Just replaced my motor encoder in my 99 Blazer and all seemed well but truck seems to have lost alot of power and keeps locking up the torque converter even in 2WD. Also will not downshift when up to speed and mash down the throttle. No 4WD light. Does engage 4WD and will go back to 2WD. Disconnect the TCCM and truck shifts fine.

 

Sounds like the encoder and/or TCCM is sending the signal to the PCM that the truck is in 4LO which alters shift points.

What transfer case do you have in your truck?

 

i believe its a 236

 

4-button switch on the dash with Auto4wd? If so, then yes, it is an NV236.

The NV236 TCCM applies +12V to connector C1, pin *F7* (gray/black wire) when 4LO is engaged. If the TCCM is somehow screwed up such that it puts +12V on this wire all the time...

But before I go further off on that tangent... How does it shift while getting up to speed initially?

*Correction* - Pin F7 (gray/black wire) for the NV236. Forgot that AllDataDIY has the NV233 & NV236 wiring schematics flip-flopped.

 

I think you mean Pin F7. I did see that and was going to check it. That wire goes to the VCM. Actually the VCM supplies 12V and the TCCM grounds it and detects the voltage. Manual says the TCCM should detect when its in the wrong state and set the Service 4WD. Likely its a faulty TCCM. One thought was to disconnect it and see if the problem goes away but then I cant use 4LO. What about the feedback from the four wheel drive front wheel lock? I dont recall the Service 4WD light on throughout the drive cycle but may need to confirm that again.

If I accelerate slow it dumps through the gears quick and bogs down the motor. You can get up to speed with some finesse.

When I started the truck it was in 2HI and I felt there was loss off power.

Do you know if the dealership can test the TCCM or reprogram it? Is there anyone out there who tests them?

 

The quick shifting is what drew me to the 4LO trigger. You could try cutting that wire or just pulling the terminal from the connector at the TCCM

Yes, sorry, I went through AllDataDIY and forgot they have the NV233 & NV236 diagrams mislabeled. It is C1 F7 for the NV236 (full 4wd wiring attached).

I do not believe that the TCCM is programmable.

 

Thanks for that diagram. I didnt see that in my service manual but it must be there somewhere. You can be sure I will post the outcome. This system seems to plague many owners.

 

Bingo! Removing that F7 wire fixed the problem. Just ordered another junkyard TCCM module for $26.95. Actually will try to repair this module I have. Dont want to open it up until I have a good one. It was warrantied by the junkyard but I figure it can be fixed. Likely some corrosion causing a short to ground. Just replaced my passenger hinge pins this AM too. Hope that solves the water getting on the TCCM. Its definitely a leak somewhere. Rainwater was all over the original module and shorted it out. Connectors were corroded as well. Cleaned them up and applied dielectric grease. Tried the junkyard TCCM after it cooked. Still had no lights on the buttons and SERVICE 4WD on. The original problem was the encoder motor. Got the encoder motor at the junky for $39.95. Might just open up the failed one to see what took it out - likely weather. Also changed to the blue transfer case fluid.

Just another one of many!!! At least there all well documented.

 

Glad that narrowed it down! The direction that I took at first seemed like a stretch even to me when I read back through the thread, but sometimes my mind works in ways I can't even explain.

 

Can you add it to your Common 4X4 problems article. I searched alot and only saw one mention about that behavior but no solution was reported. Did make an appt for a diagnostic at the dealership but will cancel tomorrow. Seems alot of posts have no outcome. My initial problem was no indicator lights on the 4WD drive selector switch. I checked the voltage from the TCCM and the return voltage when the button was pushed. That input to the TCCM was working. This should point to the interaction between motor encoder and TCCM. I found very little info about checking the encoder signals. Especially when you cant shift. I would have liked to confirm at least that the encoder was reporting to the TCCM that I was in 2HI. I also wanted to know how to bench test the encoder motor I received from the junkyard. Was wondering what gear the junkyard motor was positioned at. I have not seen any diagram indicating mechanical position vs electrical signals out.