Gas gauge sending unit? - JeepForum.com
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post #1 of 20 Old 02-24-2018, 06:50 PM Thread Starter
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Gas gauge sending unit?

The aftermarket gas gauge sending unit I put in my Jeep a couple years ago was giving me false readings. I'm in the process of replacing the tank now and decided to have a look at the sender. The FSM specs are: full 10 ohms, half tank 23 ohms and empty 73 ohms. I measured the depth of the tank and it's
9 1/2" to where the sending unit mounts. I clamped the sender to the edge of my work bench and measuring from the top of the bench, I get these readings:

Empty all the way down (9") 75 ohms
Half (4 3/4") 42 ohms
Full with the arm at the stop, 12 ohms

It's a Spectra sending unit. The half tank indication is too far out to do anything. What's a good brand to replace it with?

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post #2 of 20 Old 02-25-2018, 05:54 AM
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That is out quite a bit for the half tank reading. I set my Crown Automotive unit up the same way, on the bench with measurements and it has been working for years now.

The only thing I changed on the Crown brand sender was the float. Took the plastic one that came with it and tossed it in the trash and used either my original brass one or a brass float from a Ford Explorer.

The other difference on the Crown unit is the threads for spade connection. Its not standard threads but metric. Maybe 4 or 5mm.

This one:https://www.quadratec.com/products/55107_02.htm
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post #3 of 20 Old 02-25-2018, 07:02 AM
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I just did the math, and my guess would be the half reading should be 43.5, I would hook it up to the car and do the same test, full-half-empty, and see how the gauge reads. I don't think I've ever owned a car that the gauge was 100% accurate. You just learn what your car does and act accordingly. I've had mostly fords, and when you fill them they stay on full for a couple of days before they start to go down. If you street drive, you don't have to be very accurate, and if you off-road, the best gauge is a 2.5 gallon Jerry Can.
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post #4 of 20 Old 02-25-2018, 07:40 AM
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Iím guessing that even after replacing the sending unit, you will have the same issue. The sensor reading are pretty close to the full and empty reading in the FSM. 24 is 1/3 of 73 not 1/2. I think the gauge is bad. I would look up John Strenkís gauge testing procedures.
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post #5 of 20 Old 02-25-2018, 09:52 AM Thread Starter
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Originally Posted by OrangeCJ-5 View Post
I’m guessing that even after replacing the sending unit, you will have the same issue. The sensor reading are pretty close to the full and empty reading in the FSM. 24 is 1/3 of 73 not 1/2. I think the gauge is bad. I would look up John Strenk’s gauge testing procedures.
Just looked at the FSM again 1/2 tank should be 23 ohms. The gauge was reading properly with the original sender. A couple years ago I took the sender out to repair a broken O ring and when I removed the sender and started fooling with it, the resistive element fell apart so I stuck this aftermarket replacement in. Then the gauge readings went haywire. Now, the tank has a leak at the bottom so had to remove it and thought it would be a good time to trouble shoot the sender and gauge. I think the gauge is still good. Looking at Keith's post, I think I'll order a Crown sending unit. I have also been looking for resistors that I can use to check the gauge with also but unfortunately for me Radio Shack closed up our local store.
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post #6 of 20 Old 02-25-2018, 10:53 AM
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Garage
I saw a guy who couldnít calibrate stock gauge and put a hiddened hinged aftermarket gauge under the dash on the restoration cj. Hinges down and up

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post #7 of 20 Old 02-25-2018, 12:31 PM
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If you have a leak in the bottom of the tank, it might be time to look at an aftermarket tank. I think the aftermarket 21 gallon tank uses a different sender and may work out better. The plastic 20 gallon tank if it is protected should never leak because of rust. It is just a thought, but if you have several problems, it might be time to start new rather than putting a patch on a rusted out or punctured tank.

