Hello all,


My apologizes if this has already been discussed. I completed an extensive search and could not locate anything on it.


I have a 2001 Blazer LT 4x4 which is having a "Service 4wd" light issue. To begin with, the 4wd system works perfect (so long as the light stays off). When I push the 4HI button, it does what it is suppose to do which is go in to 4HI. I then begin to move the vehicle forward and then about one half mile later, the "Service 4wd" light illuminates. Once the light appears, the system freezes like an old computer. The light on the switch is steady for the 4HI and no other light is on. If I push 4LO or try to put it back in 2wd, nothing happens. The light for the pushed button does not illuminate at all. The truck does not try to engage in any other mode. It essentially "frozen" in time. It stays in 4HI until I turn the vehicle off and restart it. Once I restart the vehicle, the service light disappears and I can now engage in 4LO or back in 2wd as normal.


If go from 2wd to 4wd Auto, it will work as normal until I move. Immediately after the wheels begin rolling, I hear a click noise from behind the glove compartment (where the module is located) and the service light reappears and it is again frozen in time. I turn it off and restart the vehicle and I can change the modes normally again (so long as I don't move the vehicle).


I took the vehicle to a transmission shop who in turn scanned the light. They informed me the transfer case speed sensor was faulty so I bought a new one and replaced it. While I was under there, I replaced the speed sensor for the transmission as well. This did not solve my problem. The plugs looked to be in good shape.


Common sense tells me since I can hear a click from the module, I probably should test it and see if it is malfunctioning. My question is if the module is in fact the issue, can it cause a false error code or did the transmission shop not know what they were talking about? Any help will be greatly appreciated.

 

UPDATE:


I did another test drive to with windows down this time. I placed the 4wd system in 4HI, same thing. I reset and put it in 4wd auto. I moved forward and could feel the 4wd engage. It then disengaged it self and the light appeared and of course froze. Best way to describe it is the vehicle went in to 4wd mistakenly thinking it needed to go in to it because it thought I was losing traction but then realized its mistake and disengaged but became confused so it just froze it so it can't do anything else stupid. Maybe the position indicator sensor?

 

I am having different issues than you, but in my research I read that many people were able to resolve problems by removing the ATC fuse in the engine compartment for 30 seconds. There seemed to be a dispute as to whether the ignition needed to be on or not, so you had best research how to do this if you decide to try it. Another common problem was corroded terminals to the unit under the passenger kick panel.

I found the information confusing as to GMCs prior to 1999 and those post 1999. Most of the information and problems seemed to relate to vehicles that were pre-1999 and did not have the auto 4wd selection.

Here are a couple of links I fund useful:

http://www.kylestubbins.com/node/11

http://www.automotiveforums.com/t572...p_blazers.html

 

UPDATE:
AAMCO Transmissions did a complete diagnosis for free. They informed me the exact same thing, bad speed sensor. However, they noticed the sensor I put in was brand new so they told me I had a short in the wire somewhere. Now I am in process of testing the wires and trying to find the issue. I really do not want to replace the entire wire but I guess it could be worse. On a side note, they informed me of two other codes were showing on the OBDII. Now I'm replacing the O2 Bank 1-1 upstream sensor and the fuel pump. I really do not want to spend over $200 to purchase a fuel pump and sending unit at a parts store, is there any place to get AC Delco or Delphi for cheaper?

 

OK, I have two pins on the TCCM which were corroded pretty bad. One of those pins was loose, felt broken. Off to replace the TCCM in hopes that is all it is.

 

Replaced the TCCM, same problem. Took apart the harness and cleaned the female connectors good. It worked for a day perfectly but today, it shorted out again but now my selector switch is out. I'm certain the harness needs to be replaced because it was corroded so bad. I'll update when I get the new harness installed.