GM Corvette 1984-1996 Repair Guide

Diagnosis and Testing



The ABS contains sophisticated onboard diagnostics that, when accessed with a Tech 1® or equivalent "Scan" tool, are designed to identify the source of any system fault as specifically as possible, including whether or not the fault is intermittent. There are over 50 diagnostic fault codes to assist the service technician with diagnosis. The last diagnostic fault code to occur is specifically identified, and specific ABS data is stored at the time of this fault, also, the first five codes set. Additionally, using the scan tool, each input and output can be monitored, thus enabling fault confirmation and repair verification. Manual control of components and automated functional tests are also available when using a scan tool. Details of many of these functions are contained in the following sections.


Enhanced Diagnostic Information, found in the CODE HISTORY function of the scan tool is designed to provide the service technician with specific fault occurrence information. For each of the first five (5) and the very last diagnostic fault codes stored, data is stored to identify the specific fault code number, the number of failure occurrences, and the number of drive cycles since the failure first and last occurred (a drive cycle occurs when the ignition is turned ON and the vehicle is driven faster than 10 mph). However, if a fault is present, the drive cycle counter will increment by turning the ignition ON and LOCK . These first five (5) diagnostic fault codes are also stored in the order of occurrence. The order in which the first 5 faults occurred can be useful in determining if a previous fault is linked to the most recent faults, such as an intermittent wheel speed sensor which later becomes completely open.

During difficult diagnosis situations, this information can be used to identify fault occurrence trends. Does the fault occur more frequently now than it did during the last time when it only failed 1 out of 35 drive cycles- Did the fault only occur once over a large number of drive cycles, indicating unusual condition present when the fault occurred- Does the fault occur infrequently over a large number of drive cycles, indicating special diagnosis techniques may be required to identify the source of the fault-

If a fault occurred 1 out of 20 drive cycles, the fault is intermittent and has not reoccurred for 19 drive cycles. This fault may be difficult or impossible to duplicate and may have been caused by a severe vehicle impact (large pot hole, speed bump at high speed, etc.) that momentarily opened an electrical connector or caused unusual vehicle suspension movement. Problem resolution is unlikely, and the problem may never reoccur. If the fault occurred 3 out of 15 drive cycles, the odds of finding the cause are still not good, but you know how often it occurs and you can determine whether or not the fault is becoming more frequent based on an additional or past occurrences if the source of the problem can not or could not be found. If the fault occurred 10 out of 20 drive cycles, the odds of finding the cause are much better, as the fault may be easily reproduced.

By using the additional fault data, you can also determine if a failure is randomly intermittent or if it has not reoccurred for long periods of time due to weather changes or a prior repair. Say a diagnostic fault code occurred 10 of 20 drive cycles but has not reoccurred for 10 drive cycles. This means the failure occurred 10 of 10 drive cycles but has not reoccurred since. A significant environmental change or a repair may have occurred 10 drive cycles ago. A repair may not be necessary if a recent repair can be confirmed. If no repair was made, the service can focus on diagnosis techniques used to locate difficult to recreate problems.


As with most electronic systems, intermittent failures may be difficult to accurately diagnose. The following is a method to try to isolate an intermittent failure especially wheel speed circuitry failures.

If an ABS fault occurs, the ABS warning light indicator will be ON during the ignition cycle in which the fault was detected. If it is an intermittent problem which seems to have corrected itself (ABS warning light OFF), a history trouble code will be stored. Also stored will be the history data of the code at the time the fault occurred. The scan tool must be used to read ABS history data.


Most intermittents are caused by faulty electrical connections or wiring, although occasionally a sticking relay or solenoid can be a problem. Some items to check are:

  1. Poor mating of connector halves, or terminals not fully seated in the connector body (backed out).
  3. Dirt or corrosion on the terminals. The terminals must be clean and free of any foreign material which could impede proper terminal contact.
  5. Damaged connector body, exposing the terminals to moisture and dirt, as well as not maintaining proper terminal orientation with the component or mating connector.
  7. Improperly formed or damaged terminals. All connector terminals in problem circuits should be checked carefully to ensure good contact tension. Use a corresponding mating terminal to check for proper tension. Refer to Checking Terminal Contact in this section for the specific procedure.
  9. The J-35616-A Connector Test Adapter Kit must be used whenever a diagnostic procedure requests checking or probing a terminal. Using the adapter will ensure that no damage to the terminal will occur, as well as giving an idea of whether contact tension is sufficient. If contact tension seems incorrect, refer to Checking Terminal Contact in this section for specifics.
  11. Poor terminal-to-wire connection. Checking this requires removing the terminal from the connector body. Some conditions which fall under this description are poor crimps, poor solder joints, crimping over wire insulation rather than the wire itself, corrosion in the wire-to-terminal contact area, etc.
  13. Wire insulation which is rubbed through, causing an intermittent short as the bare area touches other wiring or parts of the vehicle.
  15. Wiring broken inside the insulation. This condition could cause a continuity check to show a good circuit, but if only 1 or 2 strands of a multi-strand-type wire are intact, resistance could be far too high.

