Adding a potentiometer to system to adjust fuel reading

[adam metzger]

I came across an interesting idea on how to adjust your bad senders.

It's simply adding another potentiometer into the system, and setting it at a known fuel level.

they're cheap and easy to add into the wiring.

I thought I'd share as it might be something that a perpetual tinker that owns a GMC coach might be willing to check into. I may actually try it
myeslf.

https://sites.google.com/site/shannonpowerlab2/home/calibrating-your-fuel-gauge

Thoughts?

--
1976 Palm Beach
Austin, TX

 

[a.]

> I came across an interesting idea on how to adjust your bad senders.
> It's simply adding another potentiometer into the system, and setting it at a known fuel level.
>
> they're cheap and easy to add into the wiring.
>
> I thought I'd share as it might be something that a perpetual tinker that owns a GMC coach might be willing to check into. I may actually try it
> myeslf.
>
> https://sites.google.com/site/shannonpowerlab2/home/calibrating-your-fuel-gauge
>
> Thoughts?
You can get a little improvement, but it will never be right. Your last post on your measurements said 10 - 100 ohms. 900 ohms in parallel to the
sender will get you 90 ohms at the gauge when the sender is at 100, but will only drop the 10 ohms at the low end to 9.9. Anything less than 900 will
lower the top end to less than 90. 500 ohms in parallel would give you 83 when the sender is at 100 and 9.8 when the sender is at 10. The only way to
lower the low end appreciably without messing up the top end is to get the sender low end down. There is no such thing as negative resistance. The 900
ohm resistor will make your gas gauge start to drop sooner than it would without the resistor. The 10 ohms will keep the gauge at 10% when the tank is
dry. No matter what you do, that 10% reading when the tank is empty is unavoidable unless/until you get the sender resistance down. With 100 ohms and
no parallel resistor, 10% of your gas will be gone before the gauge starts to drop. All that 900 ohm resistor buys you is a more accurate/responsive
gauge at the full end, it gets you next to nothing as you get closer to empty.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."

 

[Jim Hupy]

You guys need to bore a hole through the floor in your coaches, through the
top of the fuel tanks. Then, just use a stick of wood placed through the
hole to measure the contents. When you are through, put a cork in it. (Grin)
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> > I came across an interesting idea on how to adjust your bad senders.
> > It's simply adding another potentiometer into the system, and setting it
> at a known fuel level.
> >
> > they're cheap and easy to add into the wiring.
> >
> > I thought I'd share as it might be something that a perpetual tinker
> that owns a GMC coach might be willing to check into. I may actually try it
> > myeslf.
> >
> > https://sites.google.com/site/shannonpowerlab2/home/
> calibrating-your-fuel-gauge
> >
> > Thoughts?
> You can get a little improvement, but it will never be right. Your last
> post on your measurements said 10 - 100 ohms. 900 ohms in parallel to the
> sender will get you 90 ohms at the gauge when the sender is at 100, but
> will only drop the 10 ohms at the low end to 9.9. Anything less than 900
> will
> lower the top end to less than 90. 500 ohms in parallel would give you 83
> when the sender is at 100 and 9.8 when the sender is at 10. The only way to
> lower the low end appreciably without messing up the top end is to get the
> sender low end down. There is no such thing as negative resistance. The 900
> ohm resistor will make your gas gauge start to drop sooner than it would
> without the resistor. The 10 ohms will keep the gauge at 10% when the tank
> is
> dry. No matter what you do, that 10% reading when the tank is empty is
> unavoidable unless/until you get the sender resistance down. With 100 ohms
> and
> no parallel resistor, 10% of your gas will be gone before the gauge starts
> to drop. All that 900 ohm resistor buys you is a more accurate/responsive
> gauge at the full end, it gets you next to nothing as you get closer to
> empty.
> --
> 73 23' Sequoia 4 Sale
> 73 23' CanyonLands Parts Unit 4 Sale
> Upper Alabama
> "Highest price does not guarantee highest quality."
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[adam metzger]

