Crankshaft Position Sensor Learn Issues on Gen III and Gen IV Engines
Trifecta Performance, Inc. (www.trifectaperformance.com)
Disclaimer: The authors have taken every measure possible to ensure the accuracy of the information presented here, but cannot be held responsible for any inaccurate information presented. In other words, proceed at your own risk.
With regards to powertrain control module (PCM) custom programming, have you ever seen people discuss the dreaded “crank relearn” issue? Usually this issue is associated with frustration at having to pay the GM dealership upwards of $100 to do something to your ride so the Service Engine Soon instrument cluster light (a.k.a. SES light) stays off. And perhaps some of that performance you lost is restored. This article discusses what “crank relearn” is, why it’s a problem for custom PCM programmers, and what the dealership is doing to your vehicle to fix it.
What is “crank relearn”?
“Crank relearn” refers to a GM service procedure that has to do with the calibration of the PCM to a specific engine’s crankshaft position sensor (CKP sensor). When your Gen III (LS1, etc) or Gen IV (LS2, etc) based car or truck was put together and factory-tested, one of the procedures it went through was the “crank learn” procedure. The PCM has the ability to contain detailed information about how the specific engine in your car operates, and it must have this information in order to run the engine optimally under all conditions. One key piece of information pertains to how the CKP sensor reports its signal to the PCM.
If a vehicle’s PCM is changed out, there is a chance the new PCM will contain incorrect information about how the CKP sensor reports its signal to the PCM. If the new PCM’s information is incorrect, and the PCM cannot self-correct it, it will set the P1336 Diagnostic Trouble Code and turn the SES light on. Aside from the annoying SES light, vehicle performance may be affected, and emissions systems may be compromised.
Introducing the Dreaded P1336 Diagnostic Trouble Code (DTC)
According to the GM factory service manual, the OBD-II trouble code, P1336, is defined as follows: DTC P1336 CKP System Variation Not Learned. The factory service manual does not enumerate all of the symptoms of this trouble code, but after consulting a professional custom PCM tuner, it was learned that aside from the annoying light on the instrument cluster, the other symptoms exhibited may include: poor vehicle performance, and loss of misfire detection diagnostics.
This DTC is particularly nasty, because simply clearing it by disconnecting the battery, or clearing it using an OBD-II scan tool will not address the problem. The code will be set again later unless the vehicle is run through the actual “crank relearn” procedure.
Fixing the Dreaded P1336 DTC
There is only one way to properly fix the P1336 DTC, and that’s to perform a “crank relearn” procedure, or as the GM factory service manual procedure calls it, the CKP System Variation Learn Procedure. This procedure can be initiated and performed by GM’s be-all end-all scan tool, the Tech-2. It can also be performed by some aftermarket scan tools. Furthermore, some of the PCM tuning software applications are capable of initiating a “crank relearn” procedure.
For most enthusiasts that encounter this issue, the best course of action is to bring the vehicle to a local GM dealership and have them perform the procedure. Unfortunately most dealerships will charge a modest fee for doing this (upwards of USD $100 in some cases).
How to Avoid the Issue
Unfortunately, the only sure-fire way to avoid possibly running into the P1336 DTC is to keep the engine and PCM original. Seeing as this is unrealistic given today’s performance-minded enthusiast, there are several ways to reduce the risk of running into the issue:
This section delves into the technical details surrounding the CKP system, and discusses the actual CKP System Variation Learn Procedure, as documented by GM’s factory service manual.
How the CKP System Works
The CKP sensor, and its reluctor wheel are the cornerstone of the engine management system used in Gen III and Gen IV GM vehicles. In the Gen III engines, and the early Gen IV engines, at the end of the crankshaft, there is a “24x” (24 tooth) reluctor wheel. Around 2006, GM switched a “58x” (58 tooth) reluctor wheel in the Gen IV engines. As the crankshaft rotates, the CKP sensor, which is installed in the engine’s front cover, detects when a tooth on the reluctor wheel passes the sensor. This signal is sent to the PCM, and the PCM uses it to reference when to fire the ignition system and activate the fuel injectors. The PCM also uses this signal input to detect when unexpected changes in crankshaft rotational speed are detected, which is generally indicative of a “misfire”.
The factory service manual explains that slight manufacturing variances in the crankshaft, reluctor wheel, and/or CKP sensor can make misfire detection very difficult. Thus, the CKP System Variation Learn Procedure exists to teach the PCM how these components are unique to the engine.
Partial Text: DTC P1336 CKP System Variation Not Learned from GM Factory Service Manual
The following text is quoted from the 1999 GM F-Body Factory Service Manual:
The Crankshaft Position (CKP) sensor sends pulses to the PCM as the reluctor wheel teeth rotate past the CKP sensor. The PCM uses the CKP pulses to synchronize the ignition and fuel injector operation, and to time the interval between each CKP pulse. The PCM determines when an excessive change in crankshaft speed occurs by comparing each new time interval with the previous interval. A misfire causes an unexpected change in the crankshaft speed. A certain amount of acceleration/deceleration is expected between each firing stroke, but if the crankshaft speed changes more then an expected amount, the PCM interprets this as a misfire. The interval between the CKP sensor pulses is extremely small. At high engine speeds, slight variations in the following components make misfire detection difficult: Crankshaft, Reluctor wheel, CKP sensor. The PCM learns variations during the Crankshaft Position System Variation Learning Procedure. The PCM compensates for these variations when performing detect misfire calculations. Only a scan tool can command the PCM to perform the Crankshaft Position Variation Learning Procedure again.
Perform the learning procedure after the following actions: A PCM replacement, Any operation or repair involving the crankshaft, the CKP sensor, or the CKP to reluctor wheel gap relationship, An engine replacement, The ignition switch is in the ON position until the battery is drained.
Important: A PCM power-disconnect with the ignition ON may erase the stored pulse value and set the DTC P1336. Disconnecting the PCM will not erase the learned Crankshaft Position System Variation as long as the ignition switch is in the OFF position.
Important: Reprogramming the PCM does not require running the Crankshaft Position System Variation Learn Procedure unless the PCM is new or from another vehicle. A DTC P1336 sets if the Crankshaft Position System Variation is not within an acceptable range, or can not be learned.
Many recipients of custom PCM programming have unknowingly subjected themselves to the annoyance of dealing with CKP System Variation issues. This is not an issue that customers of PCM programming services must deal with and in many cases, represents irresponsibility on the part of the custom PCM programmer in not disclosing this risk. There are several actions customers can take to protect themselves from having to deal with this annoyance. Knowing about the risk is a big step forward.