Have you seen “SYS” intermittently showing up in your 1990-1996 Corvette’s speedometer display?
This means there is a trouble code stored in one of the on-board control modules. When the 1990 Corvette debuted, GM introduced ECM (Engine Control Module) and CCM (Central Control Module) trouble codes.
In addition, ABS/ASR (Anti-lock Brake System/Acceleration Slip Regulation) and EBTCM (Electronic Brake Traction Control Module) trouble codes and data were introduced on the 1992–1996 Corvettes. Although limited ECM data is available, it can point you in the right direction.
For example, the CCM interaction is invaluable when diagnosing interior component or courtesy light problems and the VATS (Vehicle Anti-theft System).
The ALDL (Assembly Line Diagnostic Link) is used to access the codes with a jumper wire. The jumper wire is placed in the proper connector slots and the interrogation begins. Interrogation can be a daunting task.
1990–1993 Corvettes use the first generation ALDL connector. It houses a fuel pump override lead that has battery voltage.
This connector position can harm other ALDL circuits if battery voltage is inadvertently jumped to a control circuit.
1994–1996 Corvettes with the latter designated OBDII (On-board Diagnostics) connector use battery voltage for scanner power, so it also has the potential for possible circuit damage. There is no real concern if you are careful during the installation of the jumper wire.
There is no way to damage any component during the interrogation procedures; just turn the ignition key off and start over.
CCM INTERROGATION PROCEDURES
1990-1993 Corvette ECM and CCM codes can be accessed, placing a jumper wire in ALDL positions A and G.
Position A is in the top right corner; position G is the first position at the lower left of the ALDL.
1994-1996 Corvette ECM, CCM and ASR/EBTCM codes are accessed, placing the jumper wire in ALDL connector positions 4 and 12.
GM used numerals in place of alphas on all 1994 and up Corvettes at the ALDL.
The ALDL numerals start with 1 in the upper left-hand position and 9 in the lower left position.
Install one end of the jumper wire 4 positions from the upper right. Place the other end of your jumper wire in the position directly below number 4.
The ALDL connector is located above your right knee when seated in the driver seat.
The CCM will display all module codes (sequentially) in the speedometer read-out area.
Module 1 CCM (central control module) is first, followed by module 4 ECM (engine control module) and finishing with module 9 ABS/ASR EBTCM (anti-lock brake system/acceleration slip regulation/electronic brake traction control module) codes.
1990-1991 Corvette CCMs will not display any ABS/ASR or EBTCM codes.
Module numbers 1, 4 or 9 will display directly below the speedo readout area.
The codes themselves will be displayed in place of the speedo readout with a prefix: H for History or C for Current, followed by the code.
Once all module codes are displayed, “—” displays, ending the sequence.
If any codes are missed, turn the ignition off and then back to the on position to restart the code display. There is no limit to this procedure.
With LT5 engines, the automatic or manual sequence will not display the ECM codes. A scanner must be used. All other codes will be displayed.
Example: H72 code. This may have been caused from someone driving the car with the radio out for service.
The code set and was not cleared after repairs were done. If the problem causing the code is fixed, the code must be cleared.
The CCM will not display “SYS” if the problem is repaired. However, the code will remain in history until cleared.
It’s always best to record all the codes, clear them, and run the engine or operate the system causing the code to see if any codes reoccur.
Once the automatic code read-out sequence is complete, a manual mode is entered. When a manual mode is entered, the speedometer read-out area will be blank. The trip monitor area will display “1.0”, indicating module 1 is ready for manual control using the DIC (Driver Information Center) buttons.
Push the “TRIP/ODO” button a defined number of times to select the system you would like to interrogate:
Once and “1.1” will be displayed. Fault codes could then be displayed.
Twice 1.2 to display 1.2 (CCM data).
Three times to display 1.3 (CCM inputs/status). This is an interactive display to verify that the CCM is receiving inputs from switches, etc.
Four times to display 1.4. This is also interactive, as it allows you to actuate some of the CCM control circuits.
CCM data, inputs and status will be displayed in the odometer readout area.
Once you find the mode you want to display, press “ENG/MET” to advance to that particular function.
Press “FUEL INFO” to go backwards.
Example: to determine if the door pin switches are working properly, start by pushing the “TRIP/ODO” button until reaching mode 1.3.
Then, push the “ENG/MET” button until reaching 04 in the 1.3 mode list. If the door is open, “1” should display in the odometer readout area.
When the door is closed, “0” should then be displayed next to the “1”. Each time the door is opened and closed, the respective numeral will display. “1 0 1” will display if the door was closed, opened and closed again.
TIP: There is no danger of damaging any circuits during testing. Turn the ignition off and restart the automatic sequence to enter manual mode.
