The Basic Principles Of Automotive Gauge Function And How To Troubleshoot Them

3 Sep, 2024 Automotive News Repair Tips Technical Articles Permalink

By Jim Smart September 1, 2024

in Car Culture, Classics, Hemmings Classic Car, How To, Magazine, Research, Tech 101

How your car’s instruments work and fixing malfunctions

utomotive gauges are simple and have changed little in the last 100 years. Understanding how they function can make troubleshooting a snap. Some background on the basic principles of gauge operation: Variable resistance to the flow of electricity to a negative ground (body/chassis) affects where the gauge needle moves on the instrument face. Electrical flow can be controlled by switches, resistors, diodes, fuses, and circuit breakers. In the case of analog gauges, the sending unit or “sender” is a variable-resistor contact switch to ground. When resistance to ground is high, the instrument will read low. When resistance is low, the needle will read high. On/off warning lamps are switched by a simple on/off sender. 

None of this would be possible were it not for a voltage regulator located on the back of most instrument panels. With the ignition of a nominally 12-volt vehicle on, roughly 12-14 volts of electricity flow to the instrument voltage regulator, which reduces that voltage to approximately 5 volts for the instruments. It’s then a matter of completing the electrical circuit from the gauge to ground. A faulty instrument voltage regulator will either stop current flow to the gauges, or the contact inside will stick, and all instruments (except an ammeter) will give a maximum reading. 

Coolant Temperature Gauge and Sender

The coolant temperature gauge operates based on the amount of current flowing to ground via the sender at the engine. Inside the sender is a thermal contactor, which expands or contracts with temperature change. With an increase in coolant temperature, the sender decreases resistance to ground, which increases current flow across the gauge, moving the needle higher. By contrast, cold coolant causes high resistance to ground in the sender, which moves the needle low.

This is a tab-style voltage regulator as used on pre-1969 Ford instrument panels. It converts 12-14 volt current to the 5 volts required. Beginning in 1969, Ford introduced a button-style voltage limiter with printed-circuit clusters.
This is a typical pre-1969 Ford coolant temperature gauge, which is identical to the oil-pressure and fuel gauges of the era except for the gauge face.
On the reverse are needle-adjustment holes. The screwdriver points to the hole that controls where the needle rests with the ignition off, or zero feedback from the sender. The other adjusts maximum needle travel.
Inside, the white arrow points to the resistor, or heating element, that acts on the bimetallic strip (red arrow) that acts on the needle. As current flow through the resistor increases, the strip becomes hotter and moves the needle. When the ignition is turned off, current stops passing through the resistor and the needle returns to rest.
Here are two types of coolant temperature senders: A variable resistor type for gauges (right) and a simple on/off type for warning lights.
By testing a dry coolant temperature sender with a multimeter (ohmage setting), we

Fuel Gauge and Sending Unit

The fuel sending unit controls the flow of current to ground based on how full the tank is. A float rides on the fuel’s surface and acts on an arm connected to the variable resistor to ground. When the float is high, current flow to ground is low and the gauge reads full. When the tank is empty, there’s high resistance to current flow to ground and the gauge reads low.  

Troubleshooting the fuel gauge and sending unit is similar to diagnosing the temperature gauge. Disconnect the fuel-sending unit and ground the plug. If the needle moves to full, the sending unit is faulty and should be replaced. If the gauge does not respond, there is a disconnect in the wiring between the fuel gauge and the sending unit. Alternately, if the float takes on fuel, it will sink, causing an erroneous reading at the gauge, or a corroded or damaged resistor coil may provide a faulty reading. 

Oil Pressure Guage and Sender

The oil pressure gauge works much like the fuel and coolant gauges. Inside the oil pressure sender is a spring-loaded piston that – when met with oil pressure – moves a contact back and forth across a variable resistor to ground. With higher oil pressure comes lower resistance and a higher reading, and vice versa for a lower gauge reading. Troubleshooting is identical to the temperature gauge.

Ammeter Versus Voltmeter

An ammeter shows the rate of charge, while a voltmeter registers how much voltage is at the battery. An ammeter reads high when the alternator/generator is charging, and low when the battery is losing voltage due to a draw, such as using a turn signal. Ammeter needles will sometimes dance disconcertingly as a result. A potential problem with ammeters is they are live all the time and – in the case of some vehicles – are not circuit-protected, meaning they are vulnerable to overheating.

The ammeter, once common on Ford and Chrysler vehicles, shows the rate of charge or discharge depending upon which way the needle swings. Unfused ammeters can be a fire hazard because they are live whenever the battery is connected.
Direct-connection, shunt-style Ford ammeters can overheat and fail. This unit has a broken wire, yet current continued to flow through to the main wiring loom.

Mechanical Speedometers

Vintage vehicle speedometers are mechanical and thus are not dependent on electricity. They work on the principle of magnetic field in which the faster a magnet spins inside a metal drum within the instrument, the higher the gauge reads. Making the magnet spin is a cable connection between the transmission output shaft, a drive gear and a driven gear, and the gauge. 

Mechanical speedometers largely fail due to the absence of lubrication at the speedometer head, which you can fix yourself with a liquid graphite lubricant. The gauge also quits if the cable binds and breaks, but swapping the cable is another relatively easy task. If neither remedy resurrects the gauge, it is best to entrust it to a qualified repair shop, which can also calibrate it. 

Speedometer accuracy relies on matching the rear-axle ratio to the drive and driven gears (number of teeth in each). Larger- or smaller-than-stock tire diameters will cause the speedometer to read too fast or slow, a problem that can sometimes be rectified with a new driven gear. 

The time-proven mechanical speedometer is simple in function.

Electronic Tachometers

Most factory tachometers, with few exceptions, are electrically driven by ignition pulses off the distributor or the negative side of the ignition coil. Mechanically driven tachometers work like the mechanical speedometers just mentioned but with the cable typically driven by the generator or crank snout. Electronic tachometers are not user-serviceable and will have to be sent to an instrument repair shop for restoration.

Electric-based tachometers operate on ignition pulses from the ignition system.
The faster the pulses, the higher the tachometer will read. Tachometers should be serviced by a qualified instrument repair shop.