Fuel Gauge inaccuracies

[340Challman]

Hey guys,

I spoke to Thomas at Instrument Specialties today about the problems with the aftermarket fuel sending units. We had a great discussion and I wanted to pass along the knowledge I gained from talking to him about it.

Initially I was thinking that the problem was due to the gauge not being calibrated to the new sending unit. It made sense to me that the resistance of the rheostat in the new sending units would be different than factory. So I reasoned that if you calibrated the gauge to the new sending unit, you should regain your accuracy. Unfortunately the issue is a bit more complex.

The original sending units resistance was non-linear. As the float moved from empty to full, the resistance increased more logarithmically. If you graph it, it is a curve. The aftermarket units have a completely linear resistance. Straight line resistance through the entire range of motion. So naturally, you are never going to get an accurate reading with those differences.

Thomas said that what he can do is calibrate your gauge to the min and max resistance values on your sending unit. This will give you accuracy at full and empty. The gauge will not be accurate once you come off of full, but as the float moves down the accuracy will increase until you reach empty where the gauge will accurately let you know that you need fuel.

This is acceptable to me. Since your gauge won't be off the scale on the high end, which looks crappy and by the time it is showing 1/4 tank, it will be fairly accurate. If you like walking, when it says E, you'll be walking.

I hope this sheds some light on why the new sending units give us so much grief.   Keep on

[7212Mopar]

Tanks inc meter match allows calibration at four points of gauge travel.

[340Challman]

Tanks inc meter match allows calibration at four points of gauge travel.

Yeah, and that is definitely a good solution. I just don't want to get into my wiring and also have to find a place to tuck the unit. But for those who are not concerned with that, it is probably the most accurate solution for the new sending units.

[7212Mopar]

Hire it behind the DS kick panel. The connector and the wiring to sender is also in there.

[HP2]

In my mind, it begs the question why someone can't create a non-linear winding similar to what the factory used back then. Surely the capability is there, even, or especially, 50 years after the fact.

Knowing this, I may see if I can adapt a 3/8 tube to my original 5/16 sending unit, or vice-a-versa.

[340Challman]

In my mind, it begs the question why someone can't create a non-linear winding similar to what the factory used back then. Surely the capability is there, even, or especially, 50 years after the fact.

Knowing this, I may see if I can adapt a 3/8 tube to my original 5/16 sending unit, or vice-a-versa.

Someone could, if they weren't only interested in profit. That has been a beef of mine for a long time. Very seldom do enthusiasts delve into the repro manufacturing world. It's always businessmen, and businessmen only care about profit. Oh well, what ya gonna do?

[HP2]

Well, the whole point of creating a business is usually a profit motive, unless you are in a position that you don't need it for income and you do it as a charity. I wouldn't fault anyone for profit motive.

What I suspect is happening is the senders windings are universal and most cars use linear windings, so they put it together with some Chrysler specifics pieces like the tube, housing, and mount plate, and call it good.

[tparker]

I would think you could create it fairly easy with some electrical knowledge. The problem of linear is probably that of accessible resources. A lot has changed over the last 50 years and lots of parts are made different. It might be worth looking into. My gas gauge never worked when my car was running, and it had a bad fuel pump. So I didn't really know when I was out of gas or the fuel pump was acting up. LOL

As for business and profit? Of course you need profit if you're in business. Business cost money to run and that requires profit. Without profit the dodge brothers wouldn't have been able to grow out of their first shop. Summit racing would be a tiny parts store. A company can't grow and do research without profit. If your a business owner, you are putting a lot of capital into a business. Acquiring a building, paying employees before you have sold a single item, paying taxes, buying equipment and/or inventory, buying retail equipment, marketing, web presence, etc. All this and outlay of cash and there is NO guarantee that you will make a dime back. The hope/desire for profit is what gives people the incentive to provide some service. Then as you grow as a small business you need to expand employees, facilities, product, tooling, etc. This comes from profits. Enthusiasts provide a ton of great services, but they usually don't whittle cam shafts in their spare time, pound out a hood or roof after hours, our cast rubber tires on their weekends. Don't forget, many companies participate in profit sharing. Too many people today are beating up on making money. Without profits there wouldn't be an E body nor the resources to restore them.

[70vert]

Curious why the difference, linear vs non-linear, is the MOPAR design better for some reason or just because of different engineers?
I've just assumed mine was off because it is a 50 year old car LOL.

[340Challman]

Yes, profit is necessary, but I can't believe that you cannot make a profit by manufacturing and selling correct parts that work and fit properly. 

Curious why the difference, linear vs non-linear, is the MOPAR design better for some reason or just because of different engineers?
I've just assumed mine was off because it is a 50 year old car LOL.

My guess would be due to the arc of the float travel. If the float travel was purely vertical, linear may work better.

50 years later, I would think that you should have no problem reproducing the correct resistance curve. That's where I make my profit statement. It may cost a few pennies more to do it correctly. Therefore to maximize profit they don't. In my opinion, if you are not going to make a part that fits and functions properly, why do it at all. It's very frustrating to spend hundreds of dollars on something only to find that it either doesn't fit or doesn't function properly.

But, as I said, oh well, whatcha gonna do? 

[GoodysGotaCuda]

Interesting thread.

I think you'd find that most people here aren't really concerned if the needle is just above or just below the halfway mark, in regards to accuracy.  As long as it's a consistent [didn't say accurate] gauge sweep, it works just fine for 99% of the buyers out there.

We old car folks just don't want to run it empty.


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[Bullitt-]

I've said it before & I'll say it again..... These fuel gauges were not accurate when they were new...  Every vehicle I was familiar with prior to 2000 would get many more miles out of the 1st half of the gauge indicated tank than the second half...
I had to remove and adjust my '98 GT's new sending unit 2 times to get it close to the originals reading which is there is about half a tank when the needle indicated just under 3/4 otherwise it never read full.
   

[340Challman]

  Yep. Agreed Bullit-. I remember my mother had a 4Runner that when the needle hit the half way mark you better start looking for a station because it dropped like a rock after that. 1/2 was probably 1/4 in reality. Never looked to see what the shape of the tank was, just noted it, and acted accordingly.

Absolutely Goody. As long as I know at a 1/4 tank to start thinking gas, then I'm good. Don't like hoofing it for gas anymore these days. Didn't like it then either, but now it would be a real pita. Not to mention embarrassing as hell. 

All of those reasons were why I said The solution from Thomas is acceptable to me. I just wanted to pass along the reason for the inaccuracies/differences with the new sending units.