I have gotten conflicting information on the (Lucas?) voltage stabilizer
used for the temperature and fuel gauges on my Europa, and British cars in
general.
I originally thought there was no voltage stabilizer in the car, but
subsequent analysis whilst tearing it apart proved me wrong.
The bare device has three terminals, in line:
a double, with male and female "Lucar" spade connectors
another double, with male and female Lucar connectors
a single male Lucar connector
The steel case has a mounting tab.
The schematic in the factory manual shows three connections:
a double connection with both 12v in and out to the oil pressure gauge,
green wires
single connection to temperature gauge, green/lightgreen wire
single connection to fuel gauge, green/lightgreen wire
There is no mention of a connection to the case, or any ground ("earth"
for our UK friends).
The way the car was wired by the Bonzai Electronics Tech of Oahu agreed
with the schematic he drew. Wire color is not relevant, as he made no
attempt to use the Lucas standard. The connections were:
a double connection with both 12v in and out to the oil pressure gauge
double connection to temperature gauge and fuel gauge
single connection to ground.
The case was screwed to the wood dash.
With much strain and a mirror, I was able to see the voltage stabilizer in
Terry Pitts' Europa. I could not trace the wires, but the color code is
reputed to be per the Lucas standard. The connections are:
a double connector with two green wires crimped into the same Lucar
connector
a double connector with two green/lightgreen wires crimped into the same
Lucar connector AND soldered to the terminal is what appears to be
the + end of a tiny electrolytic capacitor
a single connector with the negative end of self-same electrolytic
capacitor
I could not see the writing on the capacitor (if that is what it is), so I
can't tell you its specs. There is no other connection of the third
terminal to ground or anywhere else. The case appears to be just screwed
into the wood dash, but it is hart to see in there. Terry has had real
trouble with his temperature gauge reading very high. He has replaced
everything including the stabilizer, but hooked it up the same way it was,
which may be wrong.
Since it is apparent from all information sources that the oil pressure
gauge uses straight, unstabilized, voltage, I tend to believe it.
The temperature and fuel gauges appear to require stabilized voltage in
all cases. I guess this means that there is a device to reduce the
voltage from 12v to some lower figure and maintain it at that lower value
regardless of how the supply voltage varies. This can be done with a
Zener diode and a resistor, although the Lucas method apparently used a
bimetallic electromechanical device of some sort, which chops the voltage
up into pulses which average out to a lower voltage.
If I was to use a Zener, I would wire it like this, reading from battery
side to ground:
12v-5v variable source
resistor
connection to gauges
Zener diode
ground.
This is a voltage divider. The regulated voltage falls across the Zener
and the gauge circuits, which are in parallel with the Zener. The voltage
"left over" drops across the resistor.
I would expect the Lucas voltage stabilizer to work in much the same way,
having all its parts (including the dropping resistor) enclosed in one
can. Three terminals would be required, one for the input of the variable
voltage, one in the middle to send stabilized voltage to the gauges, and
one connected to ground.
So my question is, how can a voltage stabilizer work without a ground
connection? Voltages are relative, after all. When we talk about the
voltage levels in a car, it is always in relation to ground. So to
stabilize a voltage, I tend to believe the stabilizer circuit must have a
reference to ground, so the circuit to be stabilized (gauge and its
sensor, in series with each other) will be in parallel with the stabilizer.
This is what I think the map should look like. Obviously, Bonzai
Electronics Tech figured the resistor and regulator were both in the same can:
unstabilized voltage source
|
|
\
/ resistor
\
/
|
|
|______________________________________
| | |
| | |
| | |
(stabilizing device, (temp (fuel
Zener or otherwise) gauge) gauge)
| | |
| | |
| | |
| (temp (fuel
| sensor) sensor)
| | |
| | |
|__________________________|___________|
(chassis ground = engine block = fuel tank)
A device in a can with no connection to ground would seem to me to be in
series with the gauge circuit, and therefore capable of regulating only
the current flow, which would seem to be no help at all.
So it appears to me that Bonzai Electronics Tech might be the right one
here. I wonder if the schematic is incomplete (there are some other
errors in it) and maybe there is a connection to the case on Terry's that
I could not see.
I want to get this Lotus wired ASAP, so I'd appreciate any guidance I can get!
BTW, I have heard 10v, 6v and 4.5v as the voltage these things are
supposed to regulate to. Anybody know?
Phil Ethier, THE RIGHT LINE, 672 Orleans St, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 298-5324 phile@stpaul.gov
"There is nothing like a complete money-is-no-object restoration.
And this is going to be nothing like one." - Phil Ethier
|