MIL ON, No Self-Test Codes Are Set
Bulletin No.020/92
Issued
7/10/92
Revised
Category
F
Applicable Model/s
1992 Navajo
Subject
3-DIGIT CODE OVERVIEW
DESCRIPTION
There have been reports of the Malfunction Indicator Lamp (MIL)("Check Engine" or "Service Engine Soon") lamp being lit with no self-test codes in Continuous Memory. This bulletin contains a brief overview of the 3-digit codes as well as a list of reasons why the MIL lamp may light with no accompanying Continuous Memory self-test codes stored.
OVERVIEW OF 3-DIGIT SELF-TEST CODES
Mazda introduced 3-digit EEC self-test codes in 1992 to accommodate the increasing number of service codes required to support various government Onboard Diagnostic (OBD) regulations.
REASONS FOR MIL LAMP ACTIVATION
Following is a list of reasons why the MIL lamp may light when no Continuous Memory self-test codes are present.
1. Technician Not Familiar with Self-Test Code Output
If a technician is not familiar with the self-test code output, a detailed description of the self-tests is contained in the Workshop Manual, Section F-16 (EEC Quick Test Procedures and Appendix).
A technician that is unfamiliar with the EEC self-test can mistakenly believe that Continuous Memory codes are not present when they really are. The technician may run Key On Engine Running (KOER) self-test and get a pass code (111) and not realize that the Key On Engine Off (KOEO) self-test must be run to receive any Continuous Memory codes. Also, the technician may run the KOEO self-test while counting MIL flashes and misinterpret the repeated hard fault pass code (111) to mean that the Continuous Memory does not contain any codes.
2. Inadvertent Erasure of Continuous Memory Self-Test Codes
Continuous Memory self-test codes are erased by ungrounding the Self-Test Input (STI) connector before the KOEO self-test is complete and all KOEO and Continuous Memory codes have been displayed.
EEC codes are output in two formats: fast and slow. Fast codes are issued prior to regular service codes. These codes contain the same information as the regular service codes but are transmitted at 100 times the normal rate. These codes can only be interpreted by special equipment at the end of the assembly line or by code reading testers like the Self-Test Automatic Readout (STAR) testers. After fast codes have been output, self-test should not be exited until all the slow codes have been output.
Doing this will erase any Continuous Memory codes and could put out a pass code (11/111) next time the KOEO self-test is run.
NOTE:
Some meters detect these fast codes as a short burst of information (a slight meter deflection).
The EEC processor outputs both 2 and 3-digit self-test codes in both fast and slow formats. While all STAR type testers display 2-digit codes, the original STAR tester cannot display 3-digit service codes. If the original STAR tester is used on 3-digit code applications, the display will be blank but the tester will beep. The beeps can be counted to determine service codes. The SUPER STAR II tester will only display 3-digit service codes in "FAST" mode. If "SLOW" mode is used on 3-digit service code applications, the display will be blank but the tester will beep. The beeps can be counted to determine service codes. For more information on running self-tests, refer to the Navajo Workshop Manual, Section F-16.
The technician may also realize that the tester is in the "SLOW" mode after the KOEO test has been initiated and stop the test to change tester settings. Finally, it is possible that another person, a vehicle owner or other technician, could have erased the codes before the technician reporting the situation has run the self-test.
In any of the aforementioned situations, the vehicle must be driven until the continuous memory codes are reset.
3. The Concern That Set The Continuous Memory Code Is No Longer Present
The EEC processor will erase a Continuous Memory code if the concern that caused it has not been present for 40 or 60 warm-up cycles (the number of cycles depends on the vehicle). A warm-up cycle is defined as a time when the vehicle is started with coolant temperature below 120~F and shut down with the coolant temperature above 150~F. If a vehicle is brought in for service with a MIL complaint and the vehicle is driven or otherwise allowed to warm up before the self-test is run, the code may be cleared before the technician tests the vehicle.
