too. That gave access to the inside stuff and I put the ohm meter across the resistor and could see that it was still good.

Seeing that I have pretty well a full tank of fuel I went out and hooked up the gauge, turned on the key and set the needle on in the full position. I turned off the ignition and the needle came to rest just under the E for empty. I put the glass and bezel ring back on and after reinstalling the gauge I think it's good to go.

I think it was just a case of the needle coming loose as a result of the gauge flickering wildly way back when. I have no idea what the issue was but I am happy that it is nice and stable now.
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Old 09-22-2022, 07:55 AM   #7
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Re: Fuel gauge reads too high.

I recall putting the ohm meter across the resistor and not getting the 85 to 90 ohms reading. It was in the seventies range if I was reading it correctly. The resistor is just one of those round ones with the coloured bands on it and wires coming out of the ends.

Reading over the post again and RustyPile's comment about the possibility of the resistor value changing tells me the gauge may not be quite as it should. I'll see how it works as the fuel level in the tank runs down. Perhaps I'll be taking it apart again to change that resistor.
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Old 09-22-2022, 03:28 PM   #8
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Re: Fuel gauge reads too high.

Quote:
Originally Posted by vince1 View Post
I recall putting the ohm meter across the resistor and not getting the 85 to 90 ohms reading. It was in the seventies range if I was reading it correctly. The resistor is just one of those round ones with the coloured bands on it and wires coming out of the ends.

Reading over the post again and RustyPile's comment about the possibility of the resistor value changing tells me the gauge may not be quite as it should. I'll see how it works as the fuel level in the tank runs down. Perhaps I'll be taking it apart again to change that resistor.
Those color bands indicate the value and % of accuracy of the resistor. It's physical size indicates the power rating. A flame proof, 85 - 90 ohm, 2 watt resistor is a perfectly acceptable replacement. These resistors can be obtained from any electronics supplier.. Grangers, Mouser, and Digi key are sources I use.

As I stated in my post, you CANNOT test the resistor when it is still attached to the gauge. You must remove it for testing.. The gauge has "built-in" resistance that combines with the external resistance and will give a false reading when using your procedure.. Remove the resistor for checking.. Replace the resistor only if necessary..

Just arbitrarily replacing parts and/or guessing are the worst ways to diagnose a problem.. Use good and proper methods as I have described and you'll solve your problem.

Since you have removed the needle, here's one way to install it properly.. Disconnect the gauge wire at the tank sending unit. Obtain a 45 ohm resistor similar to the one on your gauge. Attach the 45 ohm resistor to the end of the gauge wire and short the resistor to a good ground.. Turn on the ignition switch. If the gauge reads exactly 1/2 tank, the needle is in the correct position.. If not, remove the needle and reposition it to indicate exactly 1/2 tank..

Also, as I stated, where the needle comes to rest when the ignition switch is off is totally irrelevant. It can stop in any position or even remain at its last reading.. Don't clutter and/or confuse your mind (or this thread) with where it stops when the key is turned off..

Last edited by RustyPile; 09-22-2022 at 03:44 PM.
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Old 09-23-2022, 07:20 AM   #9
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Re: Fuel gauge reads too high.

I am happy you have explained how to repair and test it.

I was wondering how a resistor could fail with such small wires attached but after reviewing this it makes sense. if Volts x Amps = Watts then Amps = Watts/Volts. 2watts/14volts = 1/7 amp and that is all a 2 watt resistor could handle. Am I correct in this assumption?
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Old 09-23-2022, 05:36 PM   #10
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Re: Fuel gauge reads too high.

Quote:
Originally Posted by vince1 View Post
I am happy you have explained how to repair and test it.

I was wondering how a resistor could fail with such small wires attached but after reviewing this it makes sense. if Volts x Amps = Watts then Amps = Watts/Volts. 2watts/14volts = 1/7 amp and that is all a 2 watt resistor could handle. Am I correct in this assumption?
Having never measured the actual current draw of a fuel gauge, I would only assume it to be in the milliamp range.. Using Ohm's Law, the current handling capacity of a 2 watt resistor is ~ 142 milliamps, well within range of a fuel gauge.

Even "back in the day" those gauge resistors were hard to come by. Dealerships and auto parts stores just didn't stock them.. I've been using common resistors as replacements for a lot of years.. In those years, I've never seen one fail..
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Old 09-23-2022, 06:30 PM   #11
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Re: Fuel gauge reads too high.

[deleted] duplicate post.........
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