GM Caprice 1990-1993 Repair Guide

Diagnosis and Testing



Diagnosis of a driveablility and/or emissions problems requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to perform any repairs before performing the preliminary diagnostic steps. In many cases this will shorten diagnostic time and often cure the problem without electronic testing.

The proper troubleshooting procedure for the vehicle is as follows:

Visual/Physical Underhood Inspection

This is possibly the most critical step of diagnosis. A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other component. Wiring should be checked carefully for any sign of strain, burning, crimping, or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit.

Diagnostic Circuit Check

This step is used to check that the on-board diagnostic system is working correctly. A system which is faulty or shorted may not yield correct codes when placed in the Diagnostic Mode. Performing this test confirms that the diagnostic system is not failed and is able to communicate through the dash warning lamp.

If the diagnostic system is not operating correctly, or if a problem exists without the dash warning lamp being lit, refer to the specific vehicle's diagnostic "A-" Charts. These charts cover such conditions as Engine Cranks but Will Not Run or No Service Engine Soon Light.

Reading Codes and Use of Scan Tool

Once the integrity of the system is confirmed, enter the Diagnostic Mode and read any stored codes. To enter the diagnostic mode:

  1. Turn the ignition switch OFF . Locate the Assembly Line Diagnostic Link (ALDL), usually under the instrument panel. It may be within a plastic cover or housing labeled DIAGNOSTIC CONNECTOR. This link is used to communicate with the ECM.
  3. The code(s) stored in memory may be read either through the flashing of the dashboard warning lamp or through the use of a hand-held scan tool. If using the scan tool, connect it correctly to the ALDL.
  5. If reading codes via the dash warning lamp, use a small jumper wire to connect Terminal B of the ALDL to Terminal A. As the ALDL connector is viewed from the front, Terminal A is on the extreme right of the upper row; Terminal B is second from the right on the upper row.
  7. After the terminals are connected, turn the ignition switch to the ON position but do not start the engine. The dash warning lamp should begin to flash Code 12. The code will display as one flash, a pause and two flashes. Code 12 is not a fault code. It is used as a system acknowledgment or handshake code; its presence indicates that the ECM can communicate as requested. Code 12 is used to begin every diagnostic sequence. Some vehicles also use Code 12 after all diagnostic codes have been sent.
  9. After Code 12 has been transmitted 3 times, the fault codes, if any, will each be transmitted 3 times. The codes are stored and transmitted in numeric order from lowest to highest.

The order of codes in the memory does not indicate the order of occurrence.

  1. If there are no codes stored, but a driveability or emissions problem is evident, refer to the Symptoms and Intermittents Chart for the specific fuel system.
  3. If one or more codes are stored, record them. At the end of the procedure, refer to the applicable Diagnostic Code chart.
  5. If no fault codes are transmitted, connect the scan tool (if not already connected). Use the scan functions to view the values being sent to the ECM. Compare the actual values to the typical or normal values for the engine.
  7. Switch the ignition OFF when finished with code retrieval or scan tool readings.

Circuit/Component Diagnosis and Repair

Using the appropriate chart(s) based on the Diagnostic Circuit Check, the fault codes and the scan tool data will lead to diagnosis and checking of a particular circuit or component. It is important to note that the fault code indicates a fault or loss of signal in an ECM-controlled system, not necessarily in the specific component. Detailed procedures to isolate the problem are included in each code chart; these procedures must be followed accurately to insure timely and correct repair. Following the procedure will also insure that only truly faulty components are replaced.


The ECM may be placed into the diagnostic mode by turning the ignition switch from OFF to ON , then grounding ALDL Terminal B to Terminal A. When in the Diagnostic Mode, the ECM will:

Display Code 12, indicating the system is operating correctly.
Display any stored fault codes 3 times in succession.
Energize all the relays controlled by the ECM except the fuel pump relay. This will allow the relays and circuits to be checked in the shop without recreating certain driving conditions.
Move the IAC valve to its fully extended position, closing the idle air passage.

Due to increased battery draw, do not allow the vehicle to remain in the Diagnostic Mode for more than 30 minutes. If longer periods are necessary, connect a battery charger.


If ALDL terminal B is grounded to terminal A with the engine running, the system enters the Field Service Mode. In this mode, the dash warning lamp will indicate whether the system is operating in open loop or closed loop.

If working in open loop, the dash warning lamp will flash rapidly 2 1 / 2 times per second. In closed loop, the flash rate slows to once per second. Additionally, if the system is running lean in closed loop, the lamp will be off most of the cycle. A rich condition in closed loop will cause the lamp to remain lit for most of the 1 second cycle.

When operating in the Field Service Mode, additional codes cannot be stored by the ECM. The closed loop timer is bypassed in this mode.


Stored fault codes may be erased from memory at any time by removing power from the ECM for at least 30 seconds. It may be necessary to clear stored codes during diagnosis to check for any recurrence during a test drive, but the stored codes must be written down when retrieved. The codes may still be required for subsequent troubleshooting. Whenever a repair is complete, the stored codes must be erased and the vehicle test driven to confirm correct operation and repair.

The ignition switch must be OFF any time power is disconnected or restored to the ECM. Severe damage may result if this precaution is not observed.

Depending on the electric distribution of the particular vehicle, power to the ECM may be disconnected by removing the ECM fuse in the fusebox, disconnecting the in-line fuse holder near the positive battery terminal or disconnecting the ECM power lead at the battery terminal. Disconnecting the negative battery cable to clear codes is not recommended as this will also clear other memory data in the vehicle such as radio presets or seat memory.

Refer to the appropriate Diagnostic Code Charts at the end of this Section for all testing and diagnosis of computer controlled components and sensors.