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    1991 GMC Truck Syclone 4.3L TBI Turbo 6cyl

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    Engine Control Computer

    Inspect/Test/Service/Replace

    Always use a wiring diagram when working on computer systems.

    • First, check to see which parts are affected.
    • When several devices are not working, look closer to the power source.
    • When only one or two are not working, look in that part of the circuit for the problem.
    • Be sure to ask yourself if the problem only occurs when hot or cold. Does it always happen or is it intermittent?

    Computers have self-diagnostic ability. The newer the computer, the more accurately it can diagnose problems. A simple problem like a loose or corroded wiring connection can cause problems that the computer tries to correct by compensating with other changes. This can make it seem like the computer is at fault.

    When diagnosing all cars with computers, a logical diagnosis sequence must be followed before checking the computer. You cannot overlook the basics and just start replacing expensive electronic parts until a fix is made. Parts stores will usually not accept the return of electronic parts.

    Be careful to eliminate all of the mechanical conditions first before going on to a computer diagnosis. A large majority of cars will not actually have computer problems but can be fixed during the inspection and maintenance process.

    CAUTION
    If you clean the engine, do not use a steam cleaner. Temperatures of more than 300°F can ruin computers and electronics. High pressure cleaning results in corrosion.

    • Poor electrical connections are the most common cause of problems in cmputer systems.
      • Damaged connections are usually the reason for failure of electronic components.
      • Connector pins on chips can also be damaged or bent during installation.
      • Pin-type connectors should fit tight to a paper clip.
    • Wiring problems include loose or corroded connections and grounded wires. An advantage of onboard diagnostics is that it helps keep the wiring connections from becoming corroded or damaged because wires are not removed. This maintains the integrity of the connectors.
    • The computer must have good power and ground connections.
    • Most computers have two or three power leads.
      • One lead will always have power. This allows the computer to keep information in volatile memory.
      • The second lead (and sometimes a third lead if used) supplies power when the key is on. Fuse links protect these wires.
      • These leads are located near the battery, starter, or starter relay.
    One computer lead always has power (BATT). The other two on this diagram are powered from the relay when the key is on.
    • The number of grounds varies from two to six. They are either power or sensor grounds.
      • Power grounds are used for actuators like motors or solenoids.
      • Sensor grounds return to the computer and then to a ground source.
    • The quality of ground connections is very important too. One of the most prominent causes of ignition module failure is loss of ground.
      • Check ground straps and wires that connect the engine and chassis.
      • The eyelet of a ground wire where it bolts to the cylinder head must be tight and its bolt must not have paint or rust under its head.
    • Examine wires to see that they are properly routed and are all original in regard to length and shape.
    • Radio frequency interference (RFI) is a problem with computer systems. If a computer feed wire is routed near the alternator or secondary ignition wires, signals that the computer receives will be jumbled. The figure below shows a DSO (digital storage oscilloscope) waveform of RFI. This can result in serious driveability problems.
    A DSO pattern showing AC superimposed over DC-causing radio interference. Reproduced with permission of Fluke Corporation.
    • If you find a bare wire wrapped around the outside of wires running from the distributor to the computer, these are shielding wires.
    • The shielding wire is connected to ground and absorbs RFI that might interfere with the rpm reference signal to the computer.

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