Hi guys. I've searched the threads about the 4x4 systems here and found a lot of great help so far. I have a '01 Blazer 4 button 4wd and was having problems getting 4x4 to engage correctly. After finding posts concerning the corrosion issue on the tccm I went to looking and found I also had the same problem. Cleaned everything up the best I could and didn't see any obviously messed up wires. Put it all back together and finally heard the tccm doing it's job. But, here's where the weirdness comes in. If I push 2hi, it goes to 4low, Auto 4wd goes fine, push 4hi and I get 2hi

Seems the switch panel isn't wired individually, I think that should rule that out. Could the tccm still be the culprit ?
My scanner doesn't go deep enough for these systems and really can't afford to pay the Chevy stealership to do it on top of buying parts. What else should I be looking for ?

 

very possible on the tccm. I have a 1995 and it was dropping into 4wd and 4w low by itself even at 60 mph. my neighbor is a head of service at out local dealership and he had it for an hour at his house. he had the box that holds the tccm out of the kick panel. with engine running he moved the box and the damn thing went in 4wd by itself. he took the board out and found a loose soldering joint / part etc. he found me a brand new one from a dealership in florida on ebay. guess they don't need 4wd down there. got it almost half price with shipping. the truck hasn't had issues since. good luck with yours. hope this helps.

 

could it be the dash board switch? they're cheap maybe $25

 

If it were the switch in the dash, it's almost like someone would have changed the poles somehow. I had the switch out and looked at the board in there and it looked ok from what I can tell. I do need new glasses but seen no obvious problems lol.

The tccm however did have at least one resister that the solder joint looks odd. Can't find my magnifying glass to get a better look so it could just be my eyes. I didn't see anything burned up, but at the low voltages these things see that might not be too obvious.
I'm just curious if the tccm could cause the oddball switching problem. We just got this Blazer thinking we got a decent deal, hoping I don't take it up the wazoo repairing it lol.
Nothing like my '71 GMC

I found a new tccm for $200 + $200 core locally. Of course it can't be returned if it's not the problem but I understand why. I've seen several different rebuild brands on the net, any ideas if one is better than the other ?

 

Each mode and range selection on the NV236 dash switch, sends a different resistance value to the TCCM, on the same wire, (circuit). The TCCM might not be receiving the correct resistance value due to corrosion or a poor connection on the signal wire. Or, there's a problem in the dash switch itself causing funky resistance values to be sent to the TCCM, or possibly a bad TCCM. This is where a bidirectional scan tool would come in real handy Short of checking resistance at the switch output, and TCCM input for each position, it may come down to having it put on a scanner to pinpoint the problem.

 

Is there a list of the known resistance values for each selection ? That I have tools to check here. It is possible that a wire or two are corroded near the tccm connector that didn't get taken care of. Going to a dealer to have it scanned will leave me without money enough to buy the parts, so I'm kinda in a corner. If I can find correct resistance at the connector I will feel much better about buying the new tccm.
This is the reason I really didn't want anything this new but my wife liked it, you know how that goes. I prefer my vehicles without any electronics to go wrong lol

 

Please view the third schematic down in post #3 of the following thread:

https://blazerforum.com/forum/2nd-ge...-blazer-89044/

The TCCM sends an 8 volt refence signal to the Shift Control on circuit 431. With no buttons pushed, this 8V reference signal goes through a resistance at all times (even with no buttons pushed). This would return a constant signal to the TCCM of some voltage less than 8V on circuit 1693. When you press different buttons, different additional resistors are temporarily put into the circuit in parellel to the constant resistance mentioned in the previous sentence, resulting in different signal voltages on circuit 1693 when each button is pushed. The TCCM interprets each voltage returned to it as a certain button being pressed - or no button pressed. Exactly what voltage means what to the TCCM, I have no idea - but if the correct constant base resistance in the controls is not there or is shorted across, that would throw all the rest off and I could see things working but going to the wrong place as you describe.

I guess the bottom line in home repair strategy (without a Tech2 scanner to aid diagnosis)...... Try testing voltage on circuits 431 and 1693 with a digital voltmeter to see if anything is obvious with the wiring or function of the switch (do this with TCCM plugged in). If nothing becomes obvious, then replace the controls because they are quite cheap. Then if that doesn't fix it replace the TCCM.

FYI I recently got a Cardone TCCM for 4 button 2001 Blazer from Summit Racing for $115 + $130 core - free shipping through price matching with another online source. Remember that Summit will adjust to beat total price to your door including the core if you call customer service and tell them you want to do a price match. Be prepared to give the link for their competition and make sure it is exactly the same manufacturer part#. In my case, Summit could not lower the price of their core to the $100 core charge listed by the other company, so they lowered the price of the actual part $30 less so total was still $245. See how that works! Then they drop shipped from the manufacturer. Far, far better price for me once I returned the core (with core shipping also gratis from Summit - I did have to call them to get a return shipping label for a drop shipped part).

I like Summit and Jegs for normal (non-race) parts as they will both price match and get it to me next day if they have it in stock - plus their return policies are much better than the smaller not so well known internet sellers. After all they do have a reputation to uphold. I use them whenever I can, unless stuff is cheaper locally.

Good luck,

Les

 

Here you go - see post #24 for resistance values.

Note the schematic referenced in this post actually has the resistance values listed for each resistor in the control. (my schematic did not). Resulting resistance when each button is pressed is calculated as described. I would test this at the TCCM connector (disconnected) so that the wiring is included.

https://blazerforum.com/forum/2nd-ge...d-24964/page3/

Let us know....

ps. You are probably going to need to check/clear codes in the TCCM after your repair. I have the raw commands to put in a OBD2 terminal to check and clear TCCM codes (NP8 auto 4WD only), as long as you have the hardware and software to do it. Looks like $29 will get you both if you are interested. See other 4WD thread I'm posting in right now. Need a Windows laptop with USB port. Don't know how software will run on Win8 up but probably will be OK. Mine is installed on WinXP in an old Asus Netbook.

Les

 

Based on the resistance info, if the internal resistance for that apparently malfunctioning button when pressed was 850 ohms too high - this is exactly what you would observe! Guess problem is not as strange as it seems!! Just figure out why this is taking place, fix it, and you're golden! Sounding more and more like a selector switch problem but could also be high resistance in wiring, connectors, or even in TCCM I suppose.

Les

 

Thanks guys, lots of good info.

Lesmyer, I just went out to test the switch and all the values were spot on, then plugged the switch back in and tested again at the tccm and the values were still spot on. Kinda glad although it may end up costing more money, I hate fixing corroded wiring lol.