ECM Replacement...?

91/4.0/AW4/D30 8.25 3:55

So I went to this local area where a bunch of my friends go off-roading and took the Jeep through it's paces as I normally do. When we went to leave, I noticed the voltmeter was barely above the red area for undercharging. Eventually a check engine light came on but I made it home.

I pulled the codes using the key on-off method and got codes 12, 21, and 41. I slept on it that night and the next day I came out to start the jeep and didn't get a check engine light, but it still wasn't charging. Took out a multimeter and hooked it up, sure enough, it was around 11 volts and slowly going down.

I then bypassed the field terminals (after disconnecting the battery) on the alternator and the battery started charging. It got the battery right up to 14.5 volts when idling so I figured the ECM crapped out. Put the thing back together, started it up, and sure enough it was charging again as I said wtf to myself.

I shut it off went inside to shower, came back out and it wasn't charging. So, I got myself a new ECM from my local auto parts store. It wasn't cheap but hopefully fixes my problem. I asked some of my mechanic friends and they said you don't have to flash these older ones as they learn everything themselves, is that true? Taking it out and putting it in is nothing just wanted to make sure. Thanks, sorry for the long post.

i've got 5 bucks you drug the alternator through some nasty stuff and it's starting to crap out. i've been through 2 alternators since i got mine because apparently they don't like swamp water.

You should have used the voltmeter to measure the voltages on the field terminals when it was supposed to be charging. That would have been the conclusive test whether the ECM was bad. The 21 means you've got a bd O2 sensor? Might be related it you have a grounding problem.