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.

Module 1 CCM
Code Definition
1.1 DISPLAY CCM fault codes
12 On-board diagnostics no codes (this is a good thing: no problem found)
13 DIC switches open or shorted to battery
14 DIC switches shorted to ground
16 Ignition 3 fuse circuit open
21 Horn relay coil shorted to battery or CCM internal open circuit
22 Rear defogger relay coil shorted to battery or CCM internal open circuit
24 Courtesy lamp relays coils shorted to battery or CCM internal open circuit
25 Courtesy lamp relay coil circuit open or shorted to ground
26 LCD blanking control circuit shorted to battery or CCM internal open
27 LCD blanking control circuit open or shorted to ground
31 LCD data circuit shorted to battery or CCM internal open
32 LCD data circuit open or shorted to ground
33 Data clock circuit shorted to battery or CCM internal open circuit
34 Data clock circuit open or shorted to ground
35 Data strobe circuit shorted to battery or CCM internal open circuit
36 Data strobe circuit open or shorted to ground
37 M clock circuit shorted to battery or CCM internal open circuit
38 M clock circuit open or shorted to ground
41 Loss of ECM serial data communications
51 Pass-key invalid key detection
52 Pass-key key detection circuit shorted
53 Pass-key – key detection circuit open or shorted to battery
54 FEDS fuel enable failure
61 Pass-key – key #1 programming resistance out of range
62 Pass-key – key #2 programming resistance out of range
63 Pass-key – key #2 programming resistance low
71 LCD dimming output circuit shorted to battery or CCM internal open circuit
72 LCD dimming output circuit open or shorted to ground
73 LED display dimming output circuit shorted to battery or CCM internal open circuit
74 LED display dimming output circuit open or shorted to ground

1.2 DISPLAY CCM data

Code Definition
01 Fuel level (gallons; tenths)
02 Dimming potentiometer (A/D) counts (see note A)
03 Ambient light sensor (A/D) counts
04 Rear defogger timer (seconds) (see note B)
05 Vehicle speed (MPH)
06 Pass-key (A/D) counts
07 Ignition voltage
08 Switched battery voltage
09 Cluster incandescent lamp dimming PWM (0-100%)
10 Cluster LCD back-light lamps dimming PWM (0-100%)
11 Radio and climate control LCD back-light lamps dimming PWM (0-100%)
12 LED dimming PWM (0-100%)
13 Oil monitor effective revolution (100,000 revolutions per count) (see note C)
14 C CM software version (this display is available on 1992 and up Corvettes)

Note:
1: headlights or parking lights must be on
2: engine must be running
3: LT1 0/200 counts/LT5 0–250 counts
1.3 DISPLAY CCM inputs/status
Input ID Number 0=disabled, English, off, closed or no
01 Pass-Key fuel 1=enabled
02 English/metric status 1=Metric
03 Door key switch alarm info 1=enabled
04 Right door ajar 1=open
05 Left door ajar 1=open
06 Key in ignition 1=yes
07 Hatch ajar 1=open
08 Power door unlock 1=yes
09 Power door lock 1=yes
10 parking lights 1=”on”
11 Rear defogger input 1=”on”
12 Seat belt switch 1=buckled
13 High beam switch input 1=”on”
14 Low oil level switch 1=low oil (this is used on 1992+ Corvettes)
1.4 CYCLE CCM outputs
01 Change oil indicator
02 Check gauges indicator (see note 1)
03 Fasten seatbelt indicator
04 Security lamp indicator
05 High beam indicator
06 Chime 1 (indicated as chime 0 1990-1991 Corvette)
07 Chime 2 (indicated as chime 1 1990-1991 Corvette)
08 LCD blanking control
09 Horn Relay (1990-1991 Corvette) Rear defogger relay (1992-1996 Corvette)
10 Rear defogger relay (1990-1991 Corvette) Courtesy lamp relay (1992-1996 Corvette)
11 Courtesy lamp relay (1990-1991 Corvette) Low oil indicator (1992-1996 Corvette)
12 Not used (1990-1991 Corvette) Starter enable relay (1992-1996 Corvette) (see note 2)
13 Starter enable relay (1990-1991 Corvette) Delayed accessory bus relay (1992-1996 Corvette)
14 Not used (1990-1991 Corvette) Horn relay (1992-1996 Corvette)

