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Alternative solution for fuel tank sender high resistance?

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MarcMGA1959 Avatar
MarcMGA1959 Marc Ewbank
Dion Valmont, Belgium   BEL
On my MGA of 1959, I have a new fuel tank sender unit that has an incorrect resistance value (replaced by previous owner). For example, when the tank is at 1/2, the resistance is 73 ohms (instead of 35) ; 1/1 to 130 ohms instead of 70 ohms). (see namely https://www.mgexp.com/phorum/read.php?2,2956266 or many others also on mgaguru)
1- In order not to replace the fuel tank sender unit, can we not add a resistance in parallel to the sender unit (or in the gauge) to reduce it to an equivalent of 70 ohms with the tank full?
In my case if the resistance with full tank is 130 ohms, a resistance of 150 ohms lowers the value close to 70 ohms.
2-What will be the impact on the current and will there not be a risk of burning the other resistors?

Depending of réactions I will try this WE...;-)

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ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
You would need to use a 1 watt resistor, to be safe.12V across a150 ohm resistor is just under 1 watt. I know that the resisitor is unlikelt to be exposed to a full 12V but be safe. Everything else, inthe circuit should be safe.

herb



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colynf Avatar
colynf Colyn Firth
South Yorkshire, UK   GBR
I recently fitted a new repro sender unit which has a fairly accurate resistance range of 0.5 to 72 ohms and which was a vast improvement over the previous sender unit which was also a repro and had a range of 15 to 98 ohms.

The old sender unit worked to a point but it only moved the gauge needle from full down to 1/3rd full and so you never really knew how empty the tank was unless you kept an eye on the mileage.
The new gauge was a big improvement but even though the resistance range was much closer to the correct range of 0 to 70 ohms, the fuel gauge would still never drop below 1/8th full and so you were never really certain how much (or how little ) fuel that you had left in the tank.

I decided to try the "Gauge Wizard" by Spyda which was supposed to let you match your sender unit with your gauge. This is the post I put in the UK forum about my experiences with it.

"Posted 13 July 2018 at 22:57:49 UK time
Colyn Firth, South Yorkshire, United Kingdom
I fitted a new fuel sender unit a year or 2 ago which was a vast improvement on the old unit because its resistance range was almost correct.
Its resistance range is 0.5 to 72 ohms which is really close to the correct range according to MGA Guru) of 0 to 70 ohms.

The old sender unit had a resistance range of 15 to 98 ohms which meant that the gauge showed full most of the time and dropped to about 1/3rd full when the tank was empty.

The new sender unit was much better in that the gauge did drop to about 1/8th full but would never quite drop to empty.

The trouble was that I started to trust the gauge a lot more than I did before which was a mistake, I normally keep an eye on the odometer to see how many miles I have driven since refilling the tank and doing this combined with an eye on the gauge has worked fine.

However, if you have ever attended any of the MGCCs MGA Weekend Tours or similar, you are often told to zero your trip at a certain points so that you can follow the route.
This meant that it is really easy to lose track of your actual mileage and this meant that I ran out of fuel on the way home from 2 of these events!


So a few months ago I bought Gauge Wizard (From Spyda) to see if its digital processor would let me correct the gauge so that it read correctly.

I had read about it in Safety Fast Magazine when the MGA Scribe Mr Edward Van Dyke wrote about the Gauge Wizard by Spyda. He had arranged with Spyda to include the MGAs sender unit (0 to 70 ohms) to be included in its range of built in pre-programmed range (or Pre-Sets)
This means that you install the gauge wizard, select the MGA pre-set and off you go.

Well, its no surprise to know that using the pre-set, my gauge didnt work any better than it did before!

So I had to then go through the Spyda recommended procedure of filling the tank in 2 1/2 gallon steps and programming the gauge wizard each step of the way.
You start with an empty tank and work your way up to the tank being full.

It is a bit tedious and I must admit that I was not totally comfortable with the process as you have to handle a lot of fuel to do this. (In did keep a couple of heavy duty fire extinguishers handy.)

You have to input the sender units output to the gauge wizard in 1/4 steps, from empty through to full so that it remembers each setting.

Then you programme the gauge wizard to move the gauge needle to match the newly memorised settings from the sender unit.
This means that when the sender unit sends empty, the needle also shows empty and when the sender unit send full, the gauge then also shows full. etc

At first I was unsure if there was any real improvement, the gauge showed full for longer than I expected. But this was because when I checked the resistance reading of the sender unit on the 3/4 full mark, there wasn`t so much difference from the full resistance reading.

However, once the fuel level dropped, the gauge began to show much more accurate readings.
I found that when the gauge showed 1/2 full there were exactly 5 gallons remaining in the tank and when the gauge showed 1/4 full there were exactly two and a half gallons remaining.

