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 Post subject: Bench test fuel gauge
PostPosted: Fri Jan 10, 2020 1:57 pm 
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2 BBL ''SuperSix''

Joined: Sun Nov 17, 2019 4:09 pm
Posts: 15
Location: Asheville NC
Car Model*: 1966 A100
Hey folks! I have a 67 a100 van and the fuel gauge does not work. I pulled the gauge out of the cluster and was going to try and bench test it first. Here is a pic of the back of it:

https://photos.app.goo.gl/dMgspu3CFU7YP4Kj9

Where do I start? Can I just run 12v to one or more of these posts along with a ground to another? Any help appreciated.

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PostPosted: Fri Jan 10, 2020 7:56 pm 
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Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3118
Location: Indianapolis
Car Model*:
About all of the gauges in vintage vehicles are 5 volt.
Put 12 v on a 5 v gauge and you will quickly fry the gauge.
There is a voltage limiter, either a module on the back of the
instrument panel or built into one of the gauges.
I would start by getting familiar with the instrument panel wiring
schematic. To do that either buy a Factory Service Manual or
find a free on line source.

Once you know what voltage your fuel gauge runs on to fully test
the gauge you will need to know the resistance swing of the fuel level
gauge. That information is also likely in the FSM. Or remove the fuel
level sender and use a multimeter to find the resistance at full and empty.


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PostPosted: Sat Jan 11, 2020 7:00 am 
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2 BBL ''SuperSix''

Joined: Sun Nov 17, 2019 4:09 pm
Posts: 15
Location: Asheville NC
Car Model*: 1966 A100
Thanks dad...thats a good start. I'm read in a couple of places that the 5v limiter may be located in the fuel gauge itself. I did see/find what looked like a limiter on the back on the gauge cluster anywhere.

From doing some research it looks like I want to run positive voltage to one of the 1/3 posts, negative to 2/3 post and them ohms/resistance to the 3rd post. Does that sound correct? Also, does anyone have suggestions on how to send ohms to the gauge? Is there a device I need to do that or is there some shade tree way to do it?

Sorry for the basic questions. I've been wrenching for a while, but electrical is a weak area for me. Always learning!

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PostPosted: Sat Jan 11, 2020 7:15 am 
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TBI Slant 6
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Joined: Fri Jul 14, 2006 6:32 pm
Posts: 175
Location: Crescent City Florida
Car Model*:
That gauge has the limiter built into it that's why it has 3 terminals on the back. There are articles on the internet on how to disable it and use an electronic one or use a external mechanical one.

Brian

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PostPosted: Sat Jan 11, 2020 5:50 pm 
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Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3118
Location: Indianapolis
Car Model*:
my 68 Barracuda has (had) the voltage limiter housed inside of the fuel gauge,

per my 68 Barracuda FSM the three terminals are
one terminal is 12v in
one terminal is 5v out to the temperature gauge and to the oil pressure gauge
one terminal is the connection to the fuel level gauge in the fuel tank.

I replaced the mechanical voltage limiter with a solid state unit.

https://www.rt-eng.com/index.php/RTE_limiter.html

There are photos on the instructions page that shows what the fuel gauge with the internal limiter looks like inside.


Quote:
does anyone have suggestions on how to send ohms


ohms is a measure of resistance,
if you think of electricity as water that was contained in a bucket flowing through a hose it can help to visualize what the various electrical terms mean.

Voltage is potential or pressure,, the larger the bucket or the higher the bucket is the more potential pressure the water flow will have, 12 V vs 5 V is a similar comparison to a bucket of water setting on a 10 foot ladder ( 12 V) and a bucket of water setting on a 5 foot tall ladder ( 5 V).
Amperage is actual flow through a circuit,, the larger the internal diameter of the hose the more water it will flow, electrical current moving through a wire is similar to the flow of water through a hose, amperage is a measurement of electrical flow through a wire.
Ohm is resistance to flow,, a restricted nozzle on the end of the hose or a kink in the hose reduces the water flow, an increase in the Ohm value means that it has more resistance to flow. Zero Ohm has no resistance to current flow. The fuel level gauge in the gas tank has variable resistance, as the amount of gas in the tank changes the ohms resistance changes and that changes the amount of current going to ground.
The above analogy is not 100% accurate, but should give an idea of what the electrical terms mean.

so to test your fuel gauge, connect the appropriate terminal to 12 V
the limiter in the gauge ( if it is working ) will deliver the 5 V as needed to the fuel gauge internals, with a multi meter you could probe the 5 V output and make a determination if the voltage limiter is functioning, if it is outputting an average of 5 V it is,(a mechanical voltage limiter will open and close the 12 V circuit rapidly, so you will probably see the voltage fluctuating) but basically leave the 5 V terminal open for your test.
connect the terminal that goes to the fuel tank gauge level unit to an appropriate variable resistor to simulate the fuel level gauge. It will need to be grounded to continue the current path.

if you connect the terminal that normally connects to the fuel level gauge directly to ground, with out a resistance in line, then there is no 'kink in the line' to limit current flow, so unless the dash gauge was made to operate with 0 ohms resistance that action will likely hurt the gauge.

So to accurately, and safely test your gauge you will need to know the ohm range of the fuel level gauge in the gas tank, then duplicate that resistance range in the test.


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