With the 1996 EEC emission regulations more demanding and tighter than ever, the module must leave nothing to chance. A host of sensors monitor ambient air temperature, the temperature of the engine, airflow, engine load and revs and vehicle speed and selects which time to inject the petrol and fire the spark (via the EI module), which gear to select in the automatic gearbox, depending of course whether Sport or Economy mode has been selected.

Having fired the mixture inside the cylinder, the module then checks the exhaust mixture as it leaves the engine, and then again after it has passed through the catalyst, and if too many hydrocarbons are present it will adjust the mixture to lean. At maximum continuous revs the EEC V on the 24V must be able to monitor the inputs and control all of the functions in order to fire the cylinders at the correct time, at the rate of 25 times per second. The module also arranges the purging of the evaporative emission canister at a suitable time, controls the variable resonance inlet valves, makes allowance for the drag of the power steering pump and aircon compressor, and arranges the introduction of exhaust gases into the inlet manifold to prevent NO_x emissions, (EGR)

How does it all work? What and where are the sensors? If you have an hour or so to spare, here are the answers.

Note: the gearbox references below are for the automatic gearbox, not the manual.

Gearbox
Powertrain Control Module
The EEC V module is located beneath the dashboard behind the glove compartment. It is pop-riveted to its bracket for security, and a port for connection to the OBD diagnostics is provided on the underside of the dashboard in the change cubby to the right of the steering column.

The module constantly monitors the signals it receives from its many sensors. Some of the sensors provide an analogue signal, and the PCM must first convert these signals to digital before they can be used.

The module usually sends a reference voltage to a sensor and then measures the return signal for change, then searches a map in its memory for that value.  It then uses mapped responses in order to control the various valves and solenoids accordingly.

These maps are In Read-Only Memory (ROM), and are referred to constantly many times a second, so that the many variables produced by the sensors can be matched with the correct timings for ignition, for fuel quantity, for gearbox pressures, gear selected and the torque clutch. Also stored here is the Limited Operation Strategy (LOS), which enables the engine to run even if some sensors fail.

Stored in the Random Access Memory (RAM) is the adaptive strategy. This is where the allowance is made for deterioration of some components, and values which are missing or suspect can be substituted for another from the LOS. This error and the substitute value is then stored for retrieval over eighty drive cycles. A self test facility also stores any intermittent fault codes for the same period. It is these codes which provide invaluable information for the WDS diagnostic system.

The PCM monitors both input and output devices for any faults. If any are detected it will turn on a warning light in the instrument panel(not installed on the Scorpio), store a fault code and display it when the WDS computer diagnostic or Vehicle Explorer is connected to the diagnostic port.

Engine
Crankshaft Position sensor
In order that the PCM can control the engine at all, the position of the crankshaft must be known so that the cylinder in compression and the timing of the next spark can be calculated. The CKP is an inductive pulse generator which scans protrusions on the flywheel or on a toothed disk attached to the crankshaft, depending on engine. One of the teeth or protrusions are missing, and this gap is situated at 90° before TDC. The pulse generated by the sensor is analogue (a sine wave varying regularly between positive and negative values) and this is converted to a digital PIP (Profile Ignition Pickup) signal by the electronic ignition system (built into the EECV on some models) and it is this PIP signal which is used by the PCM to determine engine speed and ignition timing. Monitored by: CCM

Gearbox: It is also an input for the control of gearbox main line pressure, gear shifts and the engagement of the torque converter clutch. 

Camshaft Position sensor
When the engine first starts the exact position of the crankshaft will not be known until the reference gap passes the CKP. To provide the initial starting sequence the CMP scans a reference point on the camshaft. This analogue signal is then used by the PCM to calculate the position of No 1 cylinder and together with the PIP signal the PCM can then calculate the correct sequence of the injectors.

Once the engine is running, the PCM uses the PIP signal to control the fuel injectors and the CMP signal is then used to optimise the closing time for the EI module primary coil.  Monitored by: CCM

Still with me? Don’t look for any jokes – it doesn’t get any lighter than this.

Throttle Position Sensor
Mounted on the throttle housing or sometimes on the accelerator pedal, the TP sensor is a potentiometer which is supplied with a reference voltage of 5V by the PCM. As the throttle is opened by depressing the accelerator pedal, a sliding contact moves across the resistance track inside the potentiometer, which varies the voltage returned to the PCM – throttle closed, voltage low, throttle wide open, voltage high. This voltage is then used by the PCM to determine the throttle position and is also used for control of the gearbox, see Gearbox below.

The PCM is able to determine the following TP conditions:

Together with other sensors, the PCM uses the throttle position to decide idle speed, fuel quantity and ignition timing.

