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Gauging your progress - or how to tell when it can't.

Original Author: Alan 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 and oil pressure gages 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.


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.

 

   
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