Audi tt all-digital dash
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Indicator lamps and switches (Part 1)
Regardless of which operating system is monitored by an indicator tamp, the lamp can let you only two things —‘yes’ or ‘no’. ‘Yes’ means that the coolant is too hot. ‘No’ means that the oil pressure is not too low. ‘No’ also means that the generator is not supplying enough current. The coolant-temperature and oil-pressure lamps are operated by ‘on/off’ switches.
Indicator lamps convey their information by lighting when something is wrong in the system being monitored. Because this requires nothing more than simple current flow, an indicator lamp can be controlled by a single-pole single-throw (SPST) switch. Indicator switches are usually called by the name of the system they monitor: oil-pressure switch, coolant-temperature switch, and so on.
When you took at an indicator-lamp switch, you’ll see that it has only one terminal. This is because the switch is wired in series with the lamp and forms the ground end of the circuit. The circuit is completed to ground through the body of the switch. Battery voltage is available at the lamp: current flows only when the switch closes to provide a complete path to ground. You’ll appreciate the logic of this when you wire your car and realize it would take twice as much wire if current went first to the switch, then to the lamp, and finally to ground.
Temperature switches rely on a temperature-sensitive bimetallic element operating a set of contact points. The switch is an SPST type, normally open. With increased temperature, the bimetallic element changes shape, causing the points to close at the calibration temperature.
The element is contained in the switch body and extends into the coolant to take a direct reading. By carefully choosing the shape, composition and tension of the bimetallic element, the designer can calibrate the switch, or determine its critical switch-point temperature.
The temperature switch resembles a short bolt that has a threaded portion and a hexagonal head. The sensing element is housed in an extension beyond the threaded part. At the head, you’ll find the single terminal that attaches to the ground side of the lamp.
American manufacturers use a standard-taper pipe thread (NPTF – National Pipe Thread Fuel) for temperature switches. The taper ensures a pressure-light seal without need for sealing compound, which would hamper current flow to ground.
Some European auto manufacturers also use the NPTF system, although the metric system is most common on popular imports. This isn’t a problem with warning-lamp switches, though, because you’ll likely stick with the factory switch anyway.
Pressure switches are used for oil-pressure indicator lamps. In this application, the switch is a normally closed type, i.e. unless sufficient pressure is applied, the switch remains closed (‘on’), providing a ground path for the warning lamp. In other words, oil pressure keeps the switch open (‘off’) until the pressure drops below a critical level, closing it.
The pressure switch threads into a hole intersecting an oil galley in the engine. A small hole in the switch body permits pressurized oil to enter the switch, where il acts on a flexible diaphragm. Diaphragm movement operates a set of contact points that open or close the warning-lamp circuit, depending on diaphragm position.