Fuel gauge puzzle

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Steverino
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Fuel gauge puzzle

Post by Steverino » Sun Dec 03, 2017 9:05 pm

Here's my dilemma...
If I ground my fuel gauge sender wire at the tank connector, gauge reads over Full.
The existing installed sender reads only about 7/8 tank when actually very full. It reads a bit over 1/4 tank when actually about 1/2.
I have another sender I purchased some time ago. If I connect it to the sender wire and ground the body, and hold float arm at max (full) it also only reads about 7/8 tank.

So I'm wondering if I have a gauge issue, or something else. Not sure how to correlate these data points... Maybe I just need to bend the sender arm "up" on the existing sender. But then why does the other sender not get a full reading when I know for a fact it is at max? I realize I can remove the existing unit and test "max" by hand - but of course now my tank is full... and in the past I've had trouble with the sender o-rings being a one-time only use. :roll:

Any help much appreciated!
Steve
'62 Landau
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'67 MGB - sold after 22 yrs.

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Karl
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Re: Fuel gauge puzzle

Post by Karl » Mon Dec 04, 2017 7:40 am

G'day Steverino.
My car is a 63.
I don't think the gauges are that accurate. From memory when I was a teenager, gauges on cars back then were a indication. I could be wrong though.
My car never reads full even when the tank is full. Mine is also about 7/8 full. It drops slowly until just over half and then drops quickly until just over empty. I know (from experience) that when the left hand side of the needle is touching the right hand side of the empty line I still have 20 liters in the tank and I can travel about 90 klm on the highway. I have learned to know how much fuel is in the tank by what the gauge reads. I play a game every time I fill up, guess how much fuel it will take.

In saying that, the way I understand how the fuel and temperature gauges work is like this:-
There is a positive power source (12v) that flows through a voltage regulator, split up and then through the gauges and then through a variable resistor (as in the fuel gauge) in the tank sending unit and is then grounded.
The voltage regulator is called this, but to me it is the wrong terminology. All it is, is a switch that turns on and off a certain times a minute. This is done by a bi-metal strip. When it is cold it closes the circuit. As the current flows it heats up, as it heats up due to the nature of the bi-metal strip it bends thus breaking the circuit. When the circuit is broken the bi-metal strip cools down and bends back and closes the circuit. The process happens over again. In my mind all it is doing is making and breaking the circuit. I think they call it a voltage regulator because as the circuit is closed and open, the average voltage over a certain time frame is reduced. I think they work on around 5V.
I have heard that you can't measure the voltage regulator with a digital multimeter because the time that it takes for the meter to work out the voltage of the circuit, it has changed. The meter keeps chasing itself. With a analogue meter you can see the needle fluctuating up and down.

The current then goes through the fuel gauge which is another form of wire that moves as it heats up, this has a needle attached to it. The current then travels to the sender unit in the tank, which is variable resistor. If the tank is empty the sender unit has the most resistance so little current flows, if there is little current flowing then the heat generated in the fuel gauge doesn't make the needle move much. If the tank is full then the resistance in the tank sender is low causing a lot of current to flow, which in turn causes the gauge to heat up and make the needle move.

To me this is very rudimentary system. It relies on current, voltage and heat, however in the day it was a cheap and easy way of having gauges.
A lot can go wrong, if you have to much, or to less voltage (ie the voltage regulator) it will read wrong. If you have to much or to little resistance in the tank sender, it will read wrong. If the wires are old and the connections have bad contacts, it will read wrong.
Bending the arms on the sending unit or the gauge could possibly make the problem worse.

There is a system that you connect to your fuel gauge and you can calibrate it to your tank sending unit. I don't know how it works but I have heard they are a lot more accurate, you still have your old gauge but this mini computer reads the voltage at the tank and you can change it so the gauge reads what you want it too. I think you start from a empty tank and calibrate it to read empty and then add fuel and calibrate it at half and full. Not sure.

Anyway if anybody knows that I am wrong with my description please let me know, this is how I see it.
Karl.

P.S. Sorry for the ramblings.
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bbogue
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Re: Fuel gauge puzzle

Post by bbogue » Mon Dec 04, 2017 8:37 am

The temperature and fuel gauges on my 61 were troublesome when I got the car 4 years ago. Both seemed less than responsive to actual conditions. I replaced the instrument voltage regulator (behind the clock, sometimes called the constant voltage regulator) with a solid state model from eBay that is adjustable. With the regulator adjusted to 5-5.5 volts, they both seem to work fine. With a full tank of fuel the fuel gauge needle now goes to a little past F and the temperature gauge needle is about halfway when my aftermarket gauge reads about 180F. I hope this helps.
Happy Holidays.