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Originally Posted by 80cj View Post
Just looked at the FSM again 1/2 tank should be 23 ohms. The gauge was reading properly with the original sender. A couple years ago I took the sender out to repair a broken O ring and when I removed the sender and started fooling with it, the resistive element fell apart so I stuck this aftermarket replacement in. Then the gauge readings went haywire. Now, the tank has a leak at the bottom so had to remove it and thought it would be a good time to trouble shoot the sender and gauge. I think the gauge is still good. Looking at Keith's post, I think I'll order a Crown sending unit. But I have also been looking for resistors that I can use to check the gauge with also but unfortunately for me Radio Shack closed up our local store.
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post #8 of 20 Old 02-25-2018, 12:48 PM Thread Starter
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Quote:
Originally Posted by StoneTower View Post
If you have a leak in the bottom of the tank, it might be time to look at an aftermarket tank. I think the aftermarket 21 gallon tank uses a different sender and may work out better. The plastic 20 gallon tank if it is protected should never leak because of rust. It is just a thought, but if you have several problems, it might be time to start new rather than putting a patch on a rusted out or punctured tank.
Good thoughts but I already bought a replacement 15 gallon tank. The original lasted 38 years so hopefully I get at least half that out of this one. Actually, the way that Jeep uses gas since I went to 4.10s, a 21 gallon tank would not have been a bad idea.
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post #9 of 20 Old 02-25-2018, 04:41 PM
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When you push it up, At what height will it give you 75 ohms?

Sometimes the wiper goes way past the end of the windings. This would give you an offset at the half way mark.


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post #10 of 20 Old 02-25-2018, 05:22 PM Thread Starter
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Originally Posted by John Strenk View Post
When you push it up, At what height will it give you 75 ohms?

Sometimes the wiper goes way past the end of the windings. This would give you an offset at the half way mark.
The 75 ohm E reading (actually 75.6) is with the arm at the bottom stop or 9" down from the top of the tank, just off the tank bottom which is OK since at that point you're basically empty. There are plastic stops that prevent the wiper from traveling beyond the end of the windings. It's the midscale that causes problems and I think the reason that after filling the tank up, the gauge would drop rapidly to about 3/8 full indication(just over 1/4 tank left) then slow down. I don't think theres any problem with the gauge although I found and ordered a 0-100 ohm potentiometer on Amazon that I can subsitute in place of the sender to check the gauge readings.
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post #11 of 20 Old 02-26-2018, 06:45 PM
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Quote:
Originally Posted by 80cj View Post
The 75 ohm E reading (actually 75.6) is with the arm at the bottom stop or 9" down from the top of the tank, just off the tank bottom which is OK since at that point you're basically empty. There are plastic stops that prevent the wiper from traveling beyond the end of the windings. It's the midscale that causes problems and I think the reason that after filling the tank up, the gauge would drop rapidly to about 3/8 full indication(just over 1/4 tank left) then slow down. I don't think theres any problem with the gauge although I found and ordered a 0-100 ohm potentiometer on Amazon that I can subsitute in place of the sender to check the gauge readings.
Yes I'm sorry, I mean down.

Can you actually see the wiper arm and windings? Is it rectangular vs trapazoidal?

If it's square then you have a linear sending unit wich is good for an aftermarket gauge or a gauge with a coil type movement like in a YJ. Infact if you calculate the resistance of a linear gauge at half a tank 73-((73-12)/2) = 42.5 ohms.

However our OEM gauges are not linear so the sending unit was designed to give you a non linear response. So you need a sending unit that will give you 23 ohms at 1/2 tank. This is because it takes more power to move the needle in an OEM gauge. Lower resistance, more power, more heat to move the needle.

When you wind a sending unit for an original gauge, it's a trapazoidal winding. The windings on the big end have more resistance between each wrap so it changes faster at the big end for every degree the wiper arm moves.

here is an answer on the AUtometer Website:
Quote:
The short sweep electric fuel gauge manufactured by Auto Meter for the Ford 73-10ohm range is calibrated for the original Ford factory supplied, non-linear, fuel level sender design. If you view the gauge dial itself, you will notice that the halfway point in the gauge is in fact not centered in the field in order for the calibration to match this special sender.

