Understanding Electronics Part Three

In this section we will look at the various sensors around the engine management system and understand what they do, and how they do it, there are two main types of sensors:

Variable resistors - Variable resistors work by altering their resistance in proportion to something, it could be temperature or even mechanical motion, but its the variable resistance which is measured to give the engine's ECU the correct signal, and this is usually done on the earth circuit as opposed to the live circuit.

Pulse generators - these work by generating a signal by the passing of a metallic object in close proximity within its magnetic field, these can produce a pulse in proportion to speed, basically the faster an object travels through its field the more pulses it generates within a given time. Pulse generators can also generate voltages, this will usually be a voltage which rises with speed. Pulse generators have the ability to generate both a frequency pulse and a variable voltage, and they all have to have a moving metallic object such as a gear wheel or sprocket to activate them.

Sensors come in two types, these are primary sensors, these are the ones necessary for an engine to run, and secondary sensors, these are used to fine tune an engine; they are not necessary for an engine to run, and many have a limp home facility. LHF is where a defective secondary sensor has a fixed resistance or voltage substituted for a variable value when a fault is detected. Often this will not be noticed in general driving with a hot engine, but will be noticed with a cold engine or an engine used very heavily for severe off road work or heavy towing.

Primary Sensors

SSS - Speed Signal Sensor - Pulse generator.
Speed signal sensors are used on early engines to give an exact engine speed, they can also be used on gearboxes to give the signal for the speedometers on electronic speedo's. These sensors were replaced on later systems by the integrated CAS which gives both speed and engine angle.

CAS - Crank Angle Sensor - Pulse generator
Crank angle sensors measure both time and duration on an engine to give both the engines speed and its exact location on its cycle, it does this by measuring the time between the peaks of the pulses generated by the teeth on anything they are in close proximity too. CAS's are generally located on the flywheel in proximity to the starter ring gear, the camshaft pulleys next to the teeth on a cam belt sprocket, or on the front crankshaft pulley next to the cam belt sprocket.
CAS need a reference point, this comes with a tooth missing to give it this reference point so basically it misses a pulse where they should be one, it is programmed via the ECU so it knows exactly how many teeth there are on a particular sprocket and from this it knows the exact angle between the peaks of the generated pulses.
In simple terms: if we have 60 teeth on a starter ring gear and we have 360 degrees in a full rotation of the engine, the system will calculate that each pulse peak will be exactly 6 degrees between each pulse peak, from this it can calculate the engines exact position in its rotation to give its exact position. In addition it can calculate the time between each pulse peak to give the engines exact speed.

Throttle Switch - On/off switch
This is merely a microswitch which measures when no throttle is applied and switches the ECU from its running map to its idling map, these are usually fitted to a cam on the throttle body housing, and are only fitted to early injection systems. Later systems had this switch incorporated into the throttle potentiometer.

TP - Throttle Potentiometer - Variable resistor
Throttle potentiometers are a mechanically operated variable resistor, they operate from mechanical motion and the resistance gives the exact throttle position, they are usually mounted on the end of a shaft on the throttle body housing where the throttle cable mounts. As the throttle moves it moves the butterfly shaft within the throttle body housing and the potentiometer on the end of the shaft, the resistance is read by the ECU to give the exact throttle position.
Later versions of this have this potentiometer connected directly to the throttle pedal, this is called a "fly by wire" system as it has no throttle cable connecting the accelerator pedal to the throttle body, it just uses two wires directly from the fly by wire potentiometer at the throttle pedal to the ECU.
Later TP's had the throttle switch incorporated into them, when the throttle cable is slack the moving arm moving across the resistance track hits the switch and switches the ECU onto its idling map, some models have another microswitch fitted at the other end of the resistance track, this is the full throttle position. Once this switch is hit it gives the engine full beans and maximum performance irrespective of fuel consumption or emissions.

KS - Knock sensor - Piezo sensor generating pulses and voltage
Knock sensors perform a simple task, basically they listen to the sound of the combustion within the engine, they convert this sound into a series of regular pulses and this signal allows the engine management system to retard engine timing to its maximum when idling for low fuel consumption and minimised emissions. When running it listens to the combustion chamber noise and allows the engine management system to make comparisons between cylinders and compensate between individual cylinders.

OS - Oxygen sensor - Pulse/voltage generator
Oxygen sensors are often called Lambda sensors, they contain two platinum coated wires in which one is in the exhaust gas, the other is in the exposed to the ambient atmospheric air, these measure the difference between the two and generate two pulse signals and two voltages, it is these differences which are measured to ascertain the oxygen levels in the exhaust after combustion.
Later variants of this OS are heated, they only operate at temperatures over 300 degrees centigrade, to get them to this temperature quickly they incorporate a heater, but essentially work in exactly the same way as earlier types to measure the remaining oxygen content in exhaust gases after combustion has occured.