Note A: engine must be running to see indicator cycle; it will be on without the engine running
B: the starter enables relay output will be cycled only when the proper Pass-key is in the ignition, or if Pass-key is not programmed.
Clearing the CCM codes can be done after the automatic sequence of codes has occurred with the following instructions:

Press the TRIP RESET button until “1.0” is displayed in the module number area.
Then press the TRIP/ODO button until “1.7” is displayed in the module number area.
Once there, press and hold the ENG/MET button until “—“is displayed. This will clear any CCM codes.
After “—“ is displayed, turn the key off, wait ten seconds, turn the key back on and check for any CCM codes.

CCM Input and Output Testing

Courtesy light problems are a common issue. The CCM manual control can be very helpful in diagnosing the problem. When cycling the courtesy lamps in CCM manual control mode, we can prove that the courtesy lamp relay and all courtesy lamps are functioning properly.

When there is an intermittent courtesy lamp problem, try cycling the control relay repeatedly in an attempt to coerce it into acting up. If you listen carefully, you can hear the relay click as you cycle the courtesy lamps. If the relay clicks, the CCM and wiring to the relay are working.
There is a possibility that the relay is clicking, and the lamps are not coming on. Relays use a set of points to make the high amp power connection.
The set of points can become corroded and prevent relaying the power to the lamps. If the relay does not click, check the relay itself. In most cases, the relay or possibly wiring from the CCM is to blame. Wiring issues usually come up replacing the heater core or sound system installation.

Door switch input is also helpful. Go to CCM inputs 1.3 04 and 05. Try moving the doors when they are closed to simulate bumps during travel. If the switches show that the door is open, the courtesy lights will come on unexpectedly.

TIP: the CCM uses an ambient light sensor input for courtesy light control. When the CCM is used to cycle the relay, the ambient light sensor input is not used. Whether there is enough ambient light available or not, the courtesy lights should cycle on and off during testing.
If all else fails and the courtesy lights will not come on in full daylight, test the ambient light sensor operation. Use a piece of electrical tape and a flashlight.
Place the tape over the light sensor in the DIC button panel’s upper left corner while monitoring CCM 1.2 data at 03.
You should see approximately 240 counts. Shine the flashlight on the sensor, the counts should drop to 0.
We have never seen one of the ambient light sensors fail, but as time goes by, they will.

VATS or Pass-key problems occur in the 1990–1996 Corvettes from time to time.

The CCM has more tolerance for ignition key pellet resistance changes than the 1986–1989 Corvettes, making them more reliable. The ability to access Pass-key trouble codes helps troubleshoot issues more quickly. A good example would be CCM trouble code 52 (VATS key circuit is shorted).

The most logical explanation is that the wiring insulation has been damaged, allowing the wires to touch each other. The steering column may be the culprit.
The wiring could be damaged from either tilting the column or rotating the key cylinder. CCM code 51 (invalid key detection) would point to an ignition key resistor pellet that is dirty or damaged.
CCM codes 61-65 indicate that the wrong resistance key was being used during programming.

We have found that when CCM codes 41 or 54 display, it has always been an ECM (Engine Control Module) failure. The first thought is that the CCM would be the culprit, but to date, we have never seen this happen. The law of averages shows that CCMs have an excellent track record. We have only seen one failed CCM (in a 1995 Corvette) in all the years we have worked with them.

CCM (Central Control Module) Codes 72 and 74 usually occur after the radio control head was replaced. The radio head LED and LCD open circuits are noticed by the CCM, setting the code. There is a simple fix while the radio head is being replaced.
Resistors and one diode must be installed to alleviate the problem. A 1K 1/2 watt resistor is installed in the following manner: one side of the resistor must be powered by the radio head C2 connector in position 8 (pink wire). Connect one end of the resistor to this wire.

