EEC-IV TECHNICAL NOTES by Tom C
http://web.archive.org/web/20101201011938/http://fordfuelinjection.com/files/eectch98.pdf
"...The information supplied herein was taken from technical and sales literature, email
archives, news groups, and wherever else it could be found. If it helps you,
great! Please return the favor by sharing what you learn with me and others.)
None of this data is guaranteed accurate! USE AT YOUR OWN RISK! It is a collection
of technical info, opinions and guesses. You can contribute to the effort by letting
me know what you learn so that it can be added to and corrected.
PURPOSE
This data has been collected and compiled to help you decipher the EEC-IV inner
workings. The EEC MCU probably controls one or more vehicles you own, plus it contains
all the components necessary to build an EFI system for any vehicle -- if only
we could program and modify it. That is the purpose -- to uncloak the EEC-IV so
that we can play with what we bought! Much of the empirical data in this document
is specific to the A9L EEC computer. That is the model MotorSport SVO sells for
conversion of Mustangs from SD to MAF, and it was stock on 5-speed manual transmission
Mustangs from 1989 to 1993, so consensus was reached to pursue this one
configuration until it was understood rather than divide our efforts..."
OVERVIEW
The EEC-IV design began in 1978 and was first introduced in 1983 in the 1.6L ******,
Lynx, EXP and LN7 cars. It has gone through several major physical changes, the
earliest using a fairly simple two board design with through hole soldered components
while the last are more current in technology, showing extensive use of
surface mount components and a much more finished and complex appearance. In
between, there appears to be a variety of mother/daughter board and other designs.
Still, they are all called EEC-IV, although somewhere in its life there was a Ford
P/N generational change.
The reader is referred to the SAE paper #820900, noted in the reference section at
the end of this document, for a much more detailed description of the design goals
and operation of the EEC-IV MCU.
One person wrote: "The processor used is the 8065 along with several supporting
peripheral chips like the DUCE chip which can provide up to 8 PWM outputs and the
DARC chip which has 6 channels of timer capture inputs." (Is he talking about the
EEC-V here ?)
The EEC module is rated to 80oC (185oF) continuous, 100oC intermittent, so it will
be much happier and live longer in the passenger compartment. Some of the later generation
15 and 18 MHz Motorola 8061 processors have a bus loading/edge timing
sensitivity that only gets worse at high temperature, so it’s best to keep the EEC
in a more hospitable environment. Additionally, mounting the EEC in the passenger
compartment will give you better access to the J3 test port, which is where you’ll
be plugging in an after-market module or any test / modification device.
The J3 test port on the side of the ECU box is for developers to plug into -- this
is how the after-market chipmakers and others get into the box. The test connector
has the micro-controller’s multiplexed address/data bus signals on it. It also,
very conveniently, has a PROM disable signal. So the chip makers design something
that hangs off that connector, disables the computer’s PROM, and substitutes its own
PROM in its place.
The reader is encouraged to investigate the Intel 8096 (MCS-96) literature for
greater insight into the 8061 processor used in the EEC. One document available
from their web site is "27006102.pdf" entitled "APPLICATION NOTE; AP-248; Using The
8096"; Order Number: 270061-002. There are many other related documents available
from Intel -- including the use of the A-D converter, the implementation of "fuzzy
logic", instrumenting and controlling automotive applications, and other topics.
Also, this author has posted a synopsis of the 8096 pinouts and instruction set which is available at several web sites.."
READ MUCH MORE!
--- miesk5 Note;
Here is some info that can help in troubleshooting EEC IV;
EEC IV Connector Pin Diagram
by Fireguy50 (Ryan M) at http://web.archive.org/web/20131229163930/http://oldfuelinjection.com/images/eec04.gif
EEC IV Connector Pin Outs LEGEND, Bronco & Ford Truck & Van: 4.9, 460, 5.0, 5.8; by Ryan M (Fireguy50) at http://web.archive.org/web/20131029020805/http://oldfuelinjection.com/truckpinouts.html
a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19
http://broncozone.com/topic/14269-code-reader/?pid=74587&mode=threaded
A helper is good to assist in reading Codes best is to take a cell fone vid and replay it.
