1991-1992 MARK VII 1991-1993 CONTINENTAL, COUGAR, GRAND MARQUIS, SABLE, TOPAZ, TOWN CAR, TRACER 1993 MARK VIII
1991-1993 AEROSTAR, BRONCO, ECONOLINE, EXPLORER, F SUPER DUTY, F-150-350 SERIES, RANGER
This TSB article is being republished in its entirety to include the Taurus and Sable models.
Occasionally, there are reports of the Malfunction Indicator Lamp (MIL) "Check Engine" or "Service Engine Soon" lamp being lit with no Self-Test codes in Continuous Memory. An explanation of three digit EEC IV Self-Test Codes has been developed along with reasons for the MIL lamp being lit with no accompanying Continuous Memory Self-Test codes.
Refer to the following explanation of three digit EEC IV Self Test Codes to determine why the MIL lamp is sometimes lit with no accompanying Continuous Memory Self-Test codes.
OVERVIEW OF THREE DIGIT EEC IV SELF-TEST CODES
Ford went from two digit to three digit EEC IV Self-Test codes in 1991 to service the increasing number of service codes required to support various government On-Board Diagnostic (OBD) regulations. The phase-in from two digit to three digit codes started in the 1991 model year and is largely complete except for some medium/heavy trucks that will retain two digit codes through the 1994 model year.
MIL LAMP ACTIVATION
Following is a list of reasons why a technician may see the MIL lamp lit with no accompanying Continuous Memory Self-Test codes.
1) Technician Not Familiar With Self-Test Code Output
There are two types of EEC Self-Tests, Key On Engine Off (KOEO) and Key On Engine Running (KOER). While both of these will test for various "hard faults" that are present when the test is run, the processor continuously monitors various operating parameters whenever the engine is running. If the processor detects a problem, it will store a "Continuous Memory" code and light the MIL. These Continuous Memory codes are put out during KOEO Self-Test after any codes associated with hard faults are output.
Self-Test Codes are displayed by flashing the MIL. They are also output as voltage pulses on the Self-Test Output (STO) circuit in the Self-Test connector. In either Self-Test mode, all codes are output twice and in KOEO, the hard fault codes are separated from the Continuous Memory codes by a "separator" pulse.
A technician that is unfamiliar with the EEC Self-Test can mistakenly believe that continuous Memory codes are not present when they really are. He may run KOER Self-Test and get a pass code (lll) and not realize that KOEO Self-Test must be run to receive any Continuous Memory codes. He may run KOEO Self-Test while counting MIL flashes and misinterpret the repeated hard fault pass code (lll) to mean that 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 STI before KOEO Self-Test is complete and all KOEO and Continuous Memory codes have been displayed. It is possible to inadvertently erase Continuous Memory codes by ungrounding STI without realizing that KOEO Self-Test is not complete or the processor has not finished displaying all the codes.
The EEC Self-Test codes are not only used by service technicians, they are used as a final system test in the assembly plants. To make this test as efficient as possible, Self-Test codes are output as a very fast, short pulsewidth signal before the codes are displayed by the flashing MIL. These "FAST" codes can only be interpreted by end-of-line equipment or code-reading testers like Ford's Self-Test Automatic Readout (STAR) testers.
The EEC IV processor puts out both 2-digit and 3-digit Self-Test codes in both formats, "FAST" pulsewidth mode and "SLOW" pulsewidth mode. While all "STAR" type testers display 2-digit codes, the original STAR tester cannot display 3-digit service codes. If the STAR tester 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. The SUPER STAR II tester will only display 3-digit service codes in "FAST" code mode. If slow code 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-Test, refer to the "EEC IV Quick Test Procedures and Appendix" section of the Powertrain Control/Emissions Diagnosis Service Manual.
Since certain STAR testers are capable of reading and displaying fast codes before the slow codes are finished being output on the MIL, a technician can assume that since he sees codes displayed, he can unground STI and move on. If he ungrounds STI before all slow codes are output, Continuous Memory will be erased and could put out a pass code (ll/lll) the next time KOEO Self-Test is run. The technician may also realize that his tester is in "SLOW" mode after he has initiated the KOEO test and stop the test to change tester settings. Another possibility is that another person, a vehicle owner or another technician, could have erased the codes before the technician reporting the situation has run Self-Test. In any of these 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 80 warm-up cycles, depending on the vehicle. A warm-up cycle occurs when the vehicle is started with the coolant temperature below 120° F (49° C) and then shutdown with the coolant temperature above 150° F (66° C). If a vehicle is brought in for service with a MIL complaint and the vehicle is driven or otherwise allowed to warm-up before Self-Test is run, the code may be cleared before the technician tests it.
4) Grounded STO/MIL Circuit
The processor controls the MIL by grounding the STO/MIL circuit (Pin 17). If this circuit shorts to ground, whether the processor is controlling it or not, the MIL will be lit. Starting in 1991, if the processor has lit the MIL, it will hold it on for a minimum of 10 seconds. If the MIL flashes quickly, the concern is probably the STO/MIL circuit shorting intermittently to ground.
5) Engine Running In 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 cannot be initiated. However, Continuous 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 Continuous codes are set.
8) Processor 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 adaptive parameter tables that allow the processor to adapt to different operating requirements. It also contains the Continuous Memory codes. Continuous Memory codes will be erased any time KAPWR is disconnected (i.e. battery disconnected, processor disconnected, breakout box installed, open in the wire, etc.). If KAM fails within the processor, all Continuous codes will also be erased.
9) Damaged STAR Tester
A damaged STAR tester can produce erroneous code output or accidentally erase Continuous Memory.
10) KOEO Processor 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 no codes are output.
11) Intermittent VSS Fault Detected In Wiggle Mode
If in wiggle mode (STI grounded) and an intermittent Vehicle Speed Sensor (VSS) fault is detected, the MIL can 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) IDM Pulsewidth Not Recognized By Processor (EDIS Vehicles)
EDIS vehicles can have the MIL on with no Continuous codes if the processor does not recognize the Ignition Diagnostic Monitor (IDM) pulsewidth. 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) Intermittent Ignition System Fault
Vehicles with a Cylinder Identification (CID) sensor can light the MIL with no Continuous codes present if an intermittent ignition system fault is present long enough to activate the MIL and then goes away. The CID sensor can indicate that the fault was momentary and clear the coil pack faults but the CID fault may not register if the fault goes away fast enough.
14) Intermittent Open STI Circuit
If the Self-Test Input (STI) circuit opened during KOEO Self-Test or code output, Continuous Memory would be cleared.
15) Power Lost To EEC Processor
On some applications, the processor can lose power while the MIL stays powered. The MIL 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 lamps like the amber Air Bag lamp if they are located near each other in the dash panel.
17) Development Testing Or Wrong Processor Released To Production
The MIL can be lit without Continuous codes during testing or if the wrong processor is installed.