GM Prizm/Nova 1985-1993 Repair Guide

Diagnostic System 1989-93

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Diagnostic Information

The diagnostic tree charts and functional checks in this guide are designed to locate a faulty circuit or component through logic based on the process of elimination.

Service Engine Soon Light

This light is on the instrument panel and has the following functions.



It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
 
It displays Codes stored by the ECM which help the technician diagnose system problems.
 

As a bulb and system check, the light will come ON with the key ON and the engine not running. When the engine is started, the light will turn OFF. If the light remains ON, the self-diagnostic system has detected a problem. If the problem goes away, the light will go out in most cases after 10 seconds, but a Code will remain stored in the ECM with the following exceptions:



On 1992 models, any memory of code 51 or 53 is erased as soon as the ignition switch is turned to the OFF position. In order to detect an active code 51 or 53 the ignition switch must be in the ON position (engine not running) with the diagnostic request terminal (TE1) and the diagnosis check connector grounded. If the ignition switch is turned to the OFF position at any time during this procedure, a present code 51 or 53 will be lost.
 
On 1993 models, the Diagnostic Trouble Codes (DTC's) 16, 42, 43 and 51 are not stored in the memory of the Engine Control Module (ECM) or the Powertrain Control Module (PCM), once the problem has gone away.
 

When the light remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, a Diagnostic Circuit Check must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.

Intermittent Service Engine Soon Light

In the case of an intermittent problem, the Service Engine Soon light will light for ten (10) seconds and then will go out. However, the corresponding code will be stored in the memory of the ECM until the battery voltage to the ECM has been removed, with the exceptions noted under the explanation of Service Engine Soon Light above.

When unexpected codes appear during the code reading process, one can assume that these codes were set by an intermittent malfunction and could be helpful in diagnosing the system.

An intermittent code may or may not re-set. If it is an intermittent failure, a Diagnostic Code Chart is not used. A physical inspection of the applicable sub-system most often will resolve the problem.

Reading Codes
1989-91 MODELS

See Figures 1, 2 and 3

To read the codes from the ECM:

  1. With the ignition OFF, use a service (jumper) wire to ground the diagnostic switch in the connector under the hood.
  2.  



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Fig. Fig. 1: Diagnostic monitor connector - 1989-90 VIN code 6 engine



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Fig. Fig. 2: Diagnostic monitor connector - 1990 VIN code 5 engine



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Fig. Fig. 3: Diagnostic monitor connector - 1991

  1. Without touching the accelerator pedal, turn the ignition to ON but DO NOT start the motor.
  2.  
  3. The codes will be displayed through the flashing of the Check Engine light. Count the number of flashes to determine the numerical code. If the system is normal and has no codes stored, the lamp will flash on and off several times rhythmically. If this signal is received, no further codes will be transmitted from the ECM.Stored fault codes will be displayed in numerical order from lowest to highest without regard to which code occurred first. All codes are 2 digit and will be displayed with a 1 second pause between digits. (Example: Code 21 will show 2 flashes, a 1 second pause and then 1 flash.) Each code will be displayed 3 times in a row with a 3 second pause between each code. After any 1 code has been flashed 3 times, the next stored code will be displayed 3 times .
  4.  
  5. After the code(s) have been read and recorded, turn the ignition switch to OFF and disconnect the jumper wire.
  6.  


WARNING
Disconnecting the wire with the ignition ON may cause severe damage to the ECM.

1992 MODELS

See Figure 4

The provision for communicating with the ECM is the diagnostic request terminal (TE1) in the diagnosis check connector. The diagnosis check connector is located in the left rear engine compartment on the strut tower. Codes stored in the ECM's memory can be read by counting the number of flashes of the CHECK ENGINE indicator with the ignition switch in the ON position and the diagnostic request terminal (TE1) grounded. When this terminal is grounded, the ECM enters its diagnostic mode and outputs codes to the CHECK ENGINE indicator.