> > I came across an interesting idea on how to adjust your bad senders.
> > It's simply adding another potentiometer into the system, and setting it at a known fuel level.
> >
> > they're cheap and easy to add into the wiring.
> >
> > I thought I'd share as it might be something that a perpetual tinker that owns a GMC coach might be willing to check into. I may actually try
> > it myeslf.
> >
> > https://sites.google.com/site/shannonpowerlab2/home/calibrating-your-fuel-gauge
> >
> > Thoughts?
> You can get a little improvement, but it will never be right. Your last post on your measurements said 10 - 100 ohms. 900 ohms in parallel to the
> sender will get you 90 ohms at the gauge when the sender is at 100, but will only drop the 10 ohms at the low end to 9.9. Anything less than 900
> will lower the top end to less than 90. 500 ohms in parallel would give you 83 when the sender is at 100 and 9.8 when the sender is at 10. The only
> way to lower the low end appreciably without messing up the top end is to get the sender low end down. There is no such thing as negative
> resistance. The 900 ohm resistor will make your gas gauge start to drop sooner than it would without the resistor. The 10 ohms will keep the gauge
> at 10% when the tank is dry. No matter what you do, that 10% reading when the tank is empty is unavoidable unless/until you get the sender
> resistance down. With 100 ohms and no parallel resistor, 10% of your gas will be gone before the gauge starts to drop. All that 900 ohm resistor
> buys you is a more accurate/responsive gauge at the full end, it gets you next to nothing as you get closer to empty.


fantastic! Thanks for the info. I think with the range it's showing now, plus gauging the odometer, I;ll be okay.


--
1976 Palm Beach
Austin, TX

 

[Ken B]

Fuel gauges are not a precision device and you are never going to get the gauge to read exactly what you have. As an example the rule on an aircraft
fuel gauge is only one. The gauge must read empty when there is no fuel in the tank(s). After that there are no precise rules and a lot of variance
in readings occurrs.

If you need a precise measure measurement then look at a fuel totalizer. They have a sender in the fuel line that senses the fuel flow through it on
gas going to the engine. Then that amount is subtracted from the pre-programmed amount of fuel stored in the tank and displayed to the user.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[adam metzger]

Nope, I've always driven used cars, with goofy this or not-working that on them. I'm not by any means needing a perfectly accurate gauge. The range
I've got is fine, and until it completely stops working, will suffice. I shared simply because I thought it was an interesting and creative solution
to an issue that seems to be a regular concern with these aging coaches.
--
1976 Palm Beach
Austin, TX

 

[neil martin]

Any reason this wouldn't work with an electric oil pressure sender to "dial in" higher readings?
--
Neil
76 Eleganza now sold
Los Angeles

 

[johnny]

Do you want higher readings than the sender is calibrated for? It isn't going to be accurate in that instance. The sender is built to provide a
stated resistance at full scale, full scale being the high pressure it's made for. The wiper on the resistance wire is all the way at the end when
the rated pressure is reached. Higher pressure won't move it any further since all the resistance is already in place. Adding external resistance
will - depending on the gauge - make it read higher but it will do so at the sender's rated pressure, not a higher pressure. Worse, added resistance
will keep the gauge from reading zero when there's no pressure, because there will still be the added resistance in the line.

Now, if you have a higher reading sender, but want to keep the gauge at full scale when the higher pressure is reached,padding the sender with a
resistance will accomplish this, and the zero point will remain the same. How well it might track throughout its range is open to question.

Probably the best approach is to use a matched sender and gauge, and not have to worry about adding stuff.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"The road goes on forever, and the party never ends" --Robert Earl Keen

 

[johnny]

Oh, Padding' in this case means adding resistance in parallel with the sender.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"The road goes on forever, and the party never ends" --Robert Earl Keen

 

[terry1]

"There is no such thing as negative resistance."

Negative resistance exists in plasma conduits (fluorescent light bulbs) and ionized gases (such as the junction between a brush and moving commutator
in a motor or generator, or lightning or other electric arc).

The actual unit of measurement is "mhos".
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

 

[neil martin]

Johnny

Right as always.

Truth is my question was for a non-gmc application with mismatched foreign senders and gauges where once the motor gets hot the sender reads really
low. I was just trying to sneak some general knowledge out of the brain trust. Best would be to put in a mechanical gauge but not ascetically
feasible.
--
Neil
76 Eleganza now sold
Los Angeles

 

[a.]

> "There is no such thing as negative resistance."
> Negative resistance exists in plasma conduits (fluorescent light bulbs) and ionized gases (such as the junction between a brush and moving
> commutator in a motor or generator, or lightning or other electric arc).
>
> The actual unit of measurement is "mhos".
A mho is the reciprocal of an ohm (1/ohms), not the negative. What I was trying to convey is that a passive device is not going to subtract 10 ohms
from the reading. Creating the effect of subtracting resistance from the subject sender would require active components, like an op amp.

