Series Land Rovers
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
a day of fun, switch on the key, and the gauges don't move at all even
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
inform you of certain information on your steed's health - oil
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
to the indicator built into your gauge cluster.
<>The medium in between can be mechanical (like a capillary tube
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
On the early IIA and earlier Rovers, the fuel gauge is a
wobbly) device. It consists of nothing more than a sender (a variable
resistance not unlike a volume control), hooked directly to ground on
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
(fundamentally a small electric motor with a needle on the shaft),
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
with a voltmeter can usually turn up the reason the current isn't
most likely a bad ground wire at the tank, oxidized connectors at
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
over it, and can oxidize or break leading to interesting symptoms.
The simplest test is usually to remove the wire from the
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
wire or at the gauge end - start checking for voltage there.
The late and post-IIa Rovers used a different, more
involving a voltage regulator and a hot-wire type gauge. Gone were the
fluttering fuel gauge and inaccurate readings because of voltage
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)
slowly to changes and damps the sender swings, and the voltage
maintains a constant 10 volts to the instruments, regardless of what
electrical system is doing.
Lucky for us, diagnosing these is basically the same as
older system above. In addition to the above hints, checking the input
output of the voltage regulator (12 volts in, 10 volts out to the
making sure the gauge itself is grounded properly as well as the
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
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
grounding or bad wiring connections. Oftentimes, loss of a ground lead
the gauge or at the voltage regulator will cause excessively high or
readings, leading to large amounts of panic on the part of the
The exception to this is the Smiths water-temperature sender
cars - they have a reputation for failure, usually leading to a dead
Common sense, a voltmeter and a jumper lead are your best
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
have failed, it's far more likely that the voltage regulator has gone
or a wire has become disconnected than that both senders have failed.
the most likely item first, then backtrack down the line until the
- 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.
A gauge is just like a light or motor as far as diagnosing
switch (sender) is broken, or the current path interrupted, it's not
to work. Use the same commonsense attitude that you would with
any electrical problem on the old beasts, and you'll do fine with
<>Reprinted from the Ottawa
Rovers newsletter, 15 September, 1997. Volume XIV, Number 9.
Modified 8 November 2004 - Alan J. Richer