Checking Terminal Contact

When diagnosing an electrical system that uses Metri-Pack 150/280/480/630 series terminals (refer to Terminal Repair Kit J-38125-A instruction guide J-38125-4 for terminal identification), it is important to check terminal contact between a connector and component, or between in-line connectors, before replacing a suspect component.

Frequently, a diagnostic chart leads to a step that reads Check for poor connection. Mating terminals must be inspected to ensure good terminal contact. A poor connection between the male and female terminal at a connector may be the result of contamination or deformation.

Contamination is caused by the connector halves being improperly connected, a missing or damaged connector seal, or damage to the connector itself, exposing the terminals to moisture and dirt. Contamination, usually in underhood or underbody connectors, leads to terminal corrosion, causing an open circuit or an intermittently open circuit.

Deformation is caused by probing the mating side of a connector terminal without the proper adapter, improperly joining the connector halves or repeatedly separating and joining the connector halves. Deformation, usually to the female terminal contact tang, can result in poor terminal contact causing an open or intermittently open circuit.

Follow the procedure below to check terminal contact:

  1. Separate the connector halves. Refer to Terminal Repair Kit J-38125-A instruction guide J-38125-4, if available.
  3. Inspect the connector halves for contamination. Contamination will result in a white or green buildup within the connector body or between terminals, causing high terminal resistance, intermittent contact or an open circuit. An underhood or underbody connector that shows signs of contamination should be replaced in its entirety: terminals, seals, and connector body.
  5. Using an equivalent male terminal from the Terminal Repair Kit J-38125-A, check the retention force of the female terminal in question by inserting and removing the male terminal to the female terminal in the connector body. Good terminal contact will require a certain amount of force to separate the terminals.
  7. Using an equivalent female terminal from the Terminal Repair Kit J-38125-A, compare the retention force of this terminal to the female terminal in question by joining and separating the male terminal to the female terminal in question. If the retention force is significantly different between the two female terminals, replace the female terminal in question, using a terminal from Terminal Repair Kit J-38125-A.


Flash Code Diagnosis
1986-91 Vehicles

See Figures 1 and 2

The flash code diagnosis can only be used on 1986-91 vehicles to read ABS codes except for codes 41, 45, 55, 61, and 61. Those codes can only be read with the use of a Tech 1® or equivalent scan tool, as outlined later in this section.

On these vehicles, you can "flash" the ABS trouble codes through the SERVICE ABS indicator light on the dash panel as follows:

  1. Ground terminal "H" of the ALDL connector, then turn the ignition to the ON position. The flash code diagnostics will stay enabled as long as the terminal is grounded, serial data line communication has not been initialized and the vehicle speed is less than 4 mph.

Click image to see an enlarged view

Fig. Fig. 1: View of the ALDL connector terminal identification-1986-91 vehicles

  1. About 3 seconds after the ALDL terminal "H" is grounded, the EBCM will start flashing the SERVICE ABS indicator if current codes, except 41, 45, 55, 61 and 63 are set. Current codes 41, 45, 55, 61 and 63 can only be read through the ALDL using a Tech 1® or equivalent scan tool. The EBCM will not allow flash code diagnostics is any of these codes are set as current. If these codes are present as history codes only, you can read them with flash code diagnosis.
  3. The flash sequence will begin with code 12 to signal the beginning of the trouble code display. Code 12 will flash 3 times. Each stored code will then be displayed 3 times. After all of the codes are displayed, the sequence will repeat, starting with code 12.

Click image to see an enlarged view

Fig. Fig. 2: ABS Diagnostic Trouble Code list-1986-91 vehicles

Tech 1® Diagnosis

See Figures 3 and 4

On some vehicles, a Bosch adapter is required for the ALDL connector when using the Tech 1® on the Corvette anti-lock brake system.

Some 1986-91 ABS codes, and all 1992-96 diagnostic fault codes can only be read through the use of a Tech 1® or equivalent bi-directional scan tool. There are no provisions for flash code diagnostics. When using the Tech 1®, or equivalent scan tool, for diagnosis, the ABS is disabled by the EBCM while communicating with the scan tool. The SERVICE ABS indicator light will be on, denoting that only normal power assisted braking is possible.

After completing all diagnostic procedures with the scan tool, disconnect the tool and cycle the ignition OFF for at least 10 seconds. This will return the ABS to normal system operation. Refer to Diagnostic Process for more information.

Click image to see an enlarged view

Fig. Fig. 3: ABS Diagnostic Trouble Code list-1992-94 vehicles and 1995 VIN J vehicles

Click image to see an enlarged view

Fig. Fig. 4: ABS Diagnostic Trouble Code list-1995 VIN P vehicles and 1996 vehicles


The trouble codes in EBCM memory are erased in one of three ways:

  1. Diagnostic request line procedure.
  3. Scan tool Clear Codes selection.
  5. Ignition cycle default.

These three methods are detailed below. Be sure to verify proper system operation and absence of codes when clearing procedure is completed.