I have also installed the warning light option to act as a low fuel warning light, this illuminates continually when the needle drops to the 1/4 level and then flashes when the needle drops to 1/8 full.

It is a refreshing change to be able to look at the fuel gauge and know that what you see is what you have really got!

Cheers
Colyn
"

This is the link to this post in the UK forum.

http://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=61528672800012&mode=archiveth&subject=6&subjectar=6&thread=2018071322574923959

Colyn



Edited 2 time(s). Last edit at 2019-01-05 08:01 AM by colynf.

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rainon Ray F
Ipswich, UK   GBR
Yes adding a 150 ohm resistor will improve things but be aware that although it should give correct fuel gauge readings at full and empty, it will over-estimate the fuel level by about 15% at half full because the 150 ohm shunt will provide about 45 ohms rather than the desired 35 ohms at the halfway mark. However this is much better than you have now.

The current in the sender will not be affected by this so it is quite safe. But I would echo Herbs comment about making sure the resistor is sufficient wattage and I would go further by using a 5W rated wire wound resistor as they only cost pence.

You can fit the resistor across the sender terminals or, probably easier, fit it at the gauge connecting the resistor from the sender wire terminal to earth.

Have a good weekend.

Ray

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MGARuss Avatar
MGARuss Silver Member Russ Sharples
East Windsor, NJ, USA   USA
1960 MG MGA 1600 "Bonnie"
1993 Mazda MX-5 "Bombita"
As documented in Barney's picture (first figure below), the sender is in parallel with the "right" coil and in series with the parallel combination of the "left" coil and the "upper" resistor (see http://mgaguru.com/mgtech/electric/fg_05.htm for what these components are). This combination of the left coil and upper resistor is a 60 ohm load (1 / (1/101 + 1/154)). If the sender is 130 ohms at full, the right side combination is 56 ohms. If you put 12v across the system you have about 12v / 120 ohm = 0.1 amps flowing through it with the current dividing between the paths. You are proposing to put a 150 ohm resistor in parallel with the right 99 ohm coil and the 130 ohm sender. The maximum current through that resistor will be when the sender is at 130 ohms. So that combination is 1 / (1/99 + 1/130 + 1/150) = 41 ohms. So now we have 12v across 100 ohms total for a total current of 12/100 = 120ma. 120ma passing through 60 ohms produces a voltage drop of V = I*R = 0.120 * 60 = 7.2v. This means there will be 4.8v across the right side components. The current through the 150 ohm resistor will be I = V/R = 4.8/150 = 32ma. Power dissipated by this resistor is P = V*I = 4.8*0.032 = 0.154 watts. Therefore, a standard 1/4 watt resistor will do for this job.

The next question is how will your gauge read. Assuming your gauge is calibrated normally, 17 ohms = 1/4, 35 ohms = 1/2, 52 ohms = 3/4, and 70 ohms = Full. You didn't say what your sender reads at empty but I'll assume it is zero and I'll assume it is linear reading 35 ohms at 1/4 and 105 ohms at 3/4. Plotting out the resulting resistances we get the graph shown in the 2nd figure below. The "Corrected" line is the one you get with the 150 ohm resistor while the "As-Is" line is what you have today.

Russ



Edited 2 time(s). Last edit at 2019-01-07 10:30 PM by MGARuss.


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Osiris610 Silver Member Michael Whalen
Tulsa, OK, USA   USA
The Gauge Wizard, as explained by Colyn, seems to be the perfect solution to new sending units lacking the correct 0-70 ohm resistance. I am wondering why there hasn't been more discussion of it. Are there drawbacks?

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barneymg Avatar
barneymg Barney Gaylord
(Somewhere in USA), Pick one (or more), USA   USA
1958 MG MGA "MGA With An Attitude"
I have to disagree with the last statement. The idea of spending extra money for an additional electronic gadget, and adding complexity to increase the likelihood of future failure or future maintenance just rubs me the wrong way. Why would you want to spend $50 or so for a gadget to compensate for a fault in a replacement part when the replacement part should be good and should not need to be "fixed". The correct solution is to get the vendors to supply good sender units. You do that by refusing to buy the bad ones.

The real perfect solution is to use original fuel gauge and mate it with an original sending unit, all properly calibrated. If the original sending unit has failed and must be replaced with an aftermarket part, the next best perfect solution is to buy a replacement sending unit that has resistance the same as the original one.

Before you ask the next obvious question, yes, a good sender unit is available. The current sender unit from Moss Motors seems to be a good piece of work. I have installed a few of these recently, and they work quite well. Records of some of these installations are in my travel log for: Thursday May 5, 2016; Tuesday March 31, 2018.