Some basics;
Visual Check
1.Inspect the air cleaner and inlet ducting.
2.Check all engine vacuum hoses for damage, leaks, cracks, blockage, proper routing, etc.
3.Check EEC system wiring harness for proper connections, bent or broken pins, corrosion, loose wires, proper routing, etc.
4.Check the Powertrain Control Module (PCM), sensors and actuators for physical damage. I see tPS, etc was replaced...
5.Check the engine coolant for proper level and mixture.
6.Check the transmission fluid level and quality. See E4OD Fluid Condition Check Below)
7.Make all necessary repairs before continuing
8. Check headlights
The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears including Reverse.
Make sure A/C is off and transmission is in Park (automatic); or in Neutral for a Manual & release clutch.
Turn off all accessories; radio, lights, A/C, heater, blower, fans, etc. (close driver's door)
Then turn off engine and wait 10 seconds.
Do KOEO test First
Post Code(s) here according to:
KOEO
&
KOER
The Self-Test is divided into three specialized tests: Key On Engine Off Self-Test, Engine Running Self-Test, and Continuous Self-Test. The Self-Test is not a conclusive test by itself, but is used as a part of the functional Quick-Test diagnostic procedure. The PCM stores the Self-Test program in permanent memory. When activated, Self-Test checks the EEC system by testing memory integrity and processing capability, and verifies that various sensors and actuators are connected and operating properly.
The Key On Engine Off and Engine Running Self-Tests are functional tests which only detect faults present at the time of the Self-Test. Continuous Self-Test is performed during normal vehicle operation and stores any fault information in Keep Alive Memory (KAM) for retrieval at a later time.
Key On Engine Off Self-Test
At this time, a test of the EEC system is conducted with power applied and engine at rest.
To detect errors during Key On Engine Off Self-Test, the fault must be present at the time of testing.
Continuous Memory DTCs are issued as a result of information stored during Continuous Self-Test, while the vehicle was in normal operation. These DTCs are displayed only during Key On Engine Off Self-Test and after the separator pulse. Intermittent faults that have not occurred in the last 80 warm-up cycles (40 cycles on some applications) are erased from Continuous Memory and will not produce a Continuous Memory DTC. Note: The separator pulse and Continuous Memory DTCs follow Key On Engine Off DTCs ONLY.
Engine Running Self-Test
At this time, a test of the EEC system is conducted with the engine running. The sensors are checked under actual operating conditions and at normal operating temperatures. The actuators are exercised and checked for expected results.
============
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.
EEC-IV System Has NO Control Over the Following Items; "...Fuel quantity and quality; Damaged or faulty ignition components; Internal Engine Condition - rings, valves, Timing belt, etc.; Starter & Battery Circuit; Dual Hall Sensor; TFI or DIS Module; Distributor condition or function; Camshaft Sensor; Crankshaft Sensor; Ignition or DIS Coil; Engine Governor Module..."
EEC Strategies "...Start / Crank, Cold Start & Warm Up, Cold Drive-Away, Warm Idle, Warm Cruise, Full-Throttle Acceleration, Deceleration; Failure Mode & Adaptive Strategy
Source: by Ryan M (Fireguy50) at http://web.archive.org/web/20110704113025/http://fordfuelinjection.com/?p=64
Failure Mode Effects Management (FMEM); FMEM is an alternate system strategy in the PCM designed to maintain vehicle operation should one or more sensor inputs fail. When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy will be initiated. The PCM will substitute a fixed in-limit sensor value and will continue to monitor the faulty sensor input. If the faulty sensor operates within limits, the PCM will return to the normal engine running strategy. Engine Running DTC 98 or 998 will be displayed when FMEM is in effect. The Malfunction Indicator Lamp (MIL)/Message will remain on when FMEM is in effect..."