To ground the diagnostic request terminal, connect a jumper between diagnosis check connector terminals TE1 and E1. At this point, the CHECK ENGINE indicator should either flash ON and OFF continuously or flash Code 51 once and then flash ON and OFF continuously. This would be the following flash sequence: flash, pause, flash, pause, flash, pause, etc.., or flash-flash-flash-flash-flash, pause, flash, long pause, flash, pause, flash, pause, flash, pause,etc.. Code 51 indicates the ECM's diagnostic system, the idle switch, the A/C switch and the park/neutral circuits are all functioning correctly. If Code 51 is not indicated and the CHECK ENGINE indicator is just flashing ON and OFF continuously, a problem may be present in either the idle switch, the A/C switch or the park/neutral circuits, and should be addressed by consulting Code 51's diagnostic chart in this section.

If any code exists indicating a problem, that code will be flashed instead of Code 51. If more than one code is stored in the ECM's memory, the codes will be repeatedly output from the lowest to the highest, with each code being displayed 3 times.



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Fig. Fig. 4: Diagnostic monitor connector - 1992

1993 Models

See Figure 5

The provision for communicating with the Engine Control Module (ECM) and the Powertrain Control Module (PCM) is a set of diagnostic terminals (TE1) and (TE2) in the data link connector (DLC). The DLC is located in the left rear engine compartment on the strut tower. Diagnostic trouble codes (DTC's) stored in the ECM's/PCM's memory can be read by counting the number of flashes of the malfunction indicator lamp (MIL) with the ignition switch in the ON position with diagnostic request terminal TE1 grounded. The ECM/PCM has two different diagnostic modes - normal and test. When TE1 is grounded, the ECM/PCM enters its normal diagnostic mode and emits DTC's that have been detected one time within the last two ignition cycles. When TE2 is grounded, the ECM/PCM enters its test diagnostic mode and lowers the threshold at which DTC's can be stored in the ECM's/PCM's memory. This function is helpful when diagnosing intermittent problems. With TE1 and TE2 grounded, DTCs that cannot be read in the normal diagnostic mode can be read.

To ground diagnostic request terminal TE1, connect a fused jumper between DLC terminals TE1 and E1 and turn the ignition switch to ON. At this point, the MIL should flash ON and OFF continuously. This would be the following flash sequence: flash, pause, flash, pause, flash, pause, etc..If any DTC's exits indicating a problem, that DTC will be flashed. If more than one DTC is stored in the ECM's/PCM's memory, the DTC's will be output from the lowest to the highest. If no DTC's were flashed, the test diagnostic mode must be initiated and performed. In order to initiate the test diagnostic mode the following procedure must be performed:

  1. Remove the fused jumper from terminal TE1.
  2.  
  3. Turn the ignition switch to LOCK.
  4.  
  5. Connect a fused jumper from DLC terminals TE2 to E1.
  6.  
  7. Turn the ignition switch to ON. At this point, the MIL should be flashing ON and OFF rapidly.
  8.  
  9. Start and run engine until normal operating temperature is attained.
  10.  
  11. Test drive the vehicle at the condition at which the original problem occurred.
  12.  
  13. Connect a fused jumper from terminal TE1 to E1 and note any DTC's.
  14.  

At this point, if any DTC's are present, they will be flashed. If no DTC's are present, the trouble does not lie within the ECM/PCM or its wiring.



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Fig. Fig. 5: Diagnostic monitor connector - 1993

Clearing Codes

See Figures 6 through 66

To clear the codes from the memory of the ECM, either to determine if the malfunction will occur again or because repair has been completed, the ECM power feed must be disconnected for at least thirty (30) seconds, 10 seconds on 1993 models. Ohm 1989-92 models power can be disconnected from the ECM by removing the STOP fuse from the fuse block. On 1993 models, remove power from the ECM/PCM by removing the EFI F-HTR fuse, located in fuse and relay box in the engine compartment. The negative battery terminal may be disconnected, but other on-board memory data, such as preset radio tuning, will also be lost.


CAUTION
WARNING: To prevent ECM damage, the key must be OFF when disconnecting or reconnecting ECM power.