Gearbox: Used by the PCM for control of main line pressure, gear shifts and the torque converter clutch.  Monitored by: CCM

Fault Symptoms: Incorrect shift scheduling - torque converter clutch engages/disengages early/late

Electronic Ignition System Module
Although some Ford models have the EI module built into the PCM, on the 24 valve the EI is mounted on the offside wing behind the headlamp.

The module controls the closing time of the ignition primary circuit, the opening times and hence the triggering of the ignition by using the signal from the CKP, which has be converted to a digital signal within the EI module. The EI module calculates the correct closing time of the primary circuit, closes the circuit and then opens it at the time calculated by the PCM to produce an ignition spark.

The actual ignition angle is calculated by the PCM, which sends it to the EI module in what is known as a Spark Advanced Word (SAW) signal. This SAW signal is stored by the EI module which compares it with the digitised CKP signal to determine the exact ignition timing. At exactly the appropriate moment the EI module interrupts the current to the primary coil and this triggers an ignition spark.

If for some reason a SAW signal is not received, the EI module uses the data last stored, but if five SAW signals are missed in succession the Limited Operation Strategy is implemented. The leading and trailing edge of the PIP signal is then used to trigger the ignition, giving a fixed timing of 10° BTDC. The reduction in performance is, of course, drastic.

After three successive SAW signals are identified by the EI module, normal operation is resumed.

Gearbox: It is also an input for the control of gearbox main line pressure, gear shifts and the engagement of the torque converter clutch.  Monitored by: Misfire

Mass Air Flow sensor
In order that the correct amount of fuel can be metered by the fuel injectors, the MAF sensor must measure the mass of air entering the intake system.

It uses a well established hot-wire principle. A hot wire and an air temperature probe are mounted together in the inlet duct. The hotwire is maintained at 200°C hotter than the air temperature probe, and the current needed to maintain this temperature is connected to a precision resistor.

The principle is simple. The heated probe is cooled proportionally by the mass of air which passes it in the duct, so the current required to heat the probe varies according to the mass of the air passing through. From the voltage reading in the resistor the PCM can then assign an accurate measurement of the air flow and map the quantity of fuel to be emitted by the injectors.  Monitored by: CCM

Gearbox: MAF is also an input for the control of gearbox gear shifts and the engagement of the torque converter clutch.

Fault Symptoms: Incorrect gearshift scheduling – torque converter clutch engages/disengages early/late

See also Testing the MAF' and 'Cleaning the MAF'

Intake Air Temperature sensor
The temperature of the air entering the engine has implications for fuelling, since colder air is more dense. The IAT is mounted inside the air inlet manifold and varies its resistance to a reference voltage of 5V from the PCM, and the resultant voltage signal received by the PCM is assigned a corresponding temperature value by the PCM. (Test values)  Monitored by: CCM

The signal is is one of those used by the PCM to to determine fuel metering and ignition timing.

Engine Coolant Temperature sensor
Like the IAT, the ECT sensor varies a reference voltage of 5V by a resistance in inverse proportion to temperature. It is mounted toward the front of the cylinder head.

The signal from the sensor is assigned a temperature value by the PCM, and is one of the inputs to decide idle speed, fuel quantity and ignition timing and torque converter clutch. See Test Values  Monitored by: CCM

Gearbox: It is also an input for the control of gearbox main line pressure, gear shifts and the engagement of the torque converter clutch.

Fault Symptoms: Torque converter inoperative, high fuel consumption

Idle Air Control Valve
This is a valve, not a sensor, and is an electronically controlled solenoid which allows a flow of air to by-pass the throttle plate. Since the additional air allows the engine to tick over, the valve can maintain the engine speed regardless of load by varying the amount of air admitted.

The PCM determines engine speed from the PIP signal. This is compared with ECT, TP and MAF sensor information from which the PCM can calculate the amount of IAC valve movement required. The valve receives pulses from the PCM, the length of which determines the valve position.  Monitored by: CCM

Fuel Injectors
Also valves, operated electromagnetically, INJ meter and atomise the fuel into the inlet manifold and are controlled directly from the PCM. A magnetic coil acts upon the needle jet, and the time of opening and the length of opening are determined by the PCM, triggered individually in a sequence, hence sequential fuel injection (SFI). Since the injectors are not variable, it is the duration that they remain open which provides the different quantity of fuel required.

