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Fuel ga reads too high
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59 in Calif
Posted 2017-03-22 7:08 PM (#536472)
Subject: Fuel ga reads too high


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Posts: 1102
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Location: Hayward, Calif
Hey Guys, I finally got back to the fuel ga problem. 59 chry, I got a new sender from Valley vintage as the old sender was completely unusable. They claimed this sending unit is for 57 - 59 all chry models. I did check the ga by momentarily grounding the circuit going to the tank and the needle jumped up to the full mark. I removed the sender from the tank and reconnected the wire from the ga and grounded the sender housing and manually moved the float arm. At the empty tank position the ga reads 1/2 full tank of gas. Moving the the float arm to 1/2 way position, the ga reads full tank of gas. Bending the arm to compensate is useless as the sender is telling the ga its 1/2 full when in reality the tank is empty. Is their a resister or some electronic unit I can wire into the sys to correct this false reading ? Thanks, Jerry
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59 in Calif
Posted 2017-03-22 7:58 PM (#536478 - in reply to #536472)
Subject: Re: Fuel ga reads too high


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Here's an update,,,, I connected an old Chry electronic ign. ballast resister, rated at 5.6 ohms to the ga side of the sender. With the float arm in the empty tank position the ga reads 1/8 lower than the 1/2 tank reading I was getting without the resister. I'm just guessing I'm going in the right direction. Where would I find a 10 or 20 ohm resister ??
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Powerflite
Posted 2017-03-22 9:09 PM (#536486 - in reply to #536478)
Subject: Re: Fuel ga reads too high



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As long as the sender has a larger ohm range (range = max - min) than what the gauge needs, you can always ad a combination of series and parallel resistors to trim the sender to the perfect settings. I can give you the formula if you wish to do some measurements on your gauge. As far as where to get the resistors, you probably don't need a heavy duty resistor as I don't think the current draw is more than a couple of amps. But it depends on the resistance value that you end up with. Digikey is a good place to purchase them, or even Radioshack still has a pretty good selection.
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59 in Calif
Posted 2017-03-22 9:45 PM (#536490 - in reply to #536472)
Subject: Re: Fuel ga reads too high


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Posts: 1102
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Location: Hayward, Calif
Thanks Nathan, I think I will call Valley Vintage tomorrow and see what they have to say. Probably no help. If you have time, send me that formula and I'l check with Radio Shack to see what they have. I checked out Summit Racing. They have an asortment of senders, with high ohms at the empty tank position and low ohms at the full tank. Then they have some senders that have low ohms at the empty tank with high ohms at the full tank. Makes things a bit confusing. Jerry
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57chizler
Posted 2017-03-23 3:00 PM (#536533 - in reply to #536490)
Subject: Re: Fuel ga reads too high



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Have you checked the actual resistance range with an ohm meter? I believe the range is 0 ohms= Full and 220-240 ohms = Empty

I might be wrong, but I believe that using a single fixed resistor to change the range of a variable resistor will wind up having incorrect readings at one end of the scale.
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wizard
Posted 2017-03-23 3:58 PM (#536536 - in reply to #536472)
Subject: Re: Fuel ga reads too high



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A fixed resistor should only be used for to find out what the variable resistor "should" read on for instance half tank (roundabout 29 ? on a '60 Chrysler).

Once that the proper resistance has been found, the tank sender should be removed and the arm adjusted for to give the proper resistance on for instance half tank.

Not so easy proceedure

Also, most probably the variable resistance housing is full of old gunk, gum residue and even sand - open the housing carefully and clean up the contact and resistor "coil"

As 57chizler states above, adding a fixed resistor will give false readings
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5wndwcpe
Posted 2017-03-23 9:21 PM (#536552 - in reply to #536472)
Subject: Re: Fuel ga reads too high



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If you want to know what the gauge requires, get a 250 ohm potentiometer hook it up in series with your signal wire (using the center tap and either end tap). Rotate the pot until the gauge reads "empty", disconnect it and take a reading with a VOM (volt,ohm meter).

Reconnect it and rotate the pot until the gauge reads "full" and once again disconnect and take a reading.

This will be the range your gauge requires. You will need to repair your sending unit to reflect these numbers or source a new one that does. Stock is supposed to be 15 - 210 ohms but the gauge in my '57 wanted to see 20 and 180. YMMV.

https://www.amazon.com/Potentiometers-250ohms-Round-Linear-Res/dp/B0...

Here is a source that may be helpful.

http://www.mykmlifestyle.com/1956_Chrysler_Fuel_Sender.html


Edited by 5wndwcpe 2017-03-23 9:27 PM
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60 dart
Posted 2017-03-23 11:54 PM (#536563 - in reply to #536472)
Subject: Re: Fuel ga reads too high



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do your self a favor and go here for a correct one -------------------------------------------------------later

http://www.classiccarautoparts.com/gastank_sending_units.html
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Powerflite
Posted 2017-03-24 2:13 AM (#536571 - in reply to #536533)
Subject: Re: Fuel ga reads too high



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57chizler - 2017-03-23 12:00 PM
Have you checked the actual resistance range with an ohm meter? I believe the range is 0 ohms= Full and 220-240 ohms = Empty

I might be wrong, but I believe that using a single fixed resistor to change the range of a variable resistor will wind up having incorrect readings at one end of the scale.



The actual range should be 15-220 ohms, with 15 ohms = full. You are correct in saying that using a single resistor to make the adjustment will only allow you to adjust one side, while leaving the other end wrong. But if you use 2 resistors, one in series and one in parallel, you can adjust both sides to make it perfect. However, as I will show in the math, the numbers go negative (this means no real solution) if the range on the gauge is smaller than the range on the sender. So you must use a sender with a wider range than the gauge.

If you have a guage that is expecting to see 5-75 (= 70ohm range) and a sender that is 15-220 (= 205 ohm range), then you have a situation where you can use 2 resistors to adjust the reading to exactly where you want it. The equations are quadratic and I have chosen the positive root that gives the right answer. The formula is:

P = (T*(G1+G2)+ Sqrt[(T*G)^2+4*G*T*G1*G2])/2*(G-T)
S = T1-P*G1/(P+G1)

Here, P is the parallel resistor value that you want to add and S is the series resistance that you want to add. T1 is the low value on your sender, T2 is the high value and T= T2-T1 is your sender's range. G1 is the low value your gauge wants to see, G2 is it's high value and G=G2-G1 is the range that the gauge wants to see. You can see from the term in the denominator, that the Parallel resistor goes negative once T becomes larger than G and the solution is no longer valid. Also, this only works if the sender goes in the same direction as the gauge. It won't fix putting a Chevy sender in a Chrysler car. Let me know if any of this isn't clear.

In the end though, our cars have such a large range on the gauge, that it is probably not realistic to be able to get a sender with a larger range to be able to use this procedure. Van'sAuto used to sell these aftermarket that were specifically marked with the higher range sender and I used them in my cars. They worked great for me at the time. I know that they aren't always available though, and some people have had some trouble with them.
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Powerflite
Posted 2017-03-27 2:20 PM (#536762 - in reply to #536571)
Subject: Re: Fuel ga reads too high



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Sorry, it has been a while since I have used this. As I wrote it, you need to swap the meaning of G and T. So all the T's should be the resistances for your gauge and the G's would be the sender resistances. The hazard of changing notation, and I copied it down backward.
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