Source: by miesk5 at Ford Bronco Zone Forums
Failure Mode "...is a stand in strategy in the EEC designed to maintain vehicle operation should one or more sensor inputs fail. When a sensor input is perceived to be out-of-limits by the EEC, memorized data from the computers KAM memory will be initiated. EEC continually checks the sensors against its records of normal readings during operating conditions similar to its current state. Hold on that sounds hard! Not really, randomly EEC looks at the past to see if the engine is somewhat where it usually is. If for some unknown reason a sensor is off the charts, it can’t be trusted. So instead of tuning the engine to a possible faulty sensor reading and harming the engine and drive-ability; EEC replaces the sensor reading with a best guess from a past memory. This is why unplugging the battery to fix something works against you. Unplug the battery causes EEC to memorize all the sensor data as base line and normal. If it memorizes a bad sensor it will take it longer to call it a liar. Ford did however give EEC a very broad idea of what the sensor should be reading in its permanent ROM memory, so it will catch on to failed equipment sooner..."
Source: by Ryan M (Fireguy50
EEC IV Operation & Testing. Overview; "... ECT. MAP/BARO. TPS., etc. Used on most Fords. EGR Position (EVP) Feedback (PFE) EVP Linear Potentiometer..." PowerPoint Presentation READ MORE
Source: by Ryan M (FireGuy50) via http://www.powershow.com/view/12a75-ODU3Z/Ford_EEC_IV_powerpoint_ppt_presentation
Identification, Bronco & Ford. Program Code; "Every computer has a main sticker on the 60-pin connector that identifies the computer and holds the most information about the computer. If you have an EEC in your hands and wonder what it came out of this list is for you. If you are looking for an EEC in a junkyard or swap meets this is for you too. If you just wondered what computers Ford used in which vehicle this is also for you. I would not be upset if you printed this page off for your records, in fact I recommend taking it with you so you have an advantage finding that special EEC..."
Source: by Ryan M (Fireguy50) http://web.archive.org/web/20110704113025/http://fordfuelinjection.com/?p=17
Limp Mode; "...Fail Code conditions or Limp Mode happens when the vehicle computer recognizes a problem in it's logic. When an expected signal value from a sensor is sent to the computer and is not within the computer's programmed specifications, secondary programs are activated by the computer to strive to protect the transmission from damage the improper sensor signal might cause to occur. In other words, the computer is always expecting certain signal values from certain sensors i.e. the temperature sensor, the speed sensor, the throttle position sensor, etc. As long as these signals are what it would normally expect for the conditions and is normal based on all the other signals it is receiving from other sensors, it acts normally and accordingly. If the computer, all of a sudden, receives some crazy signal from one of the sensors that is out of the normal range expected from this sensor, it will go to emergency or secondary measures. These emergency measures vary depending on the severity of the defective signal. All this is preprogrammed into the computer's logic by the manufacturer. The manufacturer has decided that as long as a certain parameter of a particular signal is sent from a sensor to the computer, all is well. The manufacturer decided that if this signal is higher than their highest parameter or lower than their lowest parameter, something is wrong with that sensor and the computer should make someone aware of the situation and take action to try to save the vehicle systems or powertrain. Perhaps the computer will simply cause the check engine light to come on. The signal variation wasn't severe or critical to cause any mechanical failures but the vehicle's operator is made aware that he or she should have the vehicle checked out electronically to see if a minor sensor has broken down or is starting to send the odd irratic signal. This type of condition is commonly referred to as a soft code. Normal functions are not affected but if the repair is not made, performance or fuel efficiencies might suffer. Perhaps the sensor only malfunctioned one time and all other times was fine. This might be an early warning of a sensor that is beginning to fail or has a loose connector or connection. Other times the signal needed to perform operations normally is so far out of specification that the computer has no choice but to go into survival mode. With transmissions, the computer will cause the internal tranny fluid line pressure to default to high to protect clutches and bands. The transmission also turns off the shift solenoids to cause the unit to default to a single gear, usually second or third. All normal instructions to control line pressure are overridden so a hazardous slipping condition cannot occur easily. This theoretically is so that the vehicle's driver can get the damaged vehicle to the next town for repairs. This condition is commonly called Limp Mode for this reason. You limp to the next town in second or third gear only, at full line pressure so the tranny guts won't slip on your trip in. By the way, interestingly and just as a side note, if the