Bill
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ICON 1961
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Re: Fuel gauge puzzle

Post by ICON 1961 » Mon Dec 04, 2017 3:29 pm

My fuel gauge reads the same 7/8 just like Karl's same reading gives me a non accurate read. even when I fill the tank to the top. It must be a gauge issue from ford or just showing it's age , I don't know if the one I have is original or a repop.
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Steverino
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Re: Fuel gauge puzzle

Post by Steverino » Mon Dec 04, 2017 7:11 pm

Thanks to each of you for the information & perspectives. This is very helpful. I think I may do some eBay looking for a solid state regulator. FWIW, I've known this car all my life (pushin' 50) and I don't think the fuel gauge was always this bad... but then maybe my memory is slipping...
Steve
'62 Landau
'72 Citroen SM 5 spd
'67 MGB - sold after 22 yrs.

There is a fine line between "hobby" and "mental illness"...
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Steverino
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Re: Fuel gauge puzzle

Post by Steverino » Mon Dec 04, 2017 8:10 pm

bbogue wrote:
Mon Dec 04, 2017 8:37 am
...I replaced the instrument voltage regulator (behind the clock, sometimes called the constant voltage regulator) with a solid state model from eBay that is adjustable. ...
Bill
Bill, if it's not too much trouble, I'd much appreciate a link or two to items similar to the solid state adjustable one you bought. I looked around a bit, but there's a pretty wide universe and I'm not sure exactly what I'm looking for here.
Thanks in advance... :)
Steve
'62 Landau
'72 Citroen SM 5 spd
'67 MGB - sold after 22 yrs.

There is a fine line between "hobby" and "mental illness"...
VTCI #11678

RAVEN
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Re: Fuel gauge puzzle

Post by RAVEN » Mon Dec 04, 2017 9:43 pm

Gentlemen, a little help and a possible savings in the process, plus an explanation of how the CVR works.
LOOK DOWN under the General Discussion section, in the General T-B Tech area, on page two (2), about one third down and read the post, on "Uncovering the Instrument Voltage Regulator" and the LM7805.
Its a very helpful story which you all may get the info needed and explained.
The SEARCH area is a wonderful and helpful item, that many do not use, or know of, that exists.

Happy hunting

W
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Steverino
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Re: Fuel gauge puzzle

Post by Steverino » Mon Dec 04, 2017 9:55 pm

RAVEN wrote:
Mon Dec 04, 2017 9:43 pm
Gentlemen, a little help and a possible savings in the process, plus an explanation of how the CVR works.
LOOK DOWN under the General Discussion section, in the General T-B Tech area, on page two (2), about one third down and read the post, on "Uncovering the Instrument Voltage Regulator" and the LM7805.
Its a very helpful story which you all may get the info needed and explained.
The SEARCH area is a wonderful and helpful item, that many do not use, or know of, that exists.

Happy hunting

W
Thank you for the link - that looks like a good option. Appreciate the find. :mrgreen:

And, FWIW, I am well aware of and regularly use the SEARCH tool. I used it before I posted this time, too. Unfortunately, search is only as good as the terms used, and I was not initially searching for Instrument Voltage Regulator, since that's not how I began this process, though that's where we've ended up now...
Steve
'62 Landau
'72 Citroen SM 5 spd
'67 MGB - sold after 22 yrs.

There is a fine line between "hobby" and "mental illness"...
VTCI #11678

bbogue
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Re: Fuel gauge puzzle

Post by bbogue » Mon Dec 04, 2017 11:03 pm

Steve,
My eBay constant voltage regulator purchase was made in 2013 for about $25. The item did not look like the original but did have the correct connections. I wasn't aware that it was adjustable until during installation. The vendor appears to no longer be in business. There are some eBayers selling similar items (for Mustangs, etc.) that might work, but might require modification for installation. At least one TBird vendor has a solid state model that looks like the original. A little pricey. Here's a link

http://www.parts123.com/parts123/dyndet ... ukey=AAAMI

Added by edit: Here’s a link to a less expensive eBay item that appears to be identical to the above item and is also solid state.
https://www.ebay.com/itm/NEW-1961-1966- ... 2568817080

If you're handy with electronics soldering and such, the link provided by Raven is a great alternative, too.

Good luck.

Bill
VTCI# 12145
1961 Thunderbird - Heritage Burgundy Metallic
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Steverino
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Re: Fuel gauge puzzle

Post by Steverino » Tue Dec 05, 2017 6:35 pm

Thanks, Bill - appreciate the additional info. Either way, looks like a nice little project for the holidays... :)
Steve
'62 Landau
'72 Citroen SM 5 spd
'67 MGB - sold after 22 yrs.

There is a fine line between "hobby" and "mental illness"...
VTCI #11678

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