Secondary Sensors

CTS - Coolant Temperature Sensor - Variable resistor
Coolant temperature sensors simply vary their resistance according to the engine temperature, the ECU earths through the CTS to measure the resistance and give an accurate temperature reading. Older engines will have one CTS, later models will have two or more.
Different configurations mean different engines will use more than one CTS for varying reasons, some will use two CTS's for the engine managent system to make a comparison between the two, if one becomes defective it has a back up system, these are mainly fitted to 4X4 engines, truck engines, and marine engines solely for reliability. Some engines may have two CTS's fitted, one may be for the engine management system and the other may be for the electronic temperature gauge, others may tap into one for the starting system to measure engine temperature for glow plug control. There are many combinations and configurations.
Being a secondary sensor means it may have a limp home facility, sometimes called limited operating system, or other similar term; in the event of this sensor failing, its variable resistance will be substituted for a fixed value to allow the engine to operate with some limitations, but to allow you to get home or to a dealers garage for repairs.

ISCV - Idle Speed Control Valve - Stepper motor
Idle speed control valves are basically valves which control the engines idling speed, they do this by allowing a varying amount of air to enter the engine by opening and closing the valve infinately within its operating range, when the ECU operates on its idling map it activates this valve to open and close it to control the idling speed. These valves were fitted to earlier engines, predominantly at the upper end of the market, and with various accessories such as air conditioning fitted, this allowed the engine to maintain a steady idling speed irrespective of what load was applied to it.
Stepper motors are motors which operate over a defined arc, they do not rotate as ordinary motors do, instead they contain many sets of magnets which can be individually energised to send the motor to a defined position and hold it there.

AAV - Auxiliary Air Valve
Auxiliary air valves are temperature controlled much like a conventional thermostat, and are not controlled by, or connected to the engines management system, they open at cold temperatures to allow a little air to bypass the engine management system to increase idling speeds during engine warm up to allow the engine to warm quicker. These were only used on earlier engines.

Good stuff. And easy to grasp the way you write it.

The company I work for imported the Nissan D22 from Cyprus, This model based on the Japanese variant and not the European model came with a switch on the dashboard that could, according to the manual "Lock an engine valve open" and increase engine tick-over rpm by a couple hundred. Very useful for warming the engine up when the temps were very cold.
First Imported models featuring the switch were designed to be manually switched on and off.
The Facelift model with revised engines by Nissan had a safety cut out, once the vehicle started moving the "Cold Start" would automatically switch off. This was because on the previous Model of D22 pick up, their Owners would forget the cold start switch was on and this lead to a small number of problems.

Would the cold start valve on the Nissan be the AAV as described in you post?

TJ

& What's the difference between a Normal Turbo and a Variable Vein Turbocharger found in modern Pick Ups?

In a normal turbocharger the vanes are fixed and provide boost from a fixed RPM, this provides boost which is controlled by a wastegate which is simply a pressure controlled valve.

In a variable vane turbo the vanes are ECU controlled and moveable to give varying boost at varying RPM instead of the fixed boost of a normal turbocharger. This means the turbocharger can spin up at lower RPM due to less air friction and provide a low boost, but at higher working RPM it can be allowed to provide an overboost for a fixed period, this means the wastegate is often ECU controlled as well.
Overboost means better power and lower emssions, and it can respond well to varying conditions, and is beneficial to 4X4's, tractors, and heavy haulage all terrain vehicles as it quickly responds to external friction such as encountering a short patch of deep mud or ploughing in variable ground conditions. Basically it maintains the torque variance to keep the vehicles speed constant without as much driver input.

This vehicle used the Nissan ECCS engine management system which was designed as a forward compatible system from the outset, and was so well designed that its still used today, and has been continually upgraded.

Two methods of idle control were used with the system, the idle control system you describe is a wax element controlled cam, much like a conventional thermostat in the cooling system, during warm up this would operate a simple cam called the fast idle cam to operate the valve to open it to increase idle speed during warm up. This would gradually close as the engine temperature increased, thus lowering the engines idling speed as the engine got warmer.

In cold markets Nissan fitted an override system which was manually controlled, this was the switch on the dashboard, or an automatic switch fitted into the cooling system on automatically controlled models, market temperatures dictated whether a manual or automatic system were fitted and this used a solenoid to hold the valve fully open by overriding the wax element.

Once the engine was at operating temperature the ISCV took over the idle speed control, and it took the information from the coolant temperature sensor.

Sweet. That explains it then.

Nice posting, It very useful. Thank you for share.
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