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Glossary of terms you will come across during your Phase 2 training.


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obd2 pinout

On-board diagnostics, or EOBD, European on-board diagnostics.

Software on a vehicle ECU that allows it to evaluate the data it receives from the various stations and sensors on a vehicle. If faults are found they are stored for subsequent retrieval via a serial interface and a suitable diagnostic tester (for example, the KTS 650).

Check here for more on the KTS 650.

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Octane is a hydrocarbon, and octane rating is a measure of a fuel's resistance to detonation.

In a petrol engine, we want to use as high a compression ratio as possible. However, the higher we go, the hotter the compressed gasses become. This leads to a situation where it is the temperature of the induced air that is firing the engine rather than the spark.

When hot air fires an engine, the fuel will detonate rather than burn evenly (even burning is what happens when a spark fires the mixture).

Detonation is extremely bad for a petrol engine, as must be avoided. Modern engines are designed with the correct compression ratio, so we just need to be careful to use fuel with the correct octane rating.

For example, the minimum octane rating required for fuel to make up a two-stroke mix for the Stihl consaw is 90.

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Typical odometerMore commonly referred to as the 'speedo', this is a device, a gauge, that records speed and distance travelled by the vehicle. American and British vehicles use the old Imperial system of miles per hour (mph). Irish and European vehicles use the metric system, and kilometers per hour (kph).

Odometer readings may be used to determine service intervals. However, you should note that many plant vehicles do a lot of work without travelling any significant distance, (for example, tracked excavators, site dumpers). These types of vehicles may use an 'hours in use' meter, powered by the vehicle battery whenever the vehicle is in use.

Finally, many plant machines have no battery fitted, and they rely on service intervals based on time: days, weeks, months etc.

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This is a measure of the resistance in an electrical circuit, and it is measured in ohms.

The Greek letter Omega, Ω, is often used to represent resistance in electrical circuits.

Check here for more information.

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Simon Ohm

One of the most fundamental laws of electricity, discovered experimentally by the German mathematician, George Simon Ohm. He discovered that for conditions of constant temperature, the current through a conductor is directly proportional to the potential difference between its ends. This is normally expressed mathematically by the famous formula V = IR, Voltage = Current x Resistance.

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In construction plant fitting, especially in small plant engines, oil alert systems are fitted to engines. They consist of a float with a permanent magnet, and a reed switch to which the primary winding of the magneto ignition circuit is connected.

When the oil level in the engine is correct, the magnet keeps the contacts in the reed switch open, so the primary ignition circuit functions normally.

In the event of a drop in oil level (often, even a small drop in level), the float and magnet sink, causing the contacts in the reed switch to close, thus running the ignition primary circuit to ground, causing the engine to stop.

This event can also be caused by operating the engine on a sloping surface. When dealing with four-stroke small plant engines, if one fails to start, it is advisable to first check the oil level, then check with the operator to ensure that the engine was being run on level ground. If the oil level is even slightly low, or the engine is operated on a sloping surface, the oil alert switch will often cause the primary winding to be sent directly to ground, causing the failure of the ignition circuit.

YouTube Video on reed switch operation:


Contributors to this glossary entry: Mark Cunningham, Phase 2, May to September, 2011

Suggestions for further study:

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If we want to introduce oil somewhere, we must leave room for it, and this room is called 'oil clearance', or room for the oil to get between two moving parts.

Oil is introduced into vehicle components for 1, 2 or all 3 of the following reasons:

  1. To lubricate moving parts
  2. To seal moving parts
  3. To cool parts
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Oil dry on the groundGranular compound spread on the floor in order to soak up oil spillages, thus minimising the chances of accidents as a result of slippy floors in the workshops.
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Oil scroll on a shaft
  • A scroll machined on to the shaft brings the oil along the shaft until it reaches the slinger.
  • Here, centrifugal force causes the oil to be thrown off the shaft. The oil then makes its way back to the sump, via the drain at the bottom of the housing.
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Greek letter used to represent resistance in electrical circuits.

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