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How it Works: Ford Power Stroke Engine Controls

Learn the basics about how your Power Stroke's computers, sensors, and injection system function.

Text By Ray T. Bohacz, Photography by Ray T. Bohacz

When Ford (in conjunction with International) converted the 7.3L IDI engine from mechanical to the hydraulically actuated electronically controlled unit injection (HEUI) system, the Power Stroke brand was created. The 7.3L Power Stroke was the first electronically controlled, direct-injected diesel engine in the light-duty market segment.

For the '031/2, the 7.3L was replaced with the 6.0L engine, and then for the '08 model, the compound-turbocharged common-rail 6.4L design debuted. Realizing the popularity of the Power Stroke engine with our readers-diesel truck sales suggest there are more Power Stroke owners in the world than there are Cummins and Duramax pickup owners-we wanted to create a primer that provided an overview of the electronics under the hood of your Ford.

Our guide isn't meant to replace the factory shop manual when it comes to repair or diagnostics, but instead to supplement it. Most shop manuals are very good at providing diagnostic steps along with remove-and-replace procedure but offer little to understand the sensors used on the engine and the control logic involved. Thus, this article will be focused on becoming familiar with the sensors used on the 7.3L, 6.0L, and 6.4L Ford engine. Due to ever-increasing diesel emissions standards, the three versions of the Power Stroke use different control strategies, and not every sensor will be represented for each engine. Always reference the appropriate shop manual for the system your engine has.

Injection Connection
The HEUI technology that debuted in the 7.3L Power Stroke was actually developed by the Caterpillar Engine Division in Pontiac, Illinois. It was the result of a joint venture between Caterpillar and International Truck. The system debuted on the International 7.3L engine before it was ever offered on a Caterpillar diesel. The original engine was known within International as the T444E (T for turbo, 444 for the number of cubic inches the engine displaced, and E for electronic controls). When it was installed in a Ford, it became known as the 7.3L Power Stroke.

A major advantage of the HEUI design was that it offered significant control over the injection rate, duration, and timing when compared to a conventional mechanical injection system. HEUI injectors are actuated by engine oil pressure to create the high-pressure injection. The electronics that manage the HEUI fueling control the actuation oil pressure through a very wide range of values. This results in precise control of injected fuel pressure, entirely independent of engine speed. With a mechanical fuel delivery system, the fuel is usually supplied by an actuator lever riding on a camshaft, so engine speed would control the fuel pressure produced. The rate-shaping ability of the HEUI design provided much greater tuning of the fuel delivery on a diesel engine than anything else available in North America in 1994.

HEUI To Common-Rail
The 7.3L Power Stroke used a Caterpillar-produced HEUI injector. When the 6.0L engine came out, its HEUI system (injectors) was produced by Siemens but used a Bosch-built high-pressure oil pump. Heavy-duty Caterpillar engines that featured the ACERT system (advanced combustion emissions reduction technology) in the '90s also used an HEUI system design. The 6.4L Power Stroke eliminated the HEUI system and went to a Siemens common-rail piezo injector that used no engine oil. The system is similar to what is found on '01-and-newer Duramax, as well as '03-and-newer Cummins engines, which use a Bosch-based common-rail injection.

The impressive part of the 6.4L's common-rail injection system is its piezo actuator. It is an electrically energized device that acts similar to a solenoid, but it's much more precise. It is comprised of piezo discs that, when electrically charged, cause them to deform, resulting in expansion. This expansion creates a longitudinal motion that controls the injection of fuel. When energized, the piezo actuator pushes downward against a valve piston (in the injector). The piezo actuator is then returned to its non-energized state via the electronic controller, switching the polarity of the electrical feed to the injector.

Due to its speed, the piezo injector can produce up to five injection cycles per combustion event. This greatly reduces exhaust emissions and helps control the cylinder pressure rise to produce less engine noise. The piezo actuator is turned on for approximately 400-millionths of a second to create two injections.