This will send the correct voltage signal to the CCM once the other side of the resistor is connected.
At radio head connector C1, locate the gray/black striped wire at position 3. Connect the other end of the resistor here. Be careful: the radio head has two gray/black striped wires in connector C1. The gray/black striped wire at position 4 is the incandescent lamp control.
It will not set a code when left disconnected.
To recap: the resistor will be installed at connector C2 position 8 on one end and connector C1 at position 3 for the other end.

LED dimming circuit

The purple/white striped wire at the radio head connector (C1 position 2) requires a 10 Ohm resistor and diode in series with the pink wire. The same pink wire from connector C2 position 8 is used for power. Connect one side of the 10 Ohm resistor to the pink wire. The diode should be installed in series with the resistor. Diodes are an electrical check valve, allowing current to flow in one direction only.

Diodes have an arrow on their barrel denoting direction of current flow. The arrow should point away from the resistor towards the purple/white striped wire. One side of the diode connects to the resistor.
The side with arrow pointing out goes to the purple/white striped wire. This will prevent “SYS” from flashing while you’re enjoying the new sound system.

Module 4 is ECM

4.0 ECM waiting for interrogation instructions
4.1 display ECM fault codes
ECM Fault Code List
1984-1991 (L98: VIN 8)
Code Definition
12 No Distributor Reference Pulse (this code is displayed only when engine is not running)
13 Oxygen Sensor Open
14 Coolant Sensor High Temp
15 Coolant sensor Low Temp
21 TPS Voltage High
22 TPS Voltage Low
23 MAT Sensor Temp Low
24 Vehicle Speed Sensor
25 MAT Sensor Temp High
32 EGR System Error
33 MAP Sensor Voltage High
34 MAP Sensor Voltage Low
41 Cylinder Select Error
42 EST System Error
43 ESC System Error
44 Oxygen Sensor (System Lean)
45 Oxygen Sensor (System Rich)
46 VATS Error
51 Mem-Cal Error
52 Engine Oil Temp Sensor Low
53 System Voltage Error
54 Fuel Pump Voltage Low
62 Engine Oil Temp Sensor High

ECM Fault Code List 1990-1993(LT5/ZR-1; VIN J)

Code Definition
13 Left Oxygen Sensor Open
14 Coolant sensor High Temp
15 Coolant Sensor Low Temp
16 DIS Fault Line Error
21 TPS Voltage High
22 TPS Voltage Low
23 MAT Sensor Temp Low
24 Vehicle Speed Sensor
25 MAT Sensor Temp High
31 Cam Sensor Missing or Too Many Pulses
33 MAP Sensor Voltage High
34 MAP Sensor Voltage Low
36 DIS Fault Line/Missing or Extra EST Signal
41 Cylinder Select Error
42 EST System Error
43 ESC System Error
44 Oxygen Sensor (Left Bank Lean)
45 Oxygen Sensor (Right Bank Rich)
46 VATS Error
51 Mem-Cal Error
52 Engine Oil Temp Sensor Low
53 System Voltage Error
54 Fuel Pump Voltage Low
55 Fuel Monitor Lean
56 Port Throttle Vacuum Sensor Low or High
61 Secondary Port Throttle System Error
62 Engine Oil Temp Sensor High
63 Right Oxygen Sensor Circuit Open
64 Right Oxygen Sensor (Right Bank Lean)
65 Right Oxygen Sensor (Right Bank Rich)

1992-1996 (LT-1 VIN P)