cable harness going to your transmission was ever to become detached, severed or damaged, your transmission would also go to limp mode. The vehicle's computer would immediately sense that it has lost contact with the transmission and would set the codes and send limp mode signals to the tranny. But because the harness is severed between the computer and the transmission, no computer signals will reach the transmission. These sent signals, however, would have had the identical affect on the transmission as what taking away supplied power to the shift and line pressure solenoids has as in the case of a transmission harness being detached or cut. Due to the engineered voltage strategies of the solenoids, the transmission simply defaults to a single gear and line pressure defaults to high, all in order to limp you home. A Throttle Position Sensor that improperly sends a reading that it is wide open when in fact it is physically closed would be detected by the computer when it compared this reading with the vehicle speed sensor that perhaps is showing very slow vehicle speed. The signal, by itself can't be considered wrong but when put against all the other sensor signals of the system might cause a computer concern. The computer, at this point, unable to trust the collection of signals because together they are not making sense in it's logic, will simply go to limp mode in the transmission to protect it and make the operator aware that something is wrong with one of the sensors and a mechanic's attention is needed to correct the situation. This Fail Code Condition will show up as a code reading on a mechanic's scanner. This code will be cross referenced to a table from the manufacturer and represent a problem with a particular sensor or a group of sensors or system. It gives the mechanic a better idea of where the problem has showed up and which systems or sensors are involved in the malfunction. The table of codes and what each one means, is commonly programmed right into the scanner tool that the mechanic uses for easy reference. i.e. the scanner tool might tell the mechanic that the computer has thrown a code 35 which is the transmission fluid temperature sensor and might give the mechanic the recommended values this sensor should provide and what it in fact provided. In your electronic transmission, many important functions are controlled by the computer. Shift timing, sequence, feel, line pressure are controlled. The information from the vehicle speed sensor affects fuel injection, fuel mixture, ABS, transmission operation, etc. Load information of your engine is commonly taken primarily from the TPS (throttle position sensor) or the MAP sensor (manifold absolute pressure). This controls transmission shifting and downshifting when stepping on the gas or climbing hills. A regular scanning of the computer for any set hard or soft codes is something routinely done by most good tuneup shops these days..."
Source: by Greg O at autotransinc.com via webarchive
http://web.archive.org/web/20101201011938/http://fordfuelinjection.com/files/eectch98.pdf
"...The information supplied herein was taken from technical and sales literature, email
archives, news groups, and wherever else it could be found. If it helps you,
great! Please return the favor by sharing what you learn with me and others.)
None of this data is guaranteed accurate! USE AT YOUR OWN RISK! It is a collection
of technical info, opinions and guesses. You can contribute to the effort by letting
me know what you learn so that it can be added to and corrected.
PURPOSE
This data has been collected and compiled to help you decipher the EEC-IV inner
workings. The EEC MCU probably controls one or more vehicles you own, plus it contains
all the components necessary to build an EFI system for any vehicle -- if only
we could program and modify it. That is the purpose -- to uncloak the EEC-IV so
that we can play with what we bought! Much of the empirical data in this document
is specific to the A9L EEC computer. That is the model MotorSport SVO sells for
conversion of Mustangs from SD to MAF, and it was stock on 5-speed manual transmission
Mustangs from 1989 to 1993, so consensus was reached to pursue this one
configuration until it was understood rather than divide our efforts..."
OVERVIEW
The EEC-IV design began in 1978 and was first introduced in 1983 in the 1.6L ******,
Lynx, EXP and LN7 cars. It has gone through several major physical changes, the
earliest using a fairly simple two board design with through hole soldered components
while the last are more current in technology, showing extensive use of
surface mount components and a much more finished and complex appearance. In
between, there appears to be a variety of mother/daughter board and other designs.
Still, they are all called EEC-IV, although somewhere in its life there was a Ford
P/N generational change.
The reader is referred to the SAE paper #820900, noted in the reference section at
the end of this document, for a much more detailed description of the design goals
and operation of the EEC-IV MCU.
One person wrote: "The processor used is the 8065 along with several supporting
peripheral chips like the DUCE chip which can provide up to 8 PWM outputs and the
DARC chip which has 6 channels of timer capture inputs." (Is he talking about the
EEC-V here ?)
