Mode' or 'Fail Code' Conditions
Almost every system in your modern automobile is controlled by onboard computers, especially your engine and transmission functions. (In this document we are focusing on the transmission system).
Nearly all automatic transmissions in vehicles these days are 'electronic'. This means your electronic transmission absolutely relies on the computer to control all its functions such as shift timing, shift sequence,
how the shift feels (abrupt or smooth), tranny fluid line pressure, etc. Other systems on the vehicle work in unison with your transmission system as well. The information from
the vehicle speed sensor (VSS) 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) controlling transmission shifting and
downshifting when stepping on the gas or climbing hills, for example. A healthy system depends on many factors and systems working well together at all times.
'Fail Code' condition (commonly known as 'Limp Mode') happens when the vehicle computer
recognizes a problem through it's coded/programmed logic. When an electric signal
value is sent to the computer from a sensor but is not
within the computer's programmed specifications (what the program expects to see and deem normal from that sensor), 'secondary'
programs are immediately activated by the computer to strive to protect
the transmission from any damage the improper sensor signal
is indicating may occur, or may be imminent, or may contribute to.
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 (TPS), or MAP sensor, etc. As long as these signals are what it
would normally expect for the current operating conditions and are normal, based
on all the other signals it is receiving from other sensors, compared to each other as a group,
then it performs normally and accordingly.
However, if the computer all
of a sudden receives some crazy signal from one of the
sensors that is out of the normal range (not within the parameters expected from this
sensor), the computer is programmed to initiate 'emergency' or 'secondary' measures.
What are these measures?
Well, these emergency measures vary depending on the severity
of the defective signal. All of this is programmed/coded into the
computer's logic by the manufacturer. The manufacturer has
decided that as long as a certain expected parameter of a particular
signal is sent from a sensor to the computer, all is well. There is usually a little 'window' of values that the computer will accept from any sensor. It expects the signal to fall within this range somewhere.
The manufacturer has also decided that if this signal is higher than
their highest expected parameter or lower than their lowest
expected parameter, something is wrong either with that sensor or the vehicle conditions that caused it to emit that particular signal. At this point the
computer strives to make someone aware of the situation or
may even be programmed to take action to try to 'save' the vehicle systems or
powertrain somewhat, by itself.
What type of action does the vehicle's computer take?
Well, perhaps the computer will simply cause the
'check engine' light to come on. The signal variation
perhaps wasn't severe or critical enough to indicate 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, is starting to deteriorate and send the
odd irratic signal or, more seriously, some abnormal mechanical condition has developed in the powertrain, causing the sensor to send unexpected signal values.
This type of condition is commonly
referred to as a 'soft code'. (This is not 'limp mode'). Normal vehicle functions may not be
affected but if the repair is not made, performance or fuel
efficiencies might suffer, or deteriorating mechanical conditions might get worse.
It might very well be that the sensor only
malfunctioned or sent an erratic signal one time and all other times was fine. This
might indicate an early warning of a sensor that is beginning to wear out or maybe it's simply a matter of a loose connector or connection.
In any case, it is well-advised to always determine the cause of a 'soft code'.
Sometimes, the sensor signal needed to perform all
operations normally is extremely out-of-specification, or re-occurring often, or is even non-existent. Now the
computer has no choice but to go into a more critical 'survival' mode. With
vehicle transmissions, when the computer detects an obvious, dangerous signal value of this sort it will cause the internal tranny-fluid-line-pressure to default to 'high pressure' (in order to protect clutches
and bands). The computer also turns off the transmission's electronic shift
solenoids which in turn causes the unit to default to a single gear only (usually 2nd or 3rd). All normal signals to vary and control line
pressure are overridden and everything defaults to 'full on' so a hazardous 'slipping' condition within the clutch pack
cannot occur easily. This theoretically (and practically) is so that the
vehicle's driver can hopefully get the damaged vehicle to the next
town for repairs.
This condition is commonly called 'Limp
Mode' for this reason --- rather than being stranded, you're able to limp to the next town in either second or
third gear, with full tranny line pressure being applied, so that the clutch guts
won't slip on your trip in and your vehicle will move along slowly but steadily to a service center... (as I said, hopefully).
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 into 'limp mode'. (A situation that occurred for a friend of mine while he was out bush trailing).
The vehicle's computer would immediately
sense that it has lost contact with the transmission and
would set the warning codes, turn on a warning light, 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
So now what?
Interestingly, manufacturers have designed transmission systems so that these sent 'limp-mode' computer signals have the identical
affect on the transmission as what simply 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, when a harness is cut or detached, the
transmission simply defaults to a single gear (2nd or 3rd)
and line pressure defaults to high, all in order to 'limp' you home.
There are also other interesting situations, when you look at the 'big picture' of the system:
A Throttle Position Sensor (TPS) that improperly sends a
reading that it is 'wide-open' when in fact it is physically
closed would be detected by the computer as being illogical when it compared
this reading with the vehicle speed sensor that perhaps is
showing very slow vehicle speed, at the time. The TPS signal by itself in this case cannot be considered wrong (as it is within its programmed parameters) but when compared by the computer with all the other sensor signals of that system, at that time, might not make sense and this may cause a concern.
The computer, at this point, unable to 'trust' the
collection of signals because together at that time they are not making
sense in it's logic, may simply go to 'limp mode' in the
transmission, as well, to protect it. It will make the operator aware that
something is wrong with one of the sensors, through a dashboard indicator light of some sort ('checkengine' light perhaps) and a mechanic's
attention is needed to correct the situation.
So what do mechanics see?
They'll probably scan your computer to pick up the code(s) it has generated using a special electronic tool they attach to your vehicle. This
'Fail Code' condition indicated by your warning light will show up as a certain 'code' reading on a
mechanic's computer scanning tool. 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, for quick and 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...
regular scanning of the computer for any set 'hard' or 'soft' codes is something
routinely done by most good tuneup shops these days during scheduled service checkups.