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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.


OBD link connectorProvides the following functions when connected to the vehicle.

  • Built in light to aid finding the OBD port
  • Test all 16 pins in the OBD port automatically
  • Test for power supply and grounds in OBD port
  • Monitor of battery voltage via inbuilt volt meter – necessary when performing diagnostics, programming and coding
  • Check charging voltage
  • Audible warning if battery voltage goes too high or too low
  • Detect communication protocol
  • As a break out box for ease of connecting oscilloscope
  • Use as a data saver when replacing battery when used in conjunction with the optional data saver cable
  • Use as a 12 volt power source and ground for other devices – max load 5 amps
  • Use to bridge pins in OBD port for reading blink codes on certain vehicles and the bridging of pins is also required in other certain cases to reset and relearn particular systems

Here, you can download the operating manual for the OBD link connector used in cp fitters.

Object code is plain text (human readable) written for a computer program that has been converted by a compiler into a form that a computer can read - into object code.

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.


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.


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.


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.


Overhead valve arrangementOverhead Valve (also referred to as a cam in crankcase arrangement) - an engine arrangement where the inlet and exhaust valves are on top of the engine, but are operated by a camshaft that is down in the engine crankcase.

Opening of the valves is achieved by means of lobes on the camshaft, cam followers or tappets following the profile of these cams, and pushrods transmitting operating forces from the camshaft to the valve rockers.

This arrangement is in contrast to an overhead cam arrangement, where the camshaft is on top of the engine and operates the valves directly.


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:


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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Please note, if you want to contact the main Training Centre in Baldoyle, you need to ring 01-8167400.

If you want to contact the CP Fitters' Section, you can Contact Us by email,, or by telephone: 00 353 1 8167543 This number gets you through to the CP Fitters' Section, Baldoyle Satellite Unit.

Our Address is : Construction Plant Fitters' Section, Unit 116, Grange Way, Baldoyle Industrial Estate, Baldoyle, Dublin D13 CA48.
We operate in a satellite unit which is about 300 metres away from the main training centre. You can see the location of this unit here, in Google Maps. You can also Check here for directions to the Baldoyle Training Centre.

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