The ground speed signal is generated (litterally) by a small permanent
magnet generator (generator thread) mounted on the back of the speedometer
(odd threaded connector thread) and driven by the speedometer cable (wobbly
needle thread). This generates a voltage (bad connections thread) which is
directly proportional to the ground speed (tire size and gear ratio
thread). As this doesn't require much current (dim bulb and dead batteries
thread) the generator is very small (generator vs alternator thread), about
the size of a large shotgun shell (gun lobby thread).
The fuel flow rate signal is created using a photoresistor (Princess Di
thread and bad/insensitive jokes thread) with a small light bulb
("dim-bulb" thread), and a little round ball on a spring (political change
of mind thread) mounted in the flexible fuel hose (hope it doesn't leak
thread) between the body and the engine (warm and cozy thread). As the
fuel flows past the ball it physically displaces the ball against the
spring (detents and carb balancer threads) with a distance that is
proportional to the flow rate. With the light source on one side of the
ball and the photoresistor on the other side (armrest in the middle
discourages intercourse thread), the position of the ball regulates the
amount of light (daylight savings time thread) getting to the
photoresistor, creating a resistance that is proportional to the fuel flow
rate (law of proportional reactions thread). The linearity of this device
is controled by the shape of the light passage apperature (small windscreen
thread). And of course this simple little device will not work on newer
cars (engineered to screw up simplicity thread) with excess flow fuel
supply systems (overpressure fuel pump thread) where the unconsumed portion
of the fuel flow is returned to the fuel tank (leaky check valve in fuel
pump thread).
The dash instrument is internally not much more than a simple galvanometer
(volt meter thread), where the indicator needle rotates proportional to the
variable voltage provided (faulty regulator thread), except that there is
some sort of current limiting circuit (faulty fuses thread) intended to
prevent the needle from hitting the hard peg at the top end of the scale
(non linear instruments thread) when the input signals want to drive it to
infinity (no ground on the fuel tank thread).
Now you simply wire these three devices in series in any order you like
(simple wiring diagrams thread), except to pay attention to the polarity of
the generator (pos/neg earth thread), and you have a functional instrument
(not yet calibrated). The small light bulb in the flow sensor is powered
from any ignition supply circuit (ignition switch thread), and no other
outside electrical connection is required, except for the instrument dash
lamp (dim bulbs and rheostats thread). Because the signal circuit is a
closed loop and not conducting current from any outside source (jumper
cables thread), any single point in the circuit can be shorted to the
chassis of the car (frayed wiring thread) with no functional effect. For
instance, one wire on the (instrument) generator can be connected to earth
at the back of the dash (good ground we hope), and one wire on the
photoresistor can be connected to earth in the engine bay (another good
ground we hope), and the chassis of the car will complete the circuit
(missing ground strap on the engine thread), eliminating the need for a
ground return wire (more bad grounds thread).
To calibrate the instrument (calibration thread) there is included in the
kit a small peg with a shoulder (plastic nail) with a precisely controlled
length (post war precision mechining thread). This peg is to be inserted
into one end of the flow sensor (fool the inspector thread) and taped into
place without installing the device in the fuel line (disconnected fuel
cut-off valve thread). This positions the small ball against the spring
(oil pressure relief valve thread) such as to provide the exact position
that would be attained with a fuel flow rate of two gallons per hour (slow
sipping econobox thread) for the high mileage unit, or 4 gallons per hour
(gas guzzling SUVs thread) for the low mileage unit (also tach reads double
RPMs thread). You then take the car for a drive and observe the position
of the instrument indicator needle (keep one eye on the road thread) as
compared to the speedometer reading (rashly assuming the speedo is correct
thread). With the flow rate sensor locked to a fixed flow rate (won't go
over 40 MPH thread), the MPG meter needle will move in linear fashion in
unison with the speedometer needle (tach same as speedo in top gear thread)
as the small generator creates a voltage proportional to the speed
(sticking voltage regulator thread). At 60 mph you want the new instrument
to indicate exactly 30 MPG, at 40 MPH exactly 20 MPG, etc, high mileage
unit example (wishful thinking thread). On the back of the MPG meter you
can adjust a tiny potentiometer (electric tach calibration thread) to
attain the desired reading on the gauge (lie as much as you like thread).
Electrically here the potentiometer is just shunting a portion of the
voltage/current around the galvanometer (ampmeter thread) to compensate for
variations in speedo cable drive speed in different cars (OD conversion
thread), so that you get the desired voltage/current for the dash
instrument regardless of the cable drive ratio (speedo conversion thread).
Once the dash instrument is calibrated you just remove the plastic peg and
install the flow sensor in the fuel line with hose clamps (emergency fuel
pump thread), and you're finished with the installation ("when is the
restoration finished" thread).
[continued in part 3]
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