4. Grounded Self-Test Output (STO)/MIL Circuit
The processor controls the MIL, by grounding the Self-Test Output (STO)/MIL circuit (pin 17). If this circuit shorts to ground, whether being controlled by the processor or not, the MIL will be lit. If the MIL is lit by the processor, the light will stay on for a minimum of 10 seconds. If the MIL flashes quickly, the concern is probably the STO/MIL circuit intermittently shorting to ground.
5. Engine Running in Hardware Limited Operation Strategy (HLOS)
The EEC processor will enter Hardware Limited Operation Strategy (HLOS) if it detects a problem that could cause further damage to the system. Under HLOS the processor modifies its operating strategy so that certain functions are disabled but the vehicle can be safely driven in for service. If the vehicle is in HLOS, Continuous Memory codes will not be set and self-test codes cannot be initiated. However, Continuous Memory codes that were set before the processor entered HLOS will be retained.
6. Misinterpretation of MIL Bulb Check
The MIL will light as a bulb check if the key is on and the engine is not running. If the engine is running and stalls or stops for any reason with the key on, the MIL will be lit and no Continuous Memory codes will be set. When the key is first turned on, the MIL will stay lit briefly after the engine is started as part of the bulb check feature.
7. MIL Flashes During Self-Test
The circuit that controls the MIL is also the Self-Test Output (STO) circuit that goes to the Self-Test connector. The MIL will flash during Self-Test as the STO circuit is cycled on and off. This is normal and no condition codes are set.
8. Processor Keep Alive Memory (KAM) is Erased or Fails
The Keep Alive Memory (KAM) within the processor must always have voltage supplied to it. This voltage is supplied by the Keep Alive Power (KAPWR) circuit (Pin 1) that connects directly to the battery. KAM contains the Continuous Memory codes and any time KAPWR is disconnected (e.g. battery disconnected, processor disconnected, breakout box installed, open in wire, etc.), Continuous Memory codes will be erased.
9. Damaged STAR Tester
A damaged STAR tester can produce erroneous code output or accidentally erase Continuous Memory.
10. Key On Engine Off (KOEO) Processor Random Access Memory (RAM) Test Failed
The processor's Random Access Memory (RAM) is tested during KOEO self-test. If the processor's RAM has failed, the MIL will light and codes will be output.
11. Intermittent Vehicle Speed Sensor (VSS) Fault Detected in Wiggle Mode
If an intermittent Vehicle Speed Sensor (VSS) fault is detected while in the "Wiggle Mode" (STI grounded), the MIL may be lit momentarily. If the VSS signal returns to normal, the associated code is erased. In normal operation, the VSS will not light the MIL.
12. Ignition Diagnostic Monitor (IDM) Pulsewidth Not Recognized by Processor
The vehicle can have the MIL lit with no Continuous Memory codes set if the processor does not recognize the Ignition Diagnostic Monitor (IDM) pulse width. In this case, coil pack failure codes may not be set since the fault filters can be erased before they reach the threshold that sets the code.
13. Cylinder Identification (CID) Intermittent Ignition System Fault
The Cylinder Identification (CID) sensor can light the MIL with no Continuous Memory codes present. This occurs when an intermittent ignition system fault is present long enough to activate the MIL, but then goes away. The CID sensor can indicate that the CID fault may not register if the fault goes away fast enough.
14. Intermittently Open Self-Test Input (STI) Circuit
If the Self-Test Input (STI) circuit has an open during the KOEO self-test of code outputs, Continuous Memory will be cleared.
15. Power Loss to EEC Processor
In some cases, the processor can lose power while the MEL stays powered. The MEL can light if a ground path is present through the processor.
16. Other Warning Lamps Mistaken for MIL
The MIL can sometimes be confused with other warning lights that are located near it on the dash.
17. Development Testing of Wrong Processor Released to Production
The MIL can be lit without Continuous Memory codes during development testing, or if the wrong processor is released to production.