Weird shifting issue

2000 Chevy Blazer 4.3 (automatic) - When I bought it used, it shifted fine, has no issues other than the transfer case wouldn’t engage every time. A few months later my overhead console display lights went out, then all the dash lights such as engine, 4x4, the gauges started lighting up and acting weird randomly, then a week later the truck started shifting funny.

Now 90% of the time, the truck won’t start in 1st gear, it starts in 2nd, then while accelerating up to 30-35mph it will shift up way too soon and bog down, then shit back down to 2nd, then shift up again, and continue until it hits 30-35, then it fails to shift into overdrive. Also when coming to a stop, it kicks down a gear (into 2nd I assume) pretty hard and clanks pretty loud around 2mph just before complete stop.

Weird thing is every once in awhile, it will go away and shift normal but the engine light is usually lit up during normal shifting.

Could it be the ECM causing all these weird things? Any ideas at all?

Yes the PCM could be the culprit. Usually on the electronic transmisions, if the PCM is having problems with sensors or output devices, it will default to limp home mode while setting the ‘Check Engine’ light. A scan for codes will bring up the offending condition. You will probably need someone with a scanner that is capable of pulling up ‘freeze frame’ data for the code and real time data that is occuring at the time of events.

With your other Body Control Module troubles, the problem might be communication failure between the BCN and the PCM on the Car Area Network. So the problem might be with the BCM not sending the correct data to the PCM, like which manual gear the truck is in.

Hope this helps.

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So that explains why the autozone scan found nothing but a generic code which was no help at all. Helps a lot thank you

After some reading it makes sense about the BCM because certain things it specifically controls are affected. Did not see anything about the shifting though.

Depending on how the software is designed, the BCM can handle reading the shifter position switches, the vehicle security protocols, the starter activating commands, etc. What you will probably find is nongeneric codes that are specific to GM. That is the reason to use a GM capable scanner, especially if BCM codes and CAN protocols are involved.

Not only do you have the BCM and PCM but also there is a Vehcile Control Module and Transfer Case Shift Control Module. What a can of worms, trying to figure out which module is miscommunicating. You probably will need some way of monitoring what is going over the CAN to narrow down the problem.

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This is what a proper scan showed, so this is where I am so far:

C0281 - Dynamic Rear Proportioning (history)
C0226 - Left front traction control system channel will not move (history)
C0237 - Traction control system throttle position signal
P0463 - Fuel level sensor 1 Circuit high voltage (history)
P1875 - Four wheel drive low switch circuit
U1026 - Lost communication with Transfer case control module
U1041 - Lost communication with electronic brake control module
U1064 - lost communication with Body control module

The TCCM makes sense as I have no 4wd, the BCM also because of the lights and key fob, there’s a lot there so it’s all connected somehow.

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You could reseat the connectors to each module. Also, check fuses and batter connections. Also, replace the battery if old. Check all frame and chassis grounds. When playing with connections, make sure battery is disconnected.

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