COMPUTERIZED ENGINE CONTROLS
One of the most important developments in the automotive service industry is the use of computerized engine controls. These systems constantly adjust fuel, ignition and emission controls. The result is better fuel economy, improved performance, and better driveability. Another result is an ever-increasing load of wiring, sensors, and "Black boxes" in the engine compartment. The aim of this section is to help untangle both the computer systems and the fear with which most service people approach them.
The traditional "tune-up" is performed to help improve engine operating efficiency. In vehicles with computerized systems, the computer is constantly tuning the engine. Some systems affect only air/fuel ratios, while others control air injection, EGR, transmission shifting and many other functions.
This introduction will cover the major components of computer systems, some basic servicing tips, and list some necessary tools.
CONTROL UNITS
The heart of any computer system is the control unit, or control module. These computers are usually sealed in a metal box and are linked to the system by a wiring harness with multi-pin connector. The computers are usually located in the passenger compartment for protection from moisture, vibration and temperature extremes.
The job of the computer is to coordinate vehicle operation and equipment. When engineers develop the systems, they simulate every conceivable driving condition and determine the best adjustments for economy, driveability and low emissions. These selected adjustments are in the computer's program and are used by the vehicle at certain times.
Systems are said to operate in either "Open Loop" or "Closed Loop". In an open loop condition, the instructions and decisions made by the designers are used to run the engine. The computer just follows their orders. In closed loop, the computer reads the values from a number of sensors and makes tuning decisions based on past experience and present conditions. This ability makes the system more responsive, and allows the computer to compensate for unusual or severe driving conditions.
SENSORS
The computer gets its information from a network of sensors. Most are simple sending units just like an oil pressure switch or fuel level gauge. Either the sensor is off/on or it gives the computer a variable resistance reading. Many systems use existing engine sensors, while some systems have specialized sensors that provide input on engine "knocking", atmospheric pressure or outside air temperature.
The control unit checks sensor inputs constantly and makes its decisions based on both pre-programmed input and the sensor readings. Common sensor inputs include
- Air/Fuel Ratio (from exhaust gas oxygen sensor)
- Engine Speed (from distributor or crankshaft sensor)
- Engine Temperature (from oil or coolant sensors)
- Engine Load (from throttle position or vacuum level sensors)
Other sensors are used by some systems to provide additional information. These may include
- Intake Air Temperature
- Intake Air Volume
- Barometric/Manifold Pressure
- Vehicle Speed
- Fuel Temperature and Pressure
- Detonation
- Transmission Gear
Two different systems may require the same kind of information, but get it from different sensors. For example, one of Ford's MCU systems uses an idle switch to tell the computer when the throttle is against the idle stop. Another MCU system uses a vacuum sensor to provide the same information. Despite the variety of sensors, computers make their decisions based on the same kind of inputs.
ENGINE CONTROLS
Computer commands are carried out by solenoids, motors and other control devices. Solenoids are used to control vacuum, air bleeds, the carburetor fuel flow, etc. Motors are used for the idle speed control and mixture adjustments.
Some computers control functions directly, while others use indirect means. For example, when a detonation sensor hears "knocking", the computer will retard timing. On some systems, it will tell a solenoid to cut off vacuum to the distributor vacuum advance solenoid. Timing is retarded. On other systems, the computer may be directly involved in controlling engine timing, and it just retards the firing slightly until "knocking" stops. The method used is different, but the effect is the same.
SERVICING
The first step in servicing is to eliminate possible causes of trouble. Be sure that mechanical systems (compression, valves, plugs, transmission) are okay before suspecting the computer system. It has no control over these problems.
Most computer control systems have no serviceable parts. The technician's job is to locate the part that doesn't work and to replace it. Since these systems can be quite complicated, manufac-turers have built in some self-diagnostic features. The computer looks for and indicates its own problems.
With these systems, the technician starts the computer diagnostic program and watches for service codes that indicate a problem. These code numbers can be indicated by flashes of a dash mounted light or pulses on a voltmeter. Small diagnostic testers are now available that will show a direct indication of the code.
Once the codes are known, trouble shooting procedures and charts are used to locate the specific component failure. Without the procedures and charts, troublesome components are harder to locate, but repairs can still be made. One danger is that traditional obvious causes of poor driveability may no longer be the cause now, and parts replacement to resolve a problem is very expensive. The diagnostic programs MUST be used to ensure profitable and accurate repairs.
TOOLS
Some systems require special testers (Ford EEC-IV), but most only require common shop equipment. The following items are necessary for most servicing
- Analog Volt-Ohmmeter
- Digital Volt-Ohmmeter
- Vacuum Gauge
- Hand Vacuum Pump
- Timing Light
- Test Light
- Tachometer/Dwell Meter
- Jumper Wires
Several aftermarket tool manufacturers now make testers that will plug into these computer systems and allow faster reading of codes and system problems. If you service systems frequently, these may save time.
The most important tool in servicing is the alertness of the service technician. Most problems are caused by poor connections, damaged wiring or leaky vacuum lines. Only a very small percentage of computers ever fail. Most of the computers returned for rebuilding are in fine working condition, which means they were replaced unnecessarily. Be sure a complete visual check is made before deciding any part of the system is faulty.
COMPONENT TESTING
Since it is not possible to test a computer in the car, most servicing approaches are designed to eliminate all other causes of trouble. If everything else tests out okay, then the computer is the last possible cause. In order to do this, some components must be tested first.
Sensors can be checked to see if they open or close, or if their resistance values are correct at specified temperatures. Solenoids can be checked by applying voltage and observing their operation. Solenoids should also have a measurable resistance across the windings. They should not be shorted to ground or open.
If sensors and solenoids are okay, the system circuits should be checked. Disconnect wiring at both ends of a circuit and measure resistance. If more than about 5 ohms is measured, then the connections need to be cleaned or wires repaired. Next, connect the ohmmeter between one end of the wire and ground. There should be at least several hundred thousand ohms resistance. If not, the wire is shorted to ground and must be repaired. Wiring diagrams are available for all systems and should be used to check connections and resistance values.
If all sensors, control devices and wiring check out okay, then the computer may be the problem. If possible, substitute a known good computer. If performance is restored, install a new unit. Once again, since control units are very expensive and RARELY fail, be sure ALL other causes are eliminated before replacing.
SUMMARY
Computerized engine controls are used on all engines. The computer is just a decision-making device that acts according to a complicated set of instructions. Although computers have taken over many of the tuning chores once performed by service people, they are not perfect and have not replaced the competent service technician. A great opportunity is waiting for people who can accurately diagnose and repair computerized engine control systems.