Computer Command Control (CCC)

The Computer Command Control (CCC) system comprises the Electronic Control Module (ECM) and information sensors which monitor various engine functions and returns data to the ECM. The data is processed by the ECM and, if necessary, the ECM issues commands to change the operating parameters of the engine to optimise performance and economy at all times.

Electronic Control Module

The ECM, commonly referred to as the "brain" of the system is, in fact, a computer and is capable of making over 600,000 commands per second. In later year systems, it is also referred to as the Engine Control Module (ECM) since some vehicles have an additional computer unit known as a Body Control Module (BCM), which is involved in the control of various non-engine or drive train functions. The Body Control Modules are not covered in this book, and should not be confused with Throttle Body Fuel Injection Systems.

The ECM receives information about the engines operating conditions from various sensors and then calculates the optimum spark timing and fuel mixture according to preprogrammed values. The ECM controls the fuel mixture by varying the duration of time that the 8 injector solenoid valves are open. This time period is referred to as the pulse length and is measured in milliseconds. General Motors smart ECMs have memory and learning ability and can remember changes that produce peak performance. The ECM is calibrated to a specific vehicle and engine combination by a removable PROM (Programmable Read Only Memory) which, in the 86 through 89 ECMs, is part of a removable insert referred to as a MEM-CAL (Memory Calibrator). The MEM-CAL contains the calibrations needed for a specific vehicle/engine combination, as well as the back-up fuel control circuitry required should the ECM unit become damaged or faulty. This back-up fuel circuitry is contained in a chip similar to the PROM, referred to as the CAL-PAK, and is also contained within the MEM-CAL assembly. The CAL-PAK is programmed to allow fuel delivery in the event of a PROM or ECM malfunction. This back-up fuel control capability is commonly referred to as the Limp-Mode.

Our information shows that, when installing TPI systems in Hot Rods, Jaguars, Four Wheel Drive and other non-standard swaps, the most practical place to mount the ECM is underneath the passengers seat. In this location it simplifies the initial wiring procedure and, later on, allows easy access for MEM-CAL or ECM replacement, if necessary, or other diagnostic and trouble-shooting purposes. Depending on the particular vehicle into which you are installing the TPI system, this location may or may not work for your application.

ALDL Connector

The Assembly Line Diagnostic Link (ALDL, also known as the ALCL or Assembly Line Communications Link) has terminals which are used in the assembly plant to check that the engine is operating properly before it is shipped. This connector also allows a technician to troubleshoot the system by attaching what is known as a SCAN tool to the ALDL connector in order to access all important information relative to the operation of the various control circuits within the ECM. These SCAN tools are available from most tool retailers such as Snap-On, MAC, etc. Prices start around $US400. Most of these units can be described as hand-held in terms of size. One of the more common units, marketed by the OTC Division of the Sealed Power Corporation and sold under the Monitor 2000 trade name, measures 33/4" (9.525cm) x 71/2" (19.05cm), is 11/2" (3.86cm) thick, and weighs only 1 lb 2 oz (0.96Kg). In addition to having Light Emitting Diode (LED) or Liquid Crystal Display (LCD) screens, which display real time information while the engine is running, most SCAN tools can be attached to printers for recording data and can even be interfaced (with special software) to your home Personal Computer.

There is, in fact, a recently released software package that comes equipped with the proper cables to allow direct connection between the ALDL connector and your PC. It takes the place of the SCAN tool itself, and uses the PC to display the information that would normally be displayed in the readouts of the SCAN tool. Because of its portability, a lap-top computer is recommended with this combination. However, it can be used with a standard desk-top PC. Through the use of specific plug-in modules, these SCAN tools can be adapted to almost any year or model vehicle which uses an ECM controlled engine. In the GM applications, the ALDL connector is usually installed underneath the dash where it can be easily accessed by a technician. We have found it simpler, and more expedient to install it near the ECM underneath the passengers seat, as this puts it out of view while still keeping it accessible for diagnostic purposes, in addition to simplifying the re-wiring process.

Mass Air Flow Sensor

The MAF sensor measures the amount of air passing through the engine. Chevrolets Tuned Port Injection system uses a hot wire type MAF sensor that determines air flow by measuring the current required to maintain a heated wire at a constant temperature as intake air passes over the wire. The MAF sensor is the chief compensating feature in Chevrolets TPI. This unit must be mounted between the air cleaner assembly and the TPI throttle body.

