----- Original Message -----
From: John Friese
To: amphicar-lovers@yahoogroups.com
Sent: Thursday, August 26, 2004 7:01 PM
Subject: [amphicar-lovers] Re: Wacky Fuel Gauge
Hello Mike,
If the gauge is reading full, the circuit to the sending unit is open.
The problem could either be the sender or the wiring to the sender.
Be sure that the wire that goes to the sender frame is making good
contact.
The sender unit is a zero - 180 ohm type. If you want to substitute a
variable resistor for the sender, the closest you're likely to find
would be a 200 ohm variable resistor. If you simply touch the two
wires together that go to the sender, the gauge should drop to empty.
If it does, chances are the sender is bad or your connections to the
sender are poor. I'd carefully check the connections first because
the senders are reasonably reliable.
John Friese
(The negative ground conversion guy)
Thanks John. I had gotten messed up in replying to Mike, confusing myself with
temp sender and gas gauge sender. They both work on the same principle; the
gauge is a balanced bridge, and the one bridge leg that goes through the sensor
(to ground) is made to vary with the parameter you want to measure.
In the temp sender, a pellet of resistive material (electrically called a
thermistor) changes its resistance with temperature. There are no moving parts,
and since the sensor is sealed, it lasts nearly forever. (Heat and vibration can
crack internal parts and connections; nothing lasts forever.)
In the gas sender, the variable resistor (properly called a rheostat) consists
of several inches of fairly high resistance wire (like a toaster element, but
thinner) wrapped around an arc-shaped insulator card. One end of the wire is
connected to the metal mechanical support strut (that holds the sender in the
tank and provides a ground path from the sender cover plate (where the brown
ground wire attaches). If you think about how the wire is wrapped on the little
card, you can see that every turn around the card needs a half-inch or so of
wire length. It takes maybe 40 turns of wire to cover the length of the card. A
little wiper arm, moved by the float, wipes along one edge of the card.
When the tank is near empty, the little wiper is near the grounded end of the
wire. As you fill the tank, the little wiper arm slides along the card edge,
progressively contacting one wire loop at a time. Each time it moves up a wire,
the resistance of all previous loops are added to the total resistance, so that
by the time the float is moved to the full position, the wiper is near the last
few loops of wire, and the total resistance is about 180 Ohms.
Since the resistance element is mechanical, it's subject to wear and corrosion.
Regardless, tank senders are still very reliable. I would suspect wiring
problems before I would check the gauge.
BTW, be very careful about making connections around the fuel tank. This is a
very bad place to create a spark. I used to wonder how an electrical current was
allowed to run through an exposed (to gas vapors) wiper inside the tank. Why
wasn't a spark and an explosion inevitable? It turns out that the fault current
is limited by the resistance within the gas gauge. Further, the vapor within a
gas tank is super-saturated with gasoline, and a mixture of air and gas will
only burn within a certain range of concentration. Explosions will only happen
within an even narrower range of concentration. In effect, the vapor above the
gasoline is safe because it has too much gas to support either a fire or an
explosion.
OTOH, if you remove the tank cap, drain the tank, or withdraw the sender unit,
you can add enough outside air to make the vapor dangerous. I would never work
on the sender with it laying atop the tank. Instead, take it a safe distance
away from the vapors.
Ed
El Cajon
67 Rust Guppy
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