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Fig. Fig. 6: Vacuum and wiring diagrams - 1989-90 VIN code 6 engine (49 state and Canada)

 


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Fig. Fig. 7: Vacuum and wiring diagrams - 1989-90 VIN code 6 engine (California)

 


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Fig. Fig. 8: 1989-90 VIN code 6 engine

 


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Fig. Fig. 9: 1989-90 VIN code 6 engine

 


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Fig. Fig. 10: 1990-91 VIN code 5 engine

 


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Fig. Fig. 11: 1990-91 VIN code 5 engine

 


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Fig. Fig. 12: 1990-91 VIN code 5 engine

 


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Fig. Fig. 13: 1990-91 VIN code 5 engine

 


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Fig. Fig. 14: 1990-91 VIN code 5 engine

 


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Fig. Fig. 15: 1990-91 VIN code 5 engine

 


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Fig. Fig. 16: 1990-91 VIN code 5 engine

 


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Fig. Fig. 17: 1991 VIN code 6 engine

 


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Fig. Fig. 18: 1991 VIN code 6 engine

 


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Fig. Fig. 19: 1991 VIN code 6 engine

 


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Fig. Fig. 20: 1991 VIN code 6 engine

 


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Fig. Fig. 21: 1991 VIN code 6 engine

 


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Fig. Fig. 22: 1991 VIN code 6 engine

 


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Fig. Fig. 23: 1992 VIN code 5 engine

 


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Fig. Fig. 24: 1992 VIN code 5 engine

 


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Fig. Fig. 25: 1992 VIN code 5 engine

 


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Fig. Fig. 26: 1992 VIN code 5 engine

 


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Fig. Fig. 27: 1992 VIN code 5 engine

 


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Fig. Fig. 28: 1992 VIN code 5 engine

 


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Fig. Fig. 29: 1992 VIN code 5 engine

 


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Fig. Fig. 30: 1992 VIN code 5 engine

 


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Fig. Fig. 31: 1992 VIN code 6 engine

 


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Fig. Fig. 32: 1992 VIN code 6 engine

 


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Fig. Fig. 33: 1992 VIN code 6 engine

 


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Fig. Fig. 34: 1992 VIN code 6 engine

 


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Fig. Fig. 35: 1992 VIN code 6 engine

 


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Fig. Fig. 36: 1992 VIN code 6 engine

 


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Fig. Fig. 37: 1993 VIN code 6 engine

 


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Fig. Fig. 38: 1993 VIN code 6 engine

 


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Fig. Fig. 39: 1993 VIN code 6 engine

 


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Fig. Fig. 40: 1993 VIN code 6 engine

 


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Fig. Fig. 41: 1993 VIN code 6 engine

 


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Fig. Fig. 42: 1993 VIN code 6 engine

 


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Fig. Fig. 43: 1993 VIN code 6 engine

 


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Fig. Fig. 44: 1993 VIN code 6 engine

 


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Fig. Fig. 45: 1993 VIN code 6 engine

 


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Fig. Fig. 46: 1993 VIN code 6 engine

 


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Fig. Fig. 47: 1993 VIN code 6 engine

 


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Fig. Fig. 48: 1993 VIN code 6 engine

 


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Fig. Fig. 49: 1993 VIN code 6 engine

 


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Fig. Fig. 50: 1993 VIN code 6 engine

 


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Fig. Fig. 51: 1993 VIN code 8 engine

 


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Fig. Fig. 52: 1993 VIN code 8 engine

 


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Fig. Fig. 53: 1993 VIN code 8 engine

 


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Fig. Fig. 54: 1993 VIN code 8 engine

 


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Fig. Fig. 55: 1993 VIN code 8 engine

 


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Fig. Fig. 56: 1993 VIN code 8 engine

 


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Fig. Fig. 57: 1993 VIN code 8 engine

 


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Fig. Fig. 58: 1993 VIN code 8 engine

 


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Fig. Fig. 59: 1993 VIN code 8 engine

 


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Fig. Fig. 60: 1993 VIN code 8 engine

 


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Fig. Fig. 61: 1993 VIN code 8 engine

 


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Fig. Fig. 62: 1993 VIN code 8 engine

 


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Fig. Fig. 63: 1993 VIN code 8 engine

 


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Fig. Fig. 64: 1993 VIN code 8 engine

 


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Fig. Fig. 65: 1993 VIN code 8 engine

 


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Fig. Fig. 66: 1993 VIN code 8 engine

 

 
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