The MAF sensor is the one piece of the TPI system that seems to give the most problems, as far as positioning and mounting, in almost all non-factory applications. Interviews with Engineers from Hot Rod Circles have informed us that by using 85-89 Pontiac Firebird air intake components, ie, air cleaner, air box, and ducting, we can end up with a "factory" look for this usually troublesome area of TPI installations. While this procedure adds some additional work, it results in a professional looking installation.

Obviously, the particular clearance problems dictated by the specific installation you are working with will determine the location of the MAF sensor and its attendant ducting. The farthest we have seen the factory mount the MAF from the throttle body is approximately 15 inches (38cm). We have always tried to maintain this as a maximum distance when repositioning the MAF sensor. Placing the MAF sensor farther away than this may create some driveability problems!

The MAF is also the sensor that is the most likely part of the entire system to be missing or damaged. Be sure to obtain one in your package deal when purchasing either a TPI unit or a complete engine with a TPI installed, as they are in excess of $US300 when purchased new from a GM dealer. Also, be sure that the MAF sensor has a 5-pin connector. We have anecdotal evidence of people attending numerous swap meets encountering quite a few unscrupulous vendors trying to sell "complete" TPI units with a V-6 3-pin type MAF sensor. They are very close in size and appearance to the V-8 MAF sensor unit, but will absolutely not work with the V-8 system! In addition to the 5pin connector, the correct MAF sensor has I.D. cast into it, indicating that it is manufactured by BOSCH of Germany, whereas the 3-pin 6-cylinder type unit has I.D. indicating it is manufactured by the A.C. Delco Division of GM.

Oxygen Sensor

The Exhaust Oxygen Sensor (EOS) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. It does this by measuring the amount of oxygen molecules in the exhaust.

This sensor allows the system to function in the feedback or closed loop mode. The oxygen sensor improves driveability by providing input to the ECM, causing the ECM to adjust the fuel injector pulse-width settings to produce an ideal 14.7:1 air/fuel ratio. When the oxygen sensor is removed, or disabled, the TPI system operates in its open loop mode at pre-programmed settings.

In the open loop mode the ECM will compensate for changes in engine and air temperature, as determined by its preset calibrations, but cannot fine-tune the fuel mixture. In way of reiterating the need for soldered connections and care in eliminating corrosion from the various connections in the TPI system, the total functional variation that the oxygen sensor circuit measures is from about 0.1 volt for a lean condition, to about 1.0 volt for a rich condition, thus spanning a total voltage variation of approximately nine-tenths of one volt!!! In the factory application the oxygen sensor is mounted in the exhaust manifold. If your level of fabrication experience is such, it is possible to remove the oxygen sensor boss from the late model factory manifold and install it in the earlier Corvette-style Rams-Horn type exhaust manifolds that work on many Street Rod applications. It is also possible to obtain an after-market weld-in type of boss that can be installed in the exhaust down-pipe, just below the exhaust manifold, or header.

Should you decide to fabricate your own mounting boss, the threads on the oxygen sensor are metric, 18mm / 1.50 to be exact, and the area around the threaded hole must be flat, since the sensor uses a sealing washer. It is important that, if the oxygen sensor cannot be mounted in the exhaust manifold, it must be mounted in the down-pipe as close as possible to the manifold since, for the system to go closed loop, the oxygen sensor must reach an operating temperature of 600F.

Fuel Injector Assembly

The system uses 8 primary injectors wired and fused in 2 groups of 4. One group is for cylinders 1, 3, 5, and 7, and the other for 2, 4, 6, and 8, referred to as odd and even injectors. GM wires and fuses four of these injectors together, and when we install the system, we duplicate the fusing on the outside chance that, should one injector short out and blow the fuse for that bank, the injectors on the other bank will continue to function, allowing the engine to continue running.

Commenting on the reliability of these units: we have never heard of anyone experiencing an injector failure, nor have we found any GM technicians who have. The biggest problem when these injection systems were first introduced, was clogged injector nozzles. This has been just about solved by fuel treatments such as those manufactured by Pro-Ma Systems, which introduces not only a very efficient detergent into the fuel system, but a top end lubricant to counter the drying effects of unleaded petrol and a surfactant to break down surface tension of water inherent in all fuel tanks, and allow the water to pass harmlessly through the fuel system, and into the combustion chamber.