When an original fuel gauge is out of calibration, it is not very difficult to adjust it to work properly. You can likely do that quicker than you can install The Gauge Wizard. When an existing original type sender unit might be faulty (but still working out of range), then The Gauge Wizard might be slightly less expensive than a new sender unit. In that case it may be owner's choice to bastardize the car and add complexity to save a little cash (with emphasis on "little"winking smiley.

If someone feels the need to install a non-original fuel gauge in the car, or if they have procured a car with the wrong fuel gauge, then it may be owner's choice to bastardize the car and add complexity with The Gauge Wizard, or maybe procure an original fuel gauge, or possibly install a sender unit from the vehicle that donated the incorrect gauge. My personal preference for correcting the problem would be to exhaust all avenues of matching the gauge to sender unit (or vice versa), and go to The Gauge Wizard only as a last resort if there is no other way to make the gauge work. The Gauge Wizard is a solution looking for a problem. You have to create the problem before you can use this as a solution.

Preferences aside, I also have answers to a couple of the issues brought out in Colyn's tech article detailing installation of The Gauge Wizard. In the end he reports that it worked perfectly at 3/4, 1/2, 1/4 tank. The issue with the gauge being stuck on "F" for too long comes from the fact that the float hits the top of the tank (or end of travel before it hits top of tank) when there is still a little air space up there. Then the gauge should be on the "F" mark and should stay there as you add the last gallon or so of fuel. When driving you have to use that much fuel before the sender float begins to move down and the gauge begins to move away from the "F" mark. Something similar happens at the "E" mark when the float hits bottom of tank and stops moving when there is still about a quart of fuel in there. Then you might drive another 5 miles or so on "E" before it runs dry. These same issues exist whether you are using The Gauge Wizard or not.

Finally, getting back to the original question from post #1, when the sender unit goes from 0 to 130 ohms, the gauge will sit on "F" until you have used half tank of fuel, and then progress downward from "F" to "E" as you use the rest of the fuel (like the blue line on the graph above). That's not so bad, as long as it reads "E" when the tank is actually empty, so maybe you won't run out of gas. If you install a 120 ohm resistor in parallel with the sender unit, you may get the gauge to read "F" when it is full. However, with the parallel resistors net resistance will then be increasing as fuel level goes down, quickly increasing the gauge reading to peg the needle past the "F" mark, and never going lower than "F". Since resistance would never go to zero, the gauge would never go to "E". There are some other ways to look at the diagram, as a matter of interpretation, but all with the same result.



Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com
(Please email me direct, do not leave a PM on the public server).



Edited 4 time(s). Last edit at 2019-02-10 07:45 PM by barneymg.

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colynf Avatar
colynf Colyn Firth
South Yorkshire, UK   GBR
Barney says It may be owner's choice to bastardize the car and add complexity to save a little cash winking smiley

Oops,looks like I may now officially be a "Bastardiser!" spinning smiley sticking its tongue out and a cheapskate at that! smiling smiley

The Gauge Wizard does work really well though and I think it will live with it until I can find a really accurate sender unit that works like it should. My new Moss sender unit (bought last year) was much closer to the original 0 to 70 ohms range, but not quite close enough for the gauge to show empty when the tank was completely empty

I didn`t have to modify the wiring in any way to fit it either, in other words, "No MGA wires were harmed during the fitting of this gadget!" tongue sticking out smiley

You are supposed to break into the wire between the sender unit and the gauge and connect the gauge wizard in between the two.

I was a little sceptical that it would work as well as advertised and so I just disconnected the ring connection from the terminal on the back of the gauge and made a loop to fit through the wizard back to the gauge.

Then if it didnt work, I could then just reconnect the ring terminal back to onto the gauge.

To be honest, I have never had a fuel gauge that is so accurate, especially when nearing empty, also I like the extra features the Wizard gives.

I do like the needle damping circuitry (anti-slosh) that prevents the needle moving about because of rapid fluctuations of the fuel level that sometimes happens when cornering or braking hard.
I also like the 2-stage LED low fuel warning light option,which works really well, but I can also understand that some owners may prefer not to have such a modern looking gadget on their MGA.

The low-fuel LED is an option that you can choose not to use on the Gauge Wizard, but if you do actually like the principle of it, but not the modernity, then you could always rig it up to an electric actuator that raises a much more, period correct, red flag on the dash with the words "Low Fuel" on it. smileys with beer

Cheers
Colyn



Edited 1 time(s). Last edit at 2019-02-11 09:27 AM by colynf.

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barneymg Avatar
barneymg Barney Gaylord
(Somewhere in USA), Pick one (or more), USA   USA
1958 MG MGA "MGA With An Attitude"
If you short the tank terminal to ground (0 ohms resistance), the gauge should go to "E". If it does not quite get to "E", even after proper adjustment, you may need to bend the needle a bit to place it in the desired position over the "E" mark.



Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com
(Please email me direct, do not leave a PM on the public server).

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