The Ford Fuel Level sensor with a 73-10ohm range operates with the following ohm ranges for Full, Half, Empty, and Below Empty:

Full: 10ohms
Half Tank: 25ohms
Empty: 50ohms
Pointer Width Below E: 73ohms

If you have replaced the original equipment Ford sending unit in your vehicle with an aftermarket sending unit, you may notice that the gauge will match the sending unit at the Full and Empty portions of the scale, but that the readings in between will not match up. The reason for this is that some aftermarket senders operating with the 73-10ohm range now utilize a linear movement instead of the original non-linear scale. The comparative ohm reading difference for Full, Half, and Empty have been supplied below for your reference.

Full: 10ohms
Half Tank: 41ohms
Empty: 73ohms

As you can see these numbers differ radically from those of the factory Ford sending units, and have a drastic effect upon the gauge readings.

In the event that you have this situation, we recommend one of four options:

OPTION 1 You can attempt to seek an original equipment fuel level sending unit directly from Ford for your vehicle.

OPTION 2 If you have an Auto Meter short sweep electric gauge, you may send it into us for service and we can modify the gauge to work with the after market linear sender. The cost is $25.00 + return shipping, as long as there is nothing else wrong with the gauge. (prices subject to change without notice)

OPTION 3 You may use the Auto Meter Full Sweep Programmable fuel level gauge and opt to use the Custom Calibration section of the gauge instructions. The Programmable version is not available in all series, but can be designed and built through the Auto Meter Custom Shop in nearly any standard incandescent lit series (meaning no through-the-dial LED lit designs in Custom Shop). See the following link for Custom Shop http://www.ampcustomshop.com or contact us directly at 866-248-3657 for alternate options:

OPTION 4 You may purchase one of our 240-33ohm scaled gauges and the matching #3262 sending unit. If this sending unit will not fit your specific tank type, you will likely have to retain your existing sender to serve as a fuel pick up source, and then create a new hole on a flat surface on the top of your tank for mounting the new sender. We recommend the 240-33ohm units because they are a broad and fairly linear scale, and provide a very accurate reading of the fuel level in the tank.


Read more at https://www.autometer.com/resources/...xurw7TGltwA.99
https://www.autometer.com/resources/...faq_view/id/12


Here is what a trapazoidal, non-linear winding looks like:




Before I go too far, look at this post:

https://www.jeepforum.com/forum/f8/cj...issue-1370200/


Ahh, just to sum it up. They sent you the wrong sending unit.
Hopefully, there are not 10,000 sending units made the wrong way or miss-labeled. I remeber turn signal lights wired wrong and other things that took years to flush out of the market....
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post #12 of 20 Old 02-26-2018, 11:53 PM Thread Starter
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John, thanks for the explanation and good references. I've attached a photo of the Spectra sending unit and you can see that the resistor element is a linear design, not trapezoidal as you explain. I ordered a Crown sending unit as reccommended by Keith460 from Quadratech. There are reviews on the web page that are positive but I'll test it before I install it.
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post #13 of 20 Old 02-27-2018, 10:00 AM
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Thanks for the picture.!!

I spent 1-1/2 hours looking for a good picture of a linear fuel gauge. Most i found were with the printed resistance elements, not windings like yours.

I would like to use this picture for future comparisons if I may.

Are you going to return the Spectra unit?
If not I'll buy it off you if it's not too expensive.
See, I have these plans...........


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post #14 of 20 Old 02-27-2018, 11:50 AM Thread Starter
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Originally Posted by John Strenk View Post
Thanks for the picture.!!

I spent 1-1/2 hours looking for a good picture of a linear fuel gauge. Most i found were with the printed resistance elements, not windings like yours.

I would like to use this picture for future comparisons if I may.

Are you going to return the Spectra unit?
If not I'll buy it off you if it's not too expensive.
See, I have these plans...........
John, no problem using the picture. I'll send you the Spectra unit as soon as I receive and verify the accuracy of the Crown unit that I ordered from Quadratec which will probably be later this week or early next week. I'll let you know by PM and you can forward your mailing address to me at that time.
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post #15 of 20 Old 02-27-2018, 02:43 PM
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OK Thanks..


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