Decision Making
Even though the 7.3L, 6.0L, and 6.4L Power Stroke engines do not share a basic fuel delivery system design, they rely on electronics to control the amount of diesel fuel injected into the combustion chamber. All of the Power Strokes are direct-injection engines, so there is no need for a pre-chamber in the cylinder head. The compression ratio is lower than an IDI design (the pre-chamber greatly increases the surface volume, and thus the thermal loss into the coolant). All three engines use a microprocessor to make the fuel delivery and timing decisions. But the similarity between the three engines' electronics ends there. The 7.3L and 6.0L employ two microprocessors to operate the injectors, while the 6.4L has one unit. The 7.3L Power Stroke used a PCM (power train control module) in conjunction with an IDM (injector driver module) to run the engine. The IDM employed an internal DC-to-DC converter that boosted the signal to the injectors to 115 volts.

The 6.0L engine uses two computers in series to control the engine. There is a PCM and an FICM (fuel injection control module). The two modules are intrinsically linked, since both have a flash memory and are involved with the decision-making process of how long and when to open the fuel injectors. The majority of the authority for the engine calibration is in the PCM, while the FICM does the actual work of opening the injectors through an electronic device called a driver. The driver can be considered a high-current switch that has no moving parts. The FICM also internally generates the 48 volts that the 6.0L HEUI system uses.

In the FICM, each individual injector is controlled with four driver inputs. There are high- and low-side drivers to open and close the coil of each injector. On later 6.0L engines, the low-side driver is actually shared among four injectors. This means an injector short to ground on one of the low-side drivers could produce four different cylinder error codes. The first ('03) engines had an individual low-side driver for each injector.

Unlike a gasoline fuel injector, the Power Stroke HEUI system requires both an open and a close command for the injection cycle. In contrast, a gasoline engine uses only an open command and, when shut off, the internal spring closes the injector. The high operating pressure of the HEUI will not allow for that. It needs to be remembered that the FICM and/or IDM (7.3L) is actually controlling the high-pressure engine oil on the top of the injector to provide fuel delivery. The PCM and FICM communicate through what is known as a CAN (controller area network) circuit.

6.4L Supercomputer
When the 6.4L was introduced, the FICM was eliminated and the injector control drivers were now in an ECM (engine control module) instead of a PCM. The piezo design used a varying command signal from 42 to 96 volts DC. The necessary voltage is created in the 6.4L's ECM.

In summation, the first Power Stroke (7.3L) employed an HEUI injector from Caterpillar that operated with 115 volts and was controlled through a combination of a PCM and an IDM. The 6.0L version had an HEUI system designed by Sturman for Siemens instead of a Caterpillar system (it's been suggested this was done to get around Caterpillar's patents) and was run via an FICM and PCM on 48 volts. The 6.4L eliminated the HEUI system and replaced it with a common-rail piezo injector operating between 42 and 96 volts using just a single ECM.

Data Gathering
Regardless of the style of fuel delivery, for the PCM and ECM to determine the requirements of the engine, sensors are required. A sensor is a device that translates a physical state, or condition, into an electrical signal. In all the Power Storke applications, the PCM and ECM use a 5-volt reference signal for most, if not all, sensors. This is a common logic employed among controllers for both gasoline and diesel engines. The sensor produces an output voltage that is then sent to the PCM or ECM.

Due to the stricter control on emissions with every passing year, the 6.4L Power Stroke has more sensors than any previous version. The ECM is also responsible for controlling the burn-off regeneration of the diesel particulate filter (DPF), so additional data is required that the earlier systems did not need. The following are the common Power Stroke sensors, but not all were used in all applications

AP Sensor (Accelerator Pedal Position)
The AP sensor incorporates three potentiometers to track throttle position. Throughout the movement of the AP, the resistance values of the three potentiometers must agree. If not, the Check Engine light will illuminate, and the vehicle will continue to perform as normal. If two signals from the AP are lost, the ECM will only allow the engine to idle.

Baro Sensor (Barometric Pressure)
A 5-volt reference signal is is supplied to the baro sensor and it produces an analog output signal. In the 6.4L, the sensor is located in the ECM, while the earlier engines use a remote sensor. The primary function of the baro sensor is to provide altitude information so the timing, fuel quantity, glow plug on time, and turbocharger boost can be altered.