The EBCM will not permit code clearing until all of the codes have been displayed. Also, codes cannot be cleared by unplugging the EBCM, disconnecting the battery cables, or turning the ignition to LOCK (except on an ignition cycle default).

Diagnostic Request Line Procedure

This method can only be used on 1986-91 vehicles.

To clear the codes using the ALDL diagnostic request line (pin "H"), perform the following:

  1. Turn the ignition to the OFF position.
  3. Attach a jumper to the ALDL connector terminal "H".
  5. Attach a jumper to the ALDL connector terminal "A".
  7. Connect the jumpers to each other.
  9. Turn the ignition switch to the RUN position. This will enable flash code diagnostics.
  11. Disconnect the jumpers for about 1 second, then reconnect for no less than 1 second. Repeat this 4 times with 10 seconds, leaving the jumpers connected after the 4th time.
  13. Check the SERVICE ABS indicator on the dash panel. Only code 12 should be flashed. If not, the trouble codes have not been properly cleared. Begin clearing the codes again at Step 1.
  15. When the codes are cleared, wait at least 15 seconds before turning the ignition OFF .

"Clear Codes" Method

After codes have been viewed completely, scan tool will ask, CLEAR ABS CODES; ANSWER YES. scan tool will then read, HISTORY DATA WILL BE LOST. CLEAR DATA- Answer YES and codes will be cleared.

Ignition Cycle Default

If no diagnostic fault code occurs for 100 drive cycles for 1986-91 vehicles, or 50 cycles for 1992-96 vehicles, (a drive cycle occurs when the ignition is turned ON and the vehicle is driven faster than 10 mph), any existing fault codes are cleared from the EBCM memory.


When servicing the ABS, the following steps should be followed in order. Failure to follow these steps may result in the loss of important diagnostic data and may lead to difficult and time consuming diagnosis procedures.

  1. Using the scan tool, read all current and history Diagnostic Trouble Codes (DTCs). Be certain to note which codes are current diagnostic code failures. DO NOT CLEAR CODES unless directed to do so.
  3. Using a scan tool, read the CODE HISTORY data. Note the diagnostic fault codes stored and their frequency of failure. Specifically note the last failure that occurred and the conditions present when this failure occurred. This "last failure" should be the starting point for diagnosis and repair.
  5. Perform a vehicle preliminary diagnosis inspection. This should include:
    1. Inspection of the compact master cylinder for proper brake fluid level.
    3. Inspection of the ABS hydraulic modulator for any leaks or wiring damage.
    5. Inspection of brake components at all four (4) wheels. Verify no drag exists. Also verify proper brake apply operation.
    7. Inspection for worn or damaged wheel bearings that allow a wheel to "wobble."
    9. Inspection of the wheel speed sensors and their wiring. Verify correct air gap range, solid sensor attachment, undamaged sensor toothed ring, and undamaged wiring, especially at vehicle attachment points.
    11. Verify tires meet legal tread depth requirements.

  7. If no codes are present, or mechanical component failure codes are present, perform the automated modulator test using the scan tool to isolate the cause of the problem. If the failure is intermittent and not reproducible, test drive the vehicle while using the automatic snapshot feature of the scan tool.

Perform normal acceleration, stopping, and turning maneuvers. If this does not reproduce the failure, perform an ABS stop, on a low coefficient surface such as gravel, from approximately 30-50 mph while triggering on any ABS code. If the failure is still not reproducible, use the enhanced diagnostic information found in CODE HISTORY to determine whether or not this failure should be further diagnosed.

  1. Once all system failures have been corrected, clear the ABS codes.

The scan tool, when plugged into the ALDL connector, becomes part of the vehicle's electronic system. The scan tool can also perform the following functions on components linked by the Serial Data Link (SDL):

Display ABS data
Display and clear ABS Diagnostic Trouble Codes (DTCs)
Control ABS components
Perform extensive ABS diagnosis
Provide diagnostic testing for Intermittent ABS conditions.

Each test mode has specific diagnosis capabilities which depend upon various keystrokes. In general, five (5) keys control sequencing: YES, NO, EXIT, UP arrow and DOWN arrow. The FO through F9 keys select operating modes, perform functions within an operating mode, or enter trouble code or model year designations.

In general, most scan tools have six (6) test modes for diagnosing the antilock brake system. The six (6) test modes are as follows:

MODE FO: DATA LIST - In this test mode, the scan tool continuously monitors wheel speed data, brake switch status and other inputs and outputs.
MODE F1: CODE HISTORY - In this mode, fault code history data is displayed. This data includes how many ignition cycles since the fault code occurred, along with other ABS information. The first five (5) and last fault codes set are included in the ABS history data.
MODE F2: TROUBLE CODES - In this test mode, trouble codes stored by the EBCM, both current ignition cycle and history, may be displayed or cleared.
MODE F3: ABS SNAPSHOT - In this test mode, scan tool captures ABS data before and after a fault occurrence or a forced manual trigger.
MODE F4: ABS TESTS - In this test mode, the scan tool performs hydraulic modulator functional tests to assist in problem isolation during troubleshooting. Included here is manual control of the motors which is used prior to bleeding the brake system.