The EEC module is rated to 80oC (185oF) continuous, 100oC intermittent, so it will
be much happier and live longer in the passenger compartment. Some of the later generation
15 and 18 MHz Motorola 8061 processors have a bus loading/edge timing
sensitivity that only gets worse at high temperature, so it’s best to keep the EEC
in a more hospitable environment. Additionally, mounting the EEC in the passenger
compartment will give you better access to the J3 test port, which is where you’ll
be plugging in an after-market module or any test / modification device.
The J3 test port on the side of the ECU box is for developers to plug into -- this
is how the after-market chipmakers and others get into the box. The test connector
has the micro-controller’s multiplexed address/data bus signals on it. It also,
very conveniently, has a PROM disable signal. So the chip makers design something
that hangs off that connector, disables the computer’s PROM, and substitutes its own
PROM in its place.
The reader is encouraged to investigate the Intel 8096 (MCS-96) literature for
greater insight into the 8061 processor used in the EEC. One document available
from their web site is "27006102.pdf" entitled "APPLICATION NOTE; AP-248; Using The
8096"; Order Number: 270061-002. There are many other related documents available
from Intel -- including the use of the A-D converter, the implementation of "fuzzy
logic", instrumenting and controlling automotive applications, and other topics.
Also, this author has posted a synopsis of the 8096 pinouts and instruction set which is available at several web sites.."
READ MUCH MORE!
--- miesk5 Note;
Here is some info that can help in troubleshooting EEC IV;
EEC IV Connector Pin Diagram
by Fireguy50 (Ryan M) at http://web.archive.org/web/20131229163930/http://oldfuelinjection.com/images/eec04.gif
EEC IV Connector Pin Outs LEGEND, Bronco & Ford Truck & Van: 4.9, 460, 5.0, 5.8; by Ryan M (Fireguy50) at http://web.archive.org/web/20131029020805/http://oldfuelinjection.com/truckpinouts.html
a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19
http://broncozone.com/topic/14269-code-reader/?pid=74587&mode=threaded
A helper is good to assist in reading Codes best is to take a cell fone vid and replay it.
Some basics;
Visual Check
1.Inspect the air cleaner and inlet ducting.
2.Check all engine vacuum hoses for damage, leaks, cracks, blockage, proper routing, etc.
3.Check EEC system wiring harness for proper connections, bent or broken pins, corrosion, loose wires, proper routing, etc.
4.Check the Powertrain Control Module (PCM), sensors and actuators for physical damage. I see tPS, etc was replaced...
5.Check the engine coolant for proper level and mixture.
6.Check the transmission fluid level and quality. See E4OD Fluid Condition Check Below)
7.Make all necessary repairs before continuing
8. Check headlights
The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears including Reverse.
Make sure A/C is off and transmission is in Park (automatic); or in Neutral for a Manual & release clutch.
Turn off all accessories; radio, lights, A/C, heater, blower, fans, etc. (close driver's door)
Then turn off engine and wait 10 seconds.
Do KOEO test First
Post Code(s) here according to:
KOEO
&
KOER
The Self-Test is divided into three specialized tests: Key On Engine Off Self-Test, Engine Running Self-Test, and Continuous Self-Test. The Self-Test is not a conclusive test by itself, but is used as a part of the functional Quick-Test diagnostic procedure. The PCM stores the Self-Test program in permanent memory. When activated, Self-Test checks the EEC system by testing memory integrity and processing capability, and verifies that various sensors and actuators are connected and operating properly.
The Key On Engine Off and Engine Running Self-Tests are functional tests which only detect faults present at the time of the Self-Test. Continuous Self-Test is performed during normal vehicle operation and stores any fault information in Keep Alive Memory (KAM) for retrieval at a later time.
Key On Engine Off Self-Test
At this time, a test of the EEC system is conducted with power applied and engine at rest.
To detect errors during Key On Engine Off Self-Test, the fault must be present at the time of testing.
Continuous Memory DTCs are issued as a result of information stored during Continuous Self-Test, while the vehicle was in normal operation. These DTCs are displayed only during Key On Engine Off Self-Test and after the separator pulse. Intermittent faults that have not occurred in the last 80 warm-up cycles (40 cycles on some applications) are erased from Continuous Memory and will not produce a Continuous Memory DTC. Note: The separator pulse and Continuous Memory DTCs follow Key On Engine Off DTCs ONLY.