CKP Sensor (Crankshaft Position)
The CKP sensor signal is created by a magnetic pickup mounted to the front of the engine block. It reacts to a trigger wheel positioned on the crankshaft. The sensor produces a sine wave that is converted by the ECM or PCM to a digital signal. Crankshaft speed is determined by the frequency of the signal output.

CMP Sensor (Camshaft Position)
The CMP sensor signal is the result of a magnetic pickup mounted on the engine block. The sensor reacts to a trigger placed on the camshaft. Camshaft position is used in conjunction with the CKP to calculate engine firing position.

ECT Sensor (Engine Coolant Temperature)
The ECT sensor is a thermistor, which is the opposite of a resistor. As it is heated, its resistance drops. It is used for fuel delivery modification based on the engine temperature.

EGRVP Sensor (EGR Valve Position)
The EGRVP sensor is a three-wire potentiometer. The engine controller uses the EGRVP sensor to tailor the amount of recirculated exhaust gas, based on engine operating conditions.

EOT Sensor (Engine Oil Temperature)
The EOT sensor is a two-wire sensor, similar to the ECT. The ECM and PCM monitor the engine oil temperature to aid in controlling fuel rail pressure and fan control. This allows the engine controller to compensate for oil viscosity variations due to temperature changes and ensure adequate engine power is produced for all operating conditions.

IAT1 Sensor (Intake Air Temperature, Number 1)
This sensor is a two-wire thermistor located inside the mass airflow sensor. Its primary function is to measure intake air temperature to aid in controlling the variable-vane turbocharger and the glow plug system.

IAT2 Sensor (Intake Air Temperature, Number 2)
Also a thermistor, the IAT2 looks different than IAT1. Its function is to monitor the air temperature in the intake manifold.

FRP Sensor (Fuel Rail Pressure)
The FRP is a three-wire variable capacitance sensor. Its function is to provide data to the controller indicating the pressure in the fuel rail. The ECM and PCM monitor the FRP sensor as the engine is operating to modulate the pressure control valve. The FRP is also used to command the proper injection timing.

MAF Sensor (Mass Airflow)
The MAF sensor uses a hot-wire sensing element to measure the amount of incoming air to the engine. The airflow cools the sensing wire, and the current required to keep it at 392 degrees greater than the IAT1 temperature is then measured.

MAP Sensor (Manifold Absolute Pressure)
A three-wire variable capacitance sensor is used to assist in the calculation of EGR control, fuel delivery, and throttle body position, along with boost pressure.

EP Sensor (Exhaust Pressure)
The EP sensor also uses a three-wire design. It measures the exhaust backpressure for EGR and high-pressure turbocharger control.

FTS Sensor (Fuel Temperature)
The FTS sensor is a two-wire thermistor, and its output is tailored to the fuel temperature. It is used to control fuel delivery.

EGRT Outlet Sensor (EGR Cooler Outlet Temperature)
This is a two-wire thermistor sensor. The engine controller monitors exhaust temperature with this sensor to aid in controlling both the EGR valve and throttle plate position.

Maintenance is Key
HEUI-equipped Power Stroke engines are very susceptible to poor injector performance caused by extended engine oil drain intervals, improper oil, and low-quality fuel. Unlike a mechanical injector, the HEUI unit can not be taken apart for cleaning and service by a diesel shop. If it fails-it must be replaced. Remanufactured HEUI injectors are offered, but it is much less costly to keep the engine oil changed and use a good quality fuel additive in the tank on every fill-up.

The piezo injectors are not serviceable at this time and need to be replaced if there is a problem. Though engine oil does not affect piezo designs, good fuel filter service practices are required for a long and trouble-free life of any engine.

In many ways, the Power Stroke engine has received a black eye for reliability. Some of that may be justified, but more is based on not understanding the system-which leads to simply replacing components that are not the cause of the problem. Hopefully, you will now have a better understanding of what is going on under the hood of your Ford diesel.

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By Ray T. Bohacz
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