Injectors can be of side or vertical feed. In vertical feed, fuel is supplied from above by means of a distributor line. Side feed injectors (used in the 24V) are positioned in the fuel rail, so that the body of the injector is surrounded in fuel. This has the advantage of cooling the injectors.  Monitored by: CCM

Vehicle Speed Sensor
The VSS is driven by a tachometer drive in the transmission and uses the Hall effect to provide a digital voltage signal to the PCM, which assigns a corresponding vehicle speed to the signal. With other inputs, the PCM uses this to calculate idle speed control, fuel enrichment during deceleration, and cuts off fuel supply on closed throttle deceleration at speeds greater than 1800 rpm and 5mph.

An example of the way in which integration between different vehicle systems is increasing is by considering this VSS signal. As well as providing vehicle speed information to the PCM, the same signal is used by the electronic Instrument cluster to display the vehicle speed, by the Trip computer to calculate distance and speed, by the speed-sensitive power steering to reduce steering assistance and by the cruise control system. The VSS signal is also used by the radio for the Automatic Volume Control if the option is present and turned on.

In an automatic Scorpio the PCM uses the VSS as an input for the control of line pressure, gear shifts and the engagement of the torque converter clutch and compares the VSS sensor data with that of the TSS to control slip.  Monitored by: CCM

This is undoubtedly why the Hall efect was chosen rather than the Inductive pulse generator used on the CKP and the CMP.

Fault Symptoms: harsh engagement, difficult engagement - sudden downshift when throttle is closed - torque converter clutch fails to engage

Heated Oxygen Sensor (Pre Catalyst & Post Catalyst)
The HO2S, (also called a Lambda sensor) is a voltage generator positioned in the exhaust flow ahead of the catalytic converter. It sends a voltage signal of 450mV to the PCM when it detects a Lambda level of 1, or an air/fuel ratio of 14.7:1, which is the ideal ratio for the combustion process.

If the mixture detected is lean, the HO2S sends a signal of 200mV and the PCM immediately sets the mixture to rich (ie longer duration opening on INJ). Conversely, if the mixture is too rich then the HO2S send a signal of 800mV and the PCM will lean out the mixture. In this way the PCM can keep a very close control of exhaust emission.

The operating temperature of the HO2S is about 200^oC, so a heating element is built into the sensor which is switched on by the ignition.

In those vehicles which meet the 1996 EEC emission standards, a Post-Catalyst HO2S is placed after the catalyst and is used to check the efficiency of the pre-catalyst HO2S.

The pre catalyst HO2S sensor is referred to as Sensor 1 and the post catalyst one is Sensor 2 when reading OBD2 data.  Monitored by: HO2S

Evaporative Emission Canister Purge Valve
Evaporated fuel from the petrol tank and the fuel lines is collected into a canister where it is stored in order to meet the 1996 emission standards. The EVAP purge valve is controlled by the PCM, which selects suitable moments to open the EVAP purge valve which connects the canister to manifold vacuum. The fumes are then drawn from the canister and used in the combustion process.  Monitored by: EVAP

A quick test that the EVAP system is working can be done by releasing the fuel cap when refilling at a Petrol Station - if it hisses then EVAP is working.

Exhaust Gas Vacuum Regulator
During part-load conditions, the pressure inside the combustion chamber may be high enough to produce unwanted NO_X emissions. (See EGR) When the PCM detects that emissions may be likely, it sends a signal to the EGR vacuum regulator, which is in the vacuum line between the inlet manifold and the EGR valve in the exhaust. The resulting vacuum acts on the EGR valve which opens, admitting exhaust gas into the combustion chamber and ‘quenches’ the combustion. At the same time, the PCM will make adjustments to timing and mixture based on the signals from the EGR Pressure Transducer. Monitored by: EGR

The valve is operated by a magnetic coil acting against a spring. The greater the strength of the signal from the PCM, the further the valve opens.

EGR Pressure Transducer
This is a ceramic resistance transducer which measures the pressure of the gas flowing from the exhaust manifold to the inlet manifold via the EGR valve by means of varying a reference voltage of 5V from the PCM. This information is then used by the PCM to decide the optimum EGR rate and ignition timing correction. Thus the amount of exhaust gas admitted to the combustion chambers can be closely controlled, since quenching the combustion entirely would release hydrocarbons and endanger the catalyst(s).  Monitored by: EGR

Variable Inlet System Vacuum Regulator
The VIS vacuum regulator is an electromagnetic switch located in the vacuum line between the inlet manifold and the VIS actuator, and is controlled by the PCM. (See variable resonance inlet) With the ignition switched on and the revs are less than 3200rpm, the VIS vacuum regulator receives a signal from the PCM and opens, exposing the regulator to vacuum and closing the butterfly valves in the secondary inlet duct. At this time, the air is drawn through the primary ducts only, which causes high turbulence and improves engine torque.