Engine Running Self-Test
At this time, a test of the EEC system is conducted with the engine running. The sensors are checked under actual operating conditions and at normal operating temperatures. The actuators are exercised and checked for expected results.
============
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.
EEC-IV System Has NO Control Over the Following Items; "...Fuel quantity and quality; Damaged or faulty ignition components; Internal Engine Condition - rings, valves, Timing belt, etc.; Starter & Battery Circuit; Dual Hall Sensor; TFI or DIS Module; Distributor condition or function; Camshaft Sensor; Crankshaft Sensor; Ignition or DIS Coil; Engine Governor Module..."
EEC Strategies "...Start / Crank, Cold Start & Warm Up, Cold Drive-Away, Warm Idle, Warm Cruise, Full-Throttle Acceleration, Deceleration; Failure Mode & Adaptive Strategy
Source: by Ryan M (Fireguy50) at http://web.archive.org/web/20110704113025/http://fordfuelinjection.com/?p=64
Failure Mode Effects Management (FMEM); FMEM is an alternate system strategy in the PCM designed to maintain vehicle operation should one or more sensor inputs fail. When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy will be initiated. The PCM will substitute a fixed in-limit sensor value and will continue to monitor the faulty sensor input. If the faulty sensor operates within limits, the PCM will return to the normal engine running strategy. Engine Running DTC 98 or 998 will be displayed when FMEM is in effect. The Malfunction Indicator Lamp (MIL)/Message will remain on when FMEM is in effect..."
Source: by miesk5 at Ford Bronco Zone Forums
Failure Mode "...is a stand in strategy in the EEC designed to maintain vehicle operation should one or more sensor inputs fail. When a sensor input is perceived to be out-of-limits by the EEC, memorized data from the computers KAM memory will be initiated. EEC continually checks the sensors against its records of normal readings during operating conditions similar to its current state. Hold on that sounds hard! Not really, randomly EEC looks at the past to see if the engine is somewhat where it usually is. If for some unknown reason a sensor is off the charts, it can’t be trusted. So instead of tuning the engine to a possible faulty sensor reading and harming the engine and drive-ability; EEC replaces the sensor reading with a best guess from a past memory. This is why unplugging the battery to fix something works against you. Unplug the battery causes EEC to memorize all the sensor data as base line and normal. If it memorizes a bad sensor it will take it longer to call it a liar. Ford did however give EEC a very broad idea of what the sensor should be reading in its permanent ROM memory, so it will catch on to failed equipment sooner..."
Source: by Ryan M (Fireguy50
EEC IV Operation & Testing. Overview; "... ECT. MAP/BARO. TPS., etc. Used on most Fords. EGR Position (EVP) Feedback (PFE) EVP Linear Potentiometer..." PowerPoint Presentation READ MORE
Source: by Ryan M (FireGuy50) via http://www.powershow.com/view/12a75-ODU3Z/Ford_EEC_IV_powerpoint_ppt_presentation
Identification, Bronco & Ford. Program Code; "Every computer has a main sticker on the 60-pin connector that identifies the computer and holds the most information about the computer. If you have an EEC in your hands and wonder what it came out of this list is for you. If you are looking for an EEC in a junkyard or swap meets this is for you too. If you just wondered what computers Ford used in which vehicle this is also for you. I would not be upset if you printed this page off for your records, in fact I recommend taking it with you so you have an advantage finding that special EEC..."
