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Gauging your progress - or how to tell when it can't.
Original Author: Alan J. Richer
It's a lovely day, a truly lovely day.
You hop into your trusty IIA or III to run off into the
wilderness
for
a day of fun, switch on the key, and the gauges don't move at all even
though
you filled the gas tank - or, more ominously, rise to the tops of their
scales and stay there.
Of course, something has come along to mess it up.
It seems like a bit of gauge work is in order here.
Let's start out with the basics. The gauges on Land-Rovers
are
designed to
inform you of certain information on your steed's health - oil
pressure,
water temperature, fuel level and the like. To do this, sensors of some
kind get activated by the force we want to check, and this gets
transmitted
to the indicator built into your gauge cluster.
<>The medium in between can be mechanical (like a capillary tube
or
an
oil-pressure sender line on IIAs and earlier), or it can be electrical
(usually the case on later IIA and Series III vehicles). We'll be
concentrating on the latter type here, as most folk have little trouble
identifying problems with the earlier type.
Fuel gauges: Pre-IIa or Post
IIA?
On the early IIA and earlier Rovers, the fuel gauge is a
simple
(though
wobbly) device. It consists of nothing more than a sender (a variable
resistance not unlike a volume control), hooked directly to ground on
one
side and to 12 volts through the gauge on the dashboard. The change in
voltage caused by the movement of the float in the gas tank directly
influences the movement of the meter.
This has several drawbacks - with the D'Arsonval type of
meter
movement
(fundamentally a small electric motor with a needle on the shaft),
there's
no lag in the movement to damp random changes - so you get the
floppy-needle effect we all live with. Also, variations in the 12-volt
supply (headlights being on, charging/not charging, accessories and so
forth) can cause inaccurate readings.
This is the simplest type of system to diagnose. A little
prodding
around
with a voltmeter can usually turn up the reason the current isn't
flowing -
most likely a bad ground wire at the tank, oxidized connectors at
either
the tank or gauge, or a bad sender. Oddball symptoms (I read OK between
half and full, but nothing below), are usually the fault of the sender
-
the resistor is simply wire wrapped around a form with a contact
passing
over it, and can oxidize or break leading to interesting symptoms.
The simplest test is usually to remove the wire from the
sender at
the
tank
end and ground it to a good electrical ground. If the gauge then reads
full, the problem's in the sender. If it doesn't then the problem's in
the
wire or at the gauge end - start checking for voltage there.
The late and post-IIa Rovers used a different, more
sophisticated
system
involving a voltage regulator and a hot-wire type gauge. Gone were the
fluttering fuel gauge and inaccurate readings because of voltage
changes -
the new hot-wire type gauge (which worked on the same principle as a
thermostat - a bimetal strip heated by a wire moved the pointer)
responds
slowly to changes and damps the sender swings, and the voltage
regulator
maintains a constant 10 volts to the instruments, regardless of what
the
electrical system is doing.
Lucky for us, diagnosing these is basically the same as
diagnosing
the
older system above. In addition to the above hints, checking the input
and
output of the voltage regulator (12 volts in, 10 volts out to the
gauge),
making sure the gauge itself is grounded properly as well as the
sender.
Again, grounding the wire at the sender should make the gauge move to
full-scale - if not, then the voltage regulator or supply (or the gauge
itself) is probably defective.
Temperature and Oil Pressure - Hot or Not?
The electrical coolant temperature gauiges used on the
late
IIA and Series III cars work like the fuel gauge - a voltage regulator
supplies 10 volts to the gauge, and a variable resistance (sender) then
regulates the current to affect the needle of the hot-wire gauge.
Most of the failures in these systems can usually be
attributed to
bad
grounding or bad wiring connections. Oftentimes, loss of a ground lead
at
the gauge or at the voltage regulator will cause excessively high or
low
readings, leading to large amounts of panic on the part of the
driver.
The exception to this is the Smiths water-temperature sender
used
on
these
cars - they have a reputation for failure, usually leading to a dead
gauge.
Common sense, a voltmeter and a jumper lead are your best
tools
here.
Looking at the schematics, simply break the failing units up into
easy-to-test sections, and work each one of them individually until the
problem is found. For example, if both of the gauges (fuel and water
temp.)
have failed, it's far more likely that the voltage regulator has gone
away,
or a wire has become disconnected than that both senders have failed.
Check
the most likely item first, then backtrack down the line until the
fault is
found.
Oil Pressure
Gauge
- Series III
Common sense does not prevail when one is working on the Series III
electrical oil pressure gauge. It is a piece of Lucas smoke-driven
arcana all its own, and diagnostic information for this gem can be
found at http://www.lrfaq.org/Series/FAQ.S.oil_pressure_gauge.html.
<>
Conclusion
A gauge is just like a light or motor as far as diagnosing
failure. If
the
switch (sender) is broken, or the current path interrupted, it's not
going
to work. Use the same commonsense attitude that you would with
diagnosing
any electrical problem on the old beasts, and you'll do fine with
it.
<>Reprinted from the Ottawa
Valley Land
Rovers newsletter, 15 September, 1997. Volume XIV, Number 9.
Modified 8 November 2004 - Alan J. Richer
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