Pulling Fuse #2 and Adaptives

By SRT-Tom · Apr 21, 2023 ·
  1. SRT-Tom
    HERE'S WHAT HAPPENS WHEN YOU PULL #2 FUSE (older Challengers)

    Pulling Fuse F2 resets:

    • All PCM DTCs erased.
    • All OBD2 monitor results erased.
    • All "long term fuel trim adaptive" values reset to Zero.
    Long term Fuel Adaptive trim will now start over the next 100 engine start cycles.

    Cycle the headlights as described above will create a Short Term Fuel Adaptive trim table over the next 50 start cycles.

    Driver Adaptive?

    Driver Adaptive and related fuel table trim is probably the least understood algorithm contained within the NGC - Next Generation Controller (also called PCM)

    There are long term fuel adaptive trim (100 start cycles) and short term fuel adaptive trim (50 start cycles). The NGC looks at what the fuel requirements to operate are during "closed loop" operations. The fuel trim algorithm is slowly and gradually attempting to bring the fuel consumption to best suit driving conditions and optimize the MPG by feedback from O2 and other sensors.

    Short term and long term fuel trim are also used in analyzing KR (Knock Retard) and attempting to have the NGC (PCM) calibrate the engine timing based on quality of fuel. With different quality fuels and octane's, the NGC tries to always adjust for the best engine timing (advance/retard) to protect the engine from knock or prevent detonation. The knock sensors on the HEMI engine are quite sensitive and can be considered to be "engine microphones".

    What is Knock Retard?

    Knock Retard (hereafter referred to as KR) is the response from the PCM to cylinder detonation. KR is the measure of the number of degrees of overall ignition timing advance that must be removed from the engine to prevent detonation from continuing, thus protecting the engine from damage.

    What is REAL KR and what is FALSE KR?

    Real KR is KR that grows with engine RPM and engine load. It depends entirely on detonation, which is dependent upon throttle position, IAT or MAF, MAP, engine load, engine temperature, and RPM. As RPM and engine load increase, the chance for KR (or higher KR) increases. As the vehicle shifts to the next gear, KR will usually make a small jump up as well due to the higher engine load.

    False knock is characterized by a sharp spike to an immediately high value of KR followed instantly by the KR Recovery Rate. It doesn't grow with engine RPM or load, it jumps to a high value on throttle input and then recovers to a low value, or zero perhaps, as engine RPM continues to increase. Note that this is exactly opposite to the characterization of REAL KR. Remember, knock is simply specific noise detected by engine microphones. Because it happens to fall within the frequency of real KR does not necessarily mean that it IS real KR.

    The PCM has the ability to do some rudimentary fuel tuning (all modern vehicles now do this) via "closed loop mode". It has two main modes of operation, closed loop and open loop. When the engine is started and heated up past 160 deg. F, the NGC (PCM) now goes into "closed loop" control whereby it takes feedback from sensors (ie. IAT, MAP, O2, Knock, etc.) and looks at the fuel efficiency and attempts to tweak out the current fuel tables to get the best MPG and engine efficiency.

    Tromping the accelerator to the floor puts the PCM into OPEN loop where it now goes to fixed tables to do fuel mixture and o2 sensors are now bypassed.

    So where does this wind up over time. Well, there is a number of "learn cycles" that The PCM goes into for fuel Adaptive, normal 100 start cycles, or a "quick learn" of 50 start cycles (using the headlight ON/OFF trick). During these times (cycles) fuel trim adaptive algorithms work on trying to get the best MPG. After the 100 cycles, the MPG magic is pretty much done until something is done to initial another "learn cycle" like clear memory.

    These start cycles are a fixed "time-cycle" effort to build a fuel trim table and the algorithm will stop fuel trim after these 50 or 100 start cycles.

    So if you have been driving around for a number of weeks, months in a laid back fashion, the PCM has learned this and attempted to get the best MPG for you and tuned down (slowly) performance. So, one day you tromp the gas hard or do some spirited driving and you notice the car "sluggish", or not quite as peppy as before. This could be the reason. Clearing out long term fuel table adaptive memory can sometimes help.

    The PCM has a number of classes and types of internal memory. The fuel table Adaptive are stored in volatile memory and when the fuse (F2) is pulled for at least 20-30 seconds, this memory is cleared and you can start over building a new set of Fuel Table Adaptive for closed loop control (next 100 start cycles).

    If you granny drive around town your car adapts to that type of driving. Then when you want to do a lot of quick aggressive driving the vehicle may seem sluggish and not "as responsive" as you remember. You may need to do something to the PCM called clearing driving fuel Adaptive.

    TCM Driver Adaptive?

    Now, what about the Automatic (NAG1) Transmission and its stored driver adaptives?

    For those who wonder about NAG1 (Auto) transmission driver adaptives, that is another animal entirely and pulling F2 does nothing whatsoever with the TCM (Transmission Control Module). if You have a Diablo Predator tuner, you are able to use this tool to reset the TCM via menu option.

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