Injectors are manufactured by Rochester, MULTEC, Lucas, and Bosch and have different fuel flow rates between the 305 cu. in. and 350 cu. in. engines, and different rates between injectors provided by one manufacturer or another for the same application. According to GM, the injectors should be retained with their specific engines and not interchanged. While we agree that the injectors should be matched to their specific engine, if at all possible, we have found that if they are interchanged in sets, ie, 305 cu. in. injectors installed in 350 cu. in. engines, they will work quite adequately, due to the adjustability engineered into the MAF controlled TPI systems. If there is any noticeable difference, it will only appear under full throttle maximum output conditions. In the real world, if engine rpm is kept below 4500, we have not seen any noticeable difference in their performance or mileage by using 305 cu. in. injectors in 350 cu. in. or even larger displacement engines. We would caution, however, that the injectors only be switched in sets, ie, do not mix 305 cu. in. and 350 cu. in. injectors in the same engine, since the system has no way of determining fuel delivery rates for individual cylinders.

Cold Start Injector

This 9th injector is located between cylinders #3 and #5, on the left side of the intake manifold, on 85 through 88 units. The 1989 and later injector systems are calibrated differently to compensate for the lack of a cold start injector. Para 31. page 10 refers. The cold start injector enriches the fuel mixture for up to 8 seconds during cold cranking. It is wired directly into the starter solenoid circuit, is controlled by the Thermal Time Sensor, and operates ONLY when the starter is cranking the engine over.

Thermal Time Sensor

This temperature sensitive sensor activates the cold start injector when cranking a cold engine, but is not used in 89 systems. The time period during which it allows the cold start injector to inject additional fuel into the engine is based, inversely, on the temperature of the engine below 95F, ie, above 95F there is no additional fuel injected by the cold start injector. As the temperature drops, the time period during which the cold start injector is engaged (to inject extra fuel) is increased to a maximum of 8 seconds at -5F. This sensor is threaded into the front of the intake manifold, in the area below the thermostat housing. It uses a 2-wire connector with a spring clip very similar to the type used on the fuel injectors.

Throttle Body

The Throttle Body attaches the Cruise Control, Accelerator and Transmission Throttle Valve Cables, and connects the Mass Air Flow Sesor via the Air Intake Duct. The Throttle Body also houses the Throttle Position Sensor and Idle Air Control valve, described in the following paragraphs.

Throttle Position Sensor

The Throttle Position Sensor (TPS) informs the ECM if the throttle blades are closed, or whether the throttle blades are open and, if open, how far they are open.

It also sends information on the rate of change in the throttle opening. This unit is mounted on the right side of the throttle body assembly, with 2 TORX head machine screws, has a 3-wire connector and is actuated by a tang on the throttle shaft that engages a lever on the TPS. Basically, the TPS is a potentiometer which informs the ECM with a voltage reading proportional to throttle opening, from 0.54 volts, at idle, to approximately 5 volts at full throttle. This is the only sensor in the TPI system that can be manually adjusted. It should be adjusted with the throttle plates closed, using a digital voltmeter or a SCAN tool for a reading of 0.54 0.08 volts. (See "Final Adjustments", Para on Page .)

Idle Air Control

The Idle Air Control (IAC) device uses a small stepper motor which operates an adjustable tapered valve. The valve maintains engine idle speed at closed throttle by controlling the amount of air allowed to bypass the closed throttle valves in the twin bores of the throttle body.

The engines idle speed is determined by the calibration in the MEM-CAL, and is not mechanically adjustable beyond setting, what is referred to as base idle, which in most stock applications is approximately 500 rpm. This base idle is set by removing the 4-wire connector from the IAC unit, to eliminate ECM control, and adjusting the throttle plate stop screw, (refer Figure 18 Page 20) which is on the top left side of the throttle body assembly. The IAC unit is on the right side of the throttle body below, and slightly forward of, the TPS sensor. (See "Final Adjustments" on Page .)

Coolant Temperature Sensor

This sensor, which is mounted alongside the Thermal Time Sensor (Para 55.), at the front of the intake manifold (below the thermostat housing), is a thermistor which sends engine temperature information to the ECM which enriches or leans the fuel mixture, as required, by predetermined calibrations stored in the ECM. It utilizes a 2-wire connector.