Source: by Ryan M (Fireguy50) http://web.archive.org/web/20110704113025/http://fordfuelinjection.com/?p=17
Limp Mode; "...Fail Code conditions or Limp Mode happens when the vehicle computer recognizes a problem in it's logic. When an expected signal value from a sensor is sent to the computer and is not within the computer's programmed specifications, secondary programs are activated by the computer to strive to protect the transmission from damage the improper sensor signal might cause to occur. In other words, the computer is always expecting certain signal values from certain sensors i.e. the temperature sensor, the speed sensor, the throttle position sensor, etc. As long as these signals are what it would normally expect for the conditions and is normal based on all the other signals it is receiving from other sensors, it acts normally and accordingly. If the computer, all of a sudden, receives some crazy signal from one of the sensors that is out of the normal range expected from this sensor, it will go to emergency or secondary measures. These emergency measures vary depending on the severity of the defective signal. All this is preprogrammed into the computer's logic by the manufacturer. The manufacturer has decided that as long as a certain parameter of a particular signal is sent from a sensor to the computer, all is well. The manufacturer decided that if this signal is higher than their highest parameter or lower than their lowest parameter, something is wrong with that sensor and the computer should make someone aware of the situation and take action to try to save the vehicle systems or powertrain. Perhaps the computer will simply cause the check engine light to come on. The signal variation wasn't severe or critical to cause any mechanical failures but the vehicle's operator is made aware that he or she should have the vehicle checked out electronically to see if a minor sensor has broken down or is starting to send the odd irratic signal. This type of condition is commonly referred to as a soft code. Normal functions are not affected but if the repair is not made, performance or fuel efficiencies might suffer. Perhaps the sensor only malfunctioned one time and all other times was fine. This might be an early warning of a sensor that is beginning to fail or has a loose connector or connection. Other times the signal needed to perform operations normally is so far out of specification that the computer has no choice but to go into survival mode. With transmissions, the computer will cause the internal tranny fluid line pressure to default to high to protect clutches and bands. The transmission also turns off the shift solenoids to cause the unit to default to a single gear, usually second or third. All normal instructions to control line pressure are overridden so a hazardous slipping condition cannot occur easily. This theoretically is so that the vehicle's driver can get the damaged vehicle to the next town for repairs. This condition is commonly called Limp Mode for this reason. You limp to the next town in second or third gear only, at full line pressure so the tranny guts won't slip on your trip in. By the way, interestingly and just as a side note, if the cable harness going to your transmission was ever to become detached, severed or damaged, your transmission would also go to limp mode. The vehicle's computer would immediately sense that it has lost contact with the transmission and would set the codes and send limp mode signals to the tranny. But because the harness is severed between the computer and the transmission, no computer signals will reach the transmission. These sent signals, however, would have had the identical affect on the transmission as what taking away supplied power to the shift and line pressure solenoids has as in the case of a transmission harness being detached or cut. Due to the engineered voltage strategies of the solenoids, the transmission simply defaults to a single gear and line pressure defaults to high, all in order to limp you home. A Throttle Position Sensor that improperly sends a reading that it is wide open when in fact it is physically closed would be detected by the computer when it compared this reading with the vehicle speed sensor that perhaps is showing very slow vehicle speed. The signal, by itself can't be considered wrong but when put against all the other sensor signals of the system might cause a computer concern. The computer, at this point, unable to trust the collection of signals because together they are not making sense in it's logic, will simply go to limp mode in the transmission to protect it and make the operator aware that something is wrong with one of the sensors and a mechanic's attention is needed to correct the situation. This Fail Code Condition will show up as a code reading on a mechanic's scanner. This code will be cross referenced to a table from the manufacturer and represent a problem with a particular sensor or a group of sensors or system. It gives the mechanic a better idea of where the problem has showed up and which systems or sensors are involved in the malfunction. The table of codes and what each one means, is commonly programmed right into the scanner tool that the mechanic uses for easy reference. i.e. the scanner tool might tell the mechanic that the computer has thrown a code 35 which is the transmission fluid temperature sensor and might give the mechanic the recommended values this sensor should provide and what it in fact provided. In your electronic transmission, many important functions are controlled by the computer. Shift timing, sequence, feel, line pressure are controlled. The information from the vehicle speed sensor affects fuel injection, fuel mixture, ABS, transmission operation, etc. Load information of your engine is commonly taken primarily from the TPS (throttle position sensor) or the MAP sensor (manifold absolute pressure). This controls transmission shifting and downshifting when stepping on the gas or climbing hills. A regular scanning of the computer for any set hard or soft codes is something routinely done by most good tuneup shops these days..."
Source: by Greg O at autotransinc.com via webarchive
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