DTC Codes 72-99 Pertain to 1994-1996 (LT-1 VIN P)
Code Definition
DTC 11 – Malfunction Indicator Lamp (MIL) Circuit
DTC 13 – Bank 1Left Front) Heated Oxygen} Oxygen Sensor (H02S) #1 Circuit (Open Circuit)
DTC 14 – Engine Coolant Temperature (ECT) Sensor Circuit (Signal Voltage Low,
High Temperature Indicated)
DTC 15 – Engine Coolant Temperature (ECT) Sensor Circuit (Signal Voltage High,
Low Temperature Indicated)
DTC 16 – Distributor Ignition System (Low Resolution Pulse)
DTC 18 – Injector Circuit(s)
DTC 21 – Throttle Position (TP) Sensor Circuit (Signal Voltage High)
DTC 22 – Throttle Position (TP) Sensor Circuit (Signal Voltage Low)
DTC 23 – Intake Air Temperature (IAT) Sensor Circuit (Signal Voltage High,
Low Temperature Indicated)
DTC 24 – Vehicle Speed Sensor (VSS) Circuit
DTC 25 – Intake Air Temperature (IAT) Sensor Circuit (Signal Voltage Low,
High Temperature Indicated)
DTC 26 – Evaporative Emission (EVAP) Canister Purge Solenoid Valve Circuit
DTC 27 – Exhaust Gas Recirculation (EGR) Vacuum Control Signal Solenoid Valve Circuit
DTC 28 – Transmission Range (TA) Pressure Switch Assembly Fault
DTC 29 – Secondary Air Injection (AIR) Pump Circuit
DTC 32 – Exhaust Gas Recirculation (EGR)
DTC 33 – Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage High – Low Vacuum)
DTC 34 – Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage Low – High Vacuum)
DTC 36 – Distributor Ignition System (Faulty High Resolution Pulse or Extra Low Resolution Pulse Detected)
DTC 37 – Brake Switch Stuck “ON
DTC 38 – Brake Switch Stuck “OFF”
DTC 41 – Ignition Control (IC) Circuit (Open Circuit)
DTC 42 – Ignition Control (IC) Circuit (Shorted or Grounded Circuit)
DTC 43 – Knock Sensor (KS) Circuit
DTC 44 – Bank 1 (Left Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Lean Exhaust Indicated)
DTC 45 – Bank 1 (Left Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Rich Exhaust Indicated)
DTC 47 – Knock Sensor (KS) Circuit or Module Missing
DTC 48 – Mass Air Flow (MAF) Sensor Circuit
DTC 50 – System Voltage Low
DTC 51 – EEPROM Programming Error
DTC 52 – Engine Oil Temperature Sensor Circuit (Signal Voltage High, Low Temperature Indicated)
DTC 53 – System Voltage High
DTC 55 – Fuel Lean Monitor
DTC 58 – Transmission Fluid Temperature (TFT) Sensor Circuit Low (High Temperature Indicated)
DTC 59 – Transmission Fluid Temperature (TFT) Sensor Circuit High (Low Temperature Indicated)
DTC 62 – Engine Oil Temperature Sensor Circuit (Signal Voltage Low, High Temperature Indicated)
DTC 63 – Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Open Circuit)
DTC 64 – Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Lean Exhaust Indicated)
DTC 65 – Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #t Circuit (Rich Exhaust Indicated)
DTC 66 – A/C Refrigerant Pressure Sensor Circuit (Open or Shorted)
DTC 67 – A/C Refrigerant Pressure Sensor Circuit (Pressure Sensor or A/C Clutch Circuit Problem)
DTC 68 – A/C Relay Circuit (Shorted Circuit)
DTC 69 – A/C Clutch Circuit
DTC 70 – A/C Clutch Relay Driver Circuit
DTC 72 – Vehicle Speed Sensor Loss
DTC 73 – Pressure Control Solenoid (PCS) Circuit (Current Error)
DTC 74 – Traction Control System (TCS) Circuit Low
DTC 75 – Transmission System Voltage Low
DTC 77 – Fan Control PCM Terminal “All”
DTC 78 – Fan Control PCM Terminal “Al0”
DTC 79 – Transmission Fluid Over-temp
DTC 81 – Transmission .2-3 Shift Solenoid Circuit
DTC 82 – Transmission 1-2 Shift Solenoid Circuit
DTC 83 – TCC PWM Solenoid Circuit Fault
DTC 84 – 3-2 Control Solenoid Circuit (Automatic Transmission)
DTC 84 – 2nd and 3rd Gear Block-out Relay Control Circuit (Manual Transmission)
DTC 85 – Transmission TCC Stuck “ON”
DTC 90 – Transmission TCC Solenoid Circuit
DTC 91 – One to Four Up shift Lamp (Manual Transmission)
DTC 97 – VSS Output Circuit
DTC 99 – Tach Output Circuit
DTC P0116 – Engine Coolant Temperature (ECT) Circuit Range/Performance Problem
DTC P0117 – Engine Coolant Temperature (ECT) Circuit Low Input
DTC P0118 – Engine Coolant Temperature (ECT) Circuit High Input
DTC P0131 – Bank 1 HO2S #1 Circuit Low Voltage (Lean
DTC P0132 – Bank 1 HO2S #1 Circuit High Voltage (Rich)
DTC P0133 – Bank 1 HO2S #1 Circuit Slow Response
DTC P0134 – Bank 1 HO2S #1 Circuit No Activity Detected
DTC P0135 – Bank 1 HO2S #1 Heater Circuit
DTC P0151 – Bank 2 HO2S #1 Circuit Low Voltage (Lean)
DTC P0152 – Bank 2 HO2S #1 Circuit High Voltage (Rich)
DTC P0153 – Bank 2 HO2S #1 Circuit Slow Response
DTC P0154 – Bank 2 HO2S #1 Circuit No Activity Detected
DTC P0155 – Bank 2 HO2S #1 Heater Circuit Malfunction
DTC P0158 – Bank 2 HO2S #2 Circuit High Voltage
DTC P0160 – Bank 2 HO2S #2 Circuit No Activity Detected
DTC P0161 – Bank 2 HO2S #2 Heater Circuit Malfunction
DTC P0171 – Bank 1 Fuel Trim System Too Lean
DTC P0172 – Bank 1 Fuel Trim System Too Rich (1 of 2)
DTC P0174 – Bank 2 Fuel Trim System Too Lean
DTC P0175 – Bank 2 Fuel Trim System Too Rich (1 of 2)
DTC PO420 – Bank 2 Catalyst System – Low Oxygen Storage Capacity Indicated
DTC P1114 – Engine Coolant Temperature (ECT) Circuit (intermittent Low Input Voltage)
DTC P1115 – Engine Coolant Temperature (ECT) Circuit (Intermittent High Input Voltage)
DTC P1133 – Bank 1 HO2S #1 Circuit Not Enough Switches
DTC P1153 – Bank 2 HO2S #1 Circuit Not Enough Switches
DTC P1158 – Bank 2 HO2S #2 Shifted Rich

