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2001 Blazer troubleshooting the TCCM


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Howdy folks,

I have a 2001 Blazer, electronic 4 button 4wd, automatic transmission, 200,000 miles.

 

The car shifts way too early, hitting third or fourth gear when I only get up to 10mph in 2wd. If I shift manually, it will shift properly. This problem started about a year ago, and before then, if shifted fine. The four wheel drive system is fully functional. I have a malfunction somewhere in the four wheel drive circuit causing the VCM to think it is in 4 Lo when it is really in 2wd. It has thrown a P1875 code, consistent with a fault in the 4Lo circuit.

 

If I pull the ATC fuse or unplug the TCCM, the problem goes away, and the transmission shifts perfectly normal. The TCCM did have some corrosion on two of the connections in the orange 32 pin plug. I cleaned them well and it did not help.

 

If I probe the 4 Lo circuit coming out of the 32 pin plug on the TCCM (circuit 1694, gray/black wire), it reads around 12 volts in 2wd, and almost zero volts in 4 Lo. From what I understand, this is normal functioning of the TCCM. If that wire was grounded somewhere, or if the TCCM was sending a constant 4 Lo signal, I would expect to be reading zero volts in 2 wd, which I am not. With this information, and the fact that the four wheel drive system is fully functional, I am lead to believe that the TCCM is not bad.

 

I did not find them relevant, but just in case, I did pull several other codes from it:

C0265 - electronic brake control module motor relay circuit

P0101 MAF sensor

P0122 - Oxygen sensor

P0410 - secondary air injection

 

So now, I don't really know where to look next. I can't seem to find much information other than what I have already said and done. I also can't find a detailed wiring diagram of the 4wd system. If anybody has any ideas to help troubleshoot this, I would greatly appreciate it!

 

 

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Try cleaning both sides of any connection that is corroded. A bad connection can give you a good reading on one side especially if your using an electronic volt meter vs. a cheap meter. Also if your want to keep the vehicle up see if you can clear the other codes. If not maybe you have a bad VCM. Were you driving with a check engine light before you had this problem? Also check all the connections to the VCM since the problem might be there especially if you have multiple codes.

 

My ABC has a separate light. Most likely the ABC relay circuit can be fixed by a rebuilt or fixed circuit board for a lot less money and headache than a totally new module that a dealer or parts house wants you to buy with no need to remove the hydraulic section. I did this using Google as my guide as have others.

 

A cheap fix is to use an inline switch and inline fuse for your TCCM. Just don't blame me if it does something bad.

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Check and make sure all of your codes are still set. In other words clear them and then drive enough to reset them as possible problems. I believe the easiest way to reset is to remove the black wire to the battery for a couple of minutes. Now drive around for a day or so and than check for codes again. If there are all the same try and fix the easiest and cheapest. If you are sure you can't fix it than get a new or rebuilt or junk VCM.

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Cable Guy, thanks for the information. Since my last post, I got a hold of a different wiring diagram and realized I was initially looking at the wrong diagram. At least I think I was. In case anyone is real savvy on the subject, my plug is coded with the letters "E" and "F", while the original diagram I was looking at has a plug coded with "C" and "D". To add to the confusion, there are two gray/black wires in the harness. The wire I was originally testing is the wire coming from the 4Lo switch on the dash

 

Unfortunately I have not had a chance to work on it since, but I am hoping that when I get a chance to test the actual 1694 circuit, it may lead me in the right direction. I will follow it to the VCM and check for corrosion on that side as well. I'll update with any new information.

 

Also, I like the idea of an in line fuse and kill switch for the TCCM. I thought of that myself, but I'm getting rid of the vehicle as soon as I get it in working order, its just taking up space.

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  • 1 month later...

Update on this thread:

After using the correct wiring diagram to find circuit 1694, I tested that wire to see what it was doing. When functioning in a normal fashion, it should read 12 volts in 2wd, and ground out when put into 4Lo. The wire was grounded out all the time.

 

Circuit 1694 runs from the TCCM to the VCM. I pulled the wiring harness off the VCM and tested the resistance of the circuit 1694; it was pretty much zero, so I felt confident that the wire was not broken or grounding out somewhere along the way.

 

At this point I was pretty confident that the TCCM was bad. If I had a diagnostic scanner, I could have confirmed that for sure, but I don't. So I went ahead and bought a new TCCM, plugged it in, and it fixed all of my problems. The TCCM cost me about $180 for a remanufactured part from advance auto.

 

Keep this one in the back of your when troubleshooting transmission problems on these systems. Remember, the four wheel drive system worked perfectly normal, yet the transmission shifted crazy in 2wd. The 4wd system definitely wasn't the first thing that came to my mind. The only reason I picked up on this was because I pulled the P1875 code. However, I had these problems for months before the code came up.

 

Hope this info helps someone else!

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