If I was going to expend any effort on it, I would build a circuit that would convert the resistance reading to gallons remaining and feed a digital
readout.

Digital readouts of the gas gauges would be fun to watch in mountains. Might be distracting enough to cause problems.

If all you want to know is a general idea of half a tank, plus or minus, then 10 - 100 is "good enough".
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."

 

[jerry work]

I approach this from a different perspective. Why bother trying to make these 40 year old senders and gauges function properly? A GPS speedometer/odometer will provide you with dead on accurate readings for miles traveled. Drive for 250 miles and refill. Your backside needs the blood and legs need to move by then, anyway. You will quickly learn how much fuel your coach/driving style burns so you can spot any radical changes easily. And, you will get a proper idea of fuel mileage. With our mechanical or electronic speedometers off from age, mis-calibration, gear changes and etc., and with the speedometer sending units a source of transmission fluid leaks, I find more gain and no pain from simply relying on an accurate measure of miles traveled instead of some guess as to how much fuel is left in which tank. My take on KISS, anyway.

Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR

glwork
http://jerrywork.com
1977/1994 Clasco (about to head for Mexico if the snow ever clears in normally sunny southern Oregon)
1978 Royale (back from 8600 mile round trip to Alaska this summer)

 

[johnny]

Neil -

In that instance, try to get the proper sender for the installed gauge - assuming the gauge is in a stock instrument panel which doesn't lend itself
to modification. Somewhere in the world, there's a sender intended for that gauge. Acquire it, and adapt it to the oil pressure port on the engine
you want to measure. In that the adapter is under the hood out of sight, you can get really silly with it if you have to.

--johnny
--
'76 23' transmode Norris upfit, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"The road goes on forever, and the party never ends" --Robert Earl Keen

 

[Matt Colie]

If you want it real right, but have to stick with an automotive grade sender, there is a way. It can be made to work with either air-core (Delco) or
thermal (Intro) gauges. It is a thing called a diode function generator. It took me two each quad op-amps (comparitors) and about another dozen
components to make the silly wire would single ended pot produce an output that fit the real world curve withing a half a needle. I would bother to
look it up and maybe even draw schematics if anybody actually cared. I do find it easier with the coach to run a log and learn what it does. The
several times I have been able to take on more than 50 gallons, I was concerned, but far from panic.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit

 

[a.]

> ...The several times I have been able to take on more than 50 gallons, I was concerned, but far from panic.
Gas pumps vary by calibration. I drove my daily driver for a couple of years and could never jam more than 12 gallons in it at my usual station when
the low fuel light came on. One time I was across the street from a station that was the same price as my regular place when the low fuel light came
on. So I pulled in a filled up. The pump said it took 13 gallons by the time it clicked off. I never bought gas there again, and every time I needed
gas after that I went to my usual place and never stuffed more than 12 gallons in it.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."

 

[Jim Hupy]

I put drivers who run their vehicles on very low fuel levels as the same
guys who play Russian roulette with all the chambers loaded. Breathe easier
guys, fill the tanks, and note your odometer reading. Drive 250 miles, then
refill the tanks. Forget those inaccurate electric fuel gages, and fuel
tanks joined at the hip with a large manifold between them. There are much
more important issues to concern you in your coach.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> > ...The several times I have been able to take on more than 50 gallons, I
> was concerned, but far from panic.
> Gas pumps vary by calibration. I drove my daily driver for a couple of
> years and could never jam more than 12 gallons in it at my usual station
> when
> the low fuel light came on. One time I was across the street from a
> station that was the same price as my regular place when the low fuel light
> came
> on. So I pulled in a filled up. The pump said it took 13 gallons by the
> time it clicked off. I never bought gas there again, and every time I needed
> gas after that I went to my usual place and never stuffed more than 12
> gallons in it.
> --
> 73 23' Sequoia 4 Sale
> 73 23' CanyonLands Parts Unit 4 Sale
> Upper Alabama
> "Highest price does not guarantee highest quality."
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Matt Colie]

Were I someplace I did not know, I might not play that game, but both times I did the 53+ the choice would have been to be late for a meeting with an
owner and/or buy fuel at the marina. ($$ Not Happanan Jack)
As to calibration, I have two registered jeep cans (confirmed by MDA) and had recently filled both at both of the subject pumps. One was dead nuts on
and the other 0.2 short (It said it pumped 4.8).
When one has had a lifetime that involved fuel inventory, managing a GMC is just no big thing.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit

 

[Kelvin]

I'm just getting into a groove at 350 miles!