1994-1995 LT5/ZR-1 Trouble codes

Due to federal regulations implementing uniform trouble codes throughout the automotive industry, LT5/ZR-1 Corvettes used many of the same 1994 and up codes, except for the codes listed below. (These trouble codes may be in addition to the possible codes above or different meaning)
DTC 31 Camshaft Position Sensor Circuit
DTC 41 Cylinder Select Error (Faulty or Incorrect PROM)
DTC 54 Fuel Pump Circuit (Low Voltage)
DTC 56 Secondary Air Inlet Valve Actuator Vacuum Sensor Circuit (Signal Voltage High “High Vacuum or Signal Voltage Low” (Low Vacuum)
DTC 61 Secondary Air Inlet Valve System
DTC 66 Engine Power Mode Switch Circuit (Signal Voltage High or Signal Voltage Low)

4.7 clear ECM fault codes
Press the TRIP RESET button until 4.0 is displayed in the module number area. Then press the TRIP/ODO button until 4.7 is displayed in the module number area. Press and hold the ENG/MET button until “—“ is displayed. This will clear any ECM codes.
After “—“ is displayed turn the key off, wait ten seconds turn the key back on and check for any ECM codes.

ABS/ASR EBTCM

Press the TRIP RESET button until 9.0 is displayed in the module number area.
Press the TRIP/ODO button until “9.7” is displayed in the module number area. Press and hold the ENG/MET button until “—“ is displayed.
This will clear any ABS/ASR codes. after “—“ is displayed, turn the key off, wait ten seconds, turn the key back on and check for any ABS/ASR codes.