Any benefit to resetting PCM?

Joseph

TJ Enthusiast
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Aug 20, 2018
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I read a few posts where people mostly argued the correct way to do it, but is there any large benefit? I've had my 3 speed auto TJ for roughly 6000 miles now. I never have had any problem with the way it drives, other than the issue of the crankcase sensor that has recently become a lot more prominent. I heard that resetting the PCM brings the driving "memory" or whatever you want to call it, back to stock performance.

After replacing my crankshaft sensor, is this something I could or should do? I also read people would do this after certain large changes, such as an exhaust, to let the Jeep adapt better to the performance of the new parts. I feel as if the sensor has been on the way out for awhile, more than a few hundred miles at least - would this be enough for the Jeep to learn how to compensate with the dying crank sensor?

Or is this just a stupid question? Feel free to discuss :partyhat:
 
I think it's a good thing to reset the PCM every other year or when you replace sensors, regear, or change what fuel octane you use.

I reset mine after regering and got rid of the pinging I was experiencing at very low revs when loading it up the my 4.0L

For the record - as someone is bound to ask...
  1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds to discharge the PCM capacitors, which maintain the Adaptive Memory.
  2. Reconnect the Battery Cable.
  3. Turn Ignition Switch to the “On” position but DO NOT start the engine.
  4. Turn Headlight On.
  5. Turn Headlights Off.
  6. Turn Ignition Key Off.
PCM is reset
LINK to quoted info
 
Truthfully, I don't see any harm in it, and I think it's not a bad idea to do every once in a while.
 
I've reset the PCM on mine a few times. All I ever noticed is that the engine ran worse on the first couple start ups, then it got better and returned to normal.
 
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I have seen multiple versions of how to reset the PCM to clear the memory and restart the learning phase. The above mentioned seems to be most popular. What I found to work the best(for me) was doing a complete discharge via disconnecting the battery over night, starting up and then let idle until it has heat cycled a couple of times(thermostat opened and closed), then drive normal around town for a while and then take it out on the highway. The learning phase actually takes many miles of driving and also quite a few fresh starts. This has worked for me and I did see a difference. Definitely do it for sensor changes, re-gearing, and anything performance related like exhaust, air filter, etc
 
I've never understood why the headlight on/off is part of the ritual. Does the PCM have knowledge of what the headlights are doing?
 
Turning the headlights on when the battery is disconnected just places an additional electrical draw/load across the PCM since it ultimately shares a power connection with the lights. Doing that simply helps to reset the PCM more quickly.

To me there's zero benefit to resetting the PCM unless something like a sensor like an O2 or TPS sensor has been replaced. The PCM is constantly updating its parameters according to how the Jeep is driven and resetting it just means it has to go through the entire adaptive learning process again causing the Jeep to not run as well until after it has relearned everything.
 
Turning the headlights on when the battery is disconnected just places an additional electrical draw/load across the PCM since it ultimately shares a power connection with the lights. Doing that simply helps to reset the PCM more quickly.

....

So, it isn't specifically the head lights that matter. The head lights are just an easy instruction to give in order to place an additional load across the PCM. That makes good sense to me.
 
I think it's a good thing to reset the PCM every other year or when you replace sensors, regear, or change what fuel octane you use.

I reset mine after regering and got rid of the pinging I was experiencing at very low revs when loading it up the my 4.0L

For the record - as someone is bound to ask...
  1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds to discharge the PCM capacitors, which maintain the Adaptive Memory.
  2. Reconnect the Battery Cable.
  3. Turn Ignition Switch to the “On” position but DO NOT start the engine.
  4. Turn Headlight On.
  5. Turn Headlights Off.
  6. Turn Ignition Key Off.
PCM is reset
LINK to quoted info

Forgive my noob question - when you say “touch it to ground” you’re not referring to the neg terminal right?


Sent from my iPhone using Tapatalk
 
With my 05 LJ with the 4.0 engine, I got a CEL. I just disconnected the battery for at least 30 seconds, and reconnected to clear the CEL.

I have since gotten a code reader to find the CEL did not register a fault code, and my problem was the result of my gas cap not on tight enough.
 
Forgive my noob question - when you say “touch it to ground” you’re not referring to the neg terminal right

Right! And nothing "n00b" about your question mate - it can be unnerving playing around with battery terminals.

Remove the POSITIVE (Red) wire from the positive terminal then move it over to connect it on the NEGATIVE (Black) terminal of the battery for 30 seconds or so.
By "connect" I mean hold the positive wire temporarily there on the negative terminal for at least 30 seconds.

Note: Disconnecting the battery for extended periods will not clear the PCM values and check any wires connected to the batteries POSITIVE terminals as there can be more than one positive terminal on upmarket batteries.
 
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So i have new o2 sensors that i am just waiting for a decent day to replace. I guess I didn't even think about resetting the PCM, but looks like I should definitely do it afterwards?
 
Actually, disconnecting for 8+ hours will completely discharge the system. From the research I've done, this will work better than discharging by touching the negative terminal.
 
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Turning the headlights on when the battery is disconnected just places an additional electrical draw/load across the PCM since it ultimately shares a power connection with the lights. Doing that simply helps to reset the PCM more quickly.

To me there's zero benefit to resetting the PCM unless something like a sensor like an O2 or TPS sensor has been replaced. The PCM is constantly updating its parameters according to how the Jeep is driven and resetting it just means it has to go through the entire adaptive learning process again causing the Jeep to not run as well until after it has relearned everything.

Agreed.

Also I had an issue with my GMC. I had disconnected the battery while replacing the engine, when everything was put back together I took it to a shop to get smogged. The guy there asked if I had just replaced the battery. He said that when the PCM was reset he could not smog it until it had been driven enough to relearn whatever it leans while you're driving. I would imagine that is an issue with most modern vehicles.
 
I don't recall that it drove any different, seemed normal.
The few times I've done a reset, intentionally or not, the first or second start up has been the roughest. After that, the learning process is subtle enough those I don't notice any changes.
 
The few times I've done a reset, intentionally or not, the first or second start up has been the roughest. After that, the learning process is subtle enough those I don't notice any changes.
Yeah, it's probably more apparent in a TJ. Mine was a 6.0 V8 in a full size pickup, combined with a failing memory about an event a few years ago.... :)
 
Did you have any performance issues driving then while it relearned?

Did exactly what Wombat recommended and at first start it was rough but after taking it around town, it settled down. Definitely a must do when changing out sensors (I’ve replaced quite a few)


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