Not all of us need to stop after 4 hours in the seat. Or we hit a rest
stop without filling up. I've gone 450miles without flipping over to
"AUX" a couple times. I think my record in the seat is about 8 hours.
Proper fluid intake (or lack thereof) is important.

It is good to know your particular coach, though. My guage (VDO) is
reasonably accurate. My tank sensors were in great shape last time I
had them out.

Kelvin
'73 23' in Eugene, OR

> I put drivers who run their vehicles on very low fuel levels as the same
> guys who play Russian roulette with all the chambers loaded. Breathe easier
> guys, fill the tanks, and note your odometer reading. Drive 250 miles, then
> refill the tanks. Forget those inaccurate electric fuel gages, and fuel
> tanks joined at the hip with a large manifold between them. There are much
> more important issues to concern you in your coach.
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403
>

>

>>> ...The several times I have been able to take on more than 50 gallons, I
>> was concerned, but far from panic.
>> Gas pumps vary by calibration. I drove my daily driver for a couple of
>> years and could never jam more than 12 gallons in it at my usual station
>> when
>> the low fuel light came on. One time I was across the street from a
>> station that was the same price as my regular place when the low fuel light
>> came
>> on. So I pulled in a filled up. The pump said it took 13 gallons by the
>> time it clicked off. I never bought gas there again, and every time I needed
>> gas after that I went to my usual place and never stuffed more than 12
>> gallons in it.
>> --
>> 73 23' Sequoia 4 Sale
>> 73 23' CanyonLands Parts Unit 4 Sale
>> Upper Alabama
>> "Highest price does not guarantee highest quality."
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[Jim Hupy]

Kelvin, I've seen your dash. Mighty nice layout. When all is normal and
well, all needles point straight up. Except for your fuel gages. Like a
number of race cars I have seen. But, your turn in the barrel is coming
when you will look forward to stopping and having a walk-about. You are
still young yet. (Grin)
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> I'm just getting into a groove at 350 miles!
>
> Not all of us need to stop after 4 hours in the seat. Or we hit a rest
> stop without filling up. I've gone 450miles without flipping over to "AUX"
> a couple times. I think my record in the seat is about 8 hours. Proper
> fluid intake (or lack thereof) is important.
>
> It is good to know your particular coach, though. My guage (VDO) is
> reasonably accurate. My tank sensors were in great shape last time I had
> them out.
>
> Kelvin
> '73 23' in Eugene, OR
>
>

>
>> I put drivers who run their vehicles on very low fuel levels as the same
>> guys who play Russian roulette with all the chambers loaded. Breathe
>> easier
>> guys, fill the tanks, and note your odometer reading. Drive 250 miles,
>> then
>> refill the tanks. Forget those inaccurate electric fuel gages, and fuel
>> tanks joined at the hip with a large manifold between them. There are much
>> more important issues to concern you in your coach.
>> Jim Hupy
>> Salem, Or
>> 78 GMC ROYALE 403
>>

>>

>>>
>>>> ...The several times I have been able to take on more than 50 gallons, I
>>>>
>>> was concerned, but far from panic.
>>> Gas pumps vary by calibration. I drove my daily driver for a couple of
>>> years and could never jam more than 12 gallons in it at my usual station
>>> when
>>> the low fuel light came on. One time I was across the street from a
>>> station that was the same price as my regular place when the low fuel
>>> light
>>> came
>>> on. So I pulled in a filled up. The pump said it took 13 gallons by the
>>> time it clicked off. I never bought gas there again, and every time I
>>> needed
>>> gas after that I went to my usual place and never stuffed more than 12
>>> gallons in it.
>>> --
>>> 73 23' Sequoia 4 Sale
>>> 73 23' CanyonLands Parts Unit 4 Sale
>>> Upper Alabama
>>> "Highest price does not guarantee highest quality."
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
> _______________________________________________
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> Unsubscribe or Change List Options:
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>