At engine speeds above 3,200rpm, the PCM sends a signal to the VIS vacuum regulator and it closes, which opens the butterfly valves in the secondary ducts of the variable inlet system. Now the engine is breathing through both primary and secondary ducts and this produces a higher engine output.  Monitored by: CCM

Power Steering Pressure Switch
Installed in the pressure pipe between the power steering pump and the steering rack, the PSP is an on/off switch activated by pressure, which allows the PCM to increase engine revs to allow for the drag of the power steering pump. The pump demand will be highest when the greatest steering maneuvers are made, and this will be at parking speeds when the engine will be on tick-over. The demand will sometimes be greater than the IAC valve can accommodate, so this signal is used by the PCM to make necessary adjustments.  Monitored by: CCM

Depending on model, the switch may be open-circuit or close-circuit on pressure.

Gearbox Only

Transmission Range Sensor
The signal from the TR sensor is used by the PCM as one input for the control of main line pressure, gear shifts and the engagement of the coast and torque converter clutches. The sensor is mounted on the top of the transmission housing beneath the gear selector. It changes a control voltage from the PCM and returns a low reading if the gear selector is in Park, and a high voltage if the selector is in 1 position, and in fixed steps between.   Monitored by: CCM

Fault Symptoms: harsh engagement – 3-4 downshift not operating – no speed drop in P or N

Overdrive Cancel Switch
The OCS is mounted on the selector lever and connects to the PCM with a signal wire and a ground. It generates a short signal which operates in the PCM to change the overdrive condition, either 3-4 or 4-3 shifts and to control the overdrive clutch. The OCS also makes or breaks the circuit to the relay operating the O/D OFF indicator in the instrument panel.

Fault Symptoms: O/D does not switch off – no engine braking in 2^nd and 3^rd gear

Economy/Sport/Winter switch
The PCM uses the signal from the ESW switch as an input for main line pressure, shift scheduling and the torque converter clutch.

The driver can select modes which have the following effect:

E is normal Drive mode, and the PCM defaults to this mode on ignition on. The Torque Converter clutch operates in 2^nd, 3^rd and 4^th gears.

S in Sport mode the PCM selects higher shift speeds (up and down) at part load, and the torque converter clutch operates in 3^rd and 4^th gear only.

W in Winter drive mode the PCM selects 2^nd as the move-off gear. Remaining gears shift in the same way as ‘E’ but with different, softer, timings.

Transmission Speed Sensor
In the A4LDE gearbox, as well as the torque converter being engaged and disengaged, there is a controlled slip to aid smoothness. The PCM uses the input from the TSS in order to compare the torque converter input speed with the transmission speed in order to control the degree of slip.

The TSS is mounted on the centre planetary gear carrier and produces a frequency signal in direct proportion to the transmission speed.

In the A4LDe (fitted to the Scorpio V6 12V) there is no controlled slip and this feature is not present.  Monitored by: CCM

Fault Symptoms: rising engine speed with all gear changes – harsh engagement – shift delayed, then harsh

 Brake On/Off switch.
The PCM uses the signal from the BOO switch in order to release the torque converter clutch when the brakes are applied. The PCM reads the voltage as 0 – brakes off and 12V – brakes applied.  Monitored by: CCM

Fault Symptoms: BOO switch stuck on: torque converter fails to engage when throttle is less than ⅓ open

BOO switch stuck off: torque converter fails to disengage when brakes are used.

Transmission Fluid Temperature Sensor
The TOT sensor is mounted in the automatic gearbox sump and its input is used by the PCM for control of the torque converter clutch. When the temperature is low (below 15^O C) and when too high (above 150^O C) the PCM prevents the Torque converter clutch from engaging.

The sensor changes a control voltage from the PCM inverse to temperature. When the temperature is low, the sensor resistance is high, and vice-versa.

This protects the transmission from malfunctioning due to excessive fluid temperatures.  Monitored by: CCM

Fault Symptoms: torque converter clutch engages early during cold starting - harsh or slipping engagement

Air Conditioning Switch
The ACS is also an input for the PCM. When the air conditioning is switched on the engine load changes because of the a/c compressor. The main line gearbox pressure is adjusted by the PCM to allow for this change in engine load and detects the system voltage on the switch, 0 for off, 12V for on.  Monitored by: CCM

Fault Symptoms: faulty engagement of the torque converter clutch.

NOTE: The EECV has fully implemented the OBDII diagnosis system (although there is no MIL or Check Engine light) and constantly checks the engine sensors as well as the emission-related systems. The sensors, how they work and their fault diagnosis is shown in detail on OBD Detail. If one of the symptoms shown above occurs in your vehicle, reading the diagnostic system for a fault code could save a great deal of expensive investigation.

EricR