Handle the coolant temperature sensor with care. Damage to this sensor will adversely effect the operation of the entire fuel injection system.

Manifold Air Temperature Sensor

The Manifold Air Temperature (MAT) sensor is located in a threaded boss on the underside of the TPI plenum. Identical (same GM part #) to the Coolant Temperature Sensor (CTS, Figure 21 ), this thermistor sends information to the ECM on changes in inlet air temperature for which the ECM delays EGR until manifold temperature reaches 40 F enriches, or leans, the fuel mixture according to predetermined calibrations stored in the ECM. It also uses a 2-wire connector similar to the one used on the Coolant Temperature Sensor.

Electronic Spark Timing

All spark timing is controlled by the ECM. Electronic Spark Timing is the control of ignition advance by the ECM, and comprises the following components;


The distributors used with TPI injector units are very similar to the HEl-type distributors used on electronically controlled throttle-body injected, or carburetted V-8 engines, in that they have no internal advance mechanisms or vacuum advance control canisters.

The distributor supplies reference signals to the ECM for spark timing and information on engine rpm. The 4-pin connector plug, with which these distributors are connected to the basic wiring harness, while similar in appearance, is different enough to prevent a non-TPI distributor from plugging into a TPI harness without changing the plug end. However, the color codes are the same and the non-TPI distributors will work with the TPI system.

There may be some minor internal electronic differences between the two, but, in the real world, we have found no problem using the non-TPI units in conjunction with the TPI systems.

1975 through '80 HEI distributors with an external vacuum advance unit WILL NOT WORK in conjunction with a TPI system.

This is because the ECM requires a specific rpm input signal from the distributor. While all Corvettes use the large integral coil type HEI distributor, 1987 and later Camaro/Firebird units use a new type of distributor quite similar in size to the earlier point and condenser-type distributors that GM used prior to 1974. While smaller than the previous HEI-type distributors, it is still considered a HEI distributor, the main difference from the earlier HEI distributors being the use of an externally mounted coil. In most cases, you will use the distributor that comes with your TPI unit. We prefer, whenever possible, to use the Corvette style, larger HEI with the integral coil simply because it cleans up the installation and eliminates the coil, coil mounting bracket, and extra wiring.

Any distributor used on the '87 and later roller lifter-type camshaft equipped engines must Use a distributor gear designed to be compatible with the billet steel camshaft used in these engines.

GM #10456413 is the correct gear for use on the earlier HEI large integral coil-type distributor. GM #10495062 is the correct gear to use on the later style, smaller HEI distributor that use a separate remotely mounted coil. If the distributor drive gear is not changed, the steel cam will chew up the stock iron distributor gear normally used on the earlier pre-roller cam distributors!

Knock Sensor

The knock sensor, located on the lower right side of the block, detects vibrations that are the acoustic signature of detonation and informs the Electronic Spark Control module (following) which, in turn, directs the ECM to retard the timing in an attempt to eliminate the detonation.

The combined effect of these two units can retard the ignition timing up to 20 degrees to compensate for bad fuel, high engine temperature, or any other combination of factors that produce detonation. This unit is normally mounted on the right side of the engine, in the coolant drain hole, just above the oil pan rail in front of the starter motor.

Electronic Spark Control (ESC) Module

The Electronic Spark Control (ESC) has the capability of retarding the spark timing by up to 20 degrees when the knock sensor detects detonation.

This unit, which is slightly smaller and thinner than a pack of cigarettes and has a 5-pin wiring connector, is mounted by two small bolts, in most GM applications, in a group with the 3 relays that are used for the fuel pump and the MAF power and burn-off circuitry, on the cowl, on the right side of the evaporator blower or inner fender. We recommend mounting this unit along with the ECM, (eg underneath the passenger seat) as it again simplifies the wiring process and eliminates clutter in the under hood area.

Electronic Spark Timing operation

To optimise engine performance, fuel economy and emissions, the ECM controls distributor spark advance (ignition timing) with the EST system.

The ECM receives a reference pulse from the distributor, indicating engine RPM and crankshaft position. The ECM then sets the correct spark advance by way of the EST reference pulse to the distributor.

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