Hi Guys,
I got a problem with my 2004 Chevy Blazer. The "Service 4WD Light randomly comes up while driving in either Auto4WD or 4Hi.

Initial situation:
I bought my Blazer 3 months ago and unfortunately I didn't notice that the Auto4WD light was not working. It would switch though (the 2Hi light would go out and stay out). After a while (sometimes after a few minutes, sometimes not at all), the "service 4WD light" would come on and I wouldn't be able to switch back to 2Hi or any other mode. After turning the truck off and back on the light would be gone and and I would be able to switch modes again. 4Hi would engage and work. Because of the light in the switch not working I thought my dash switch would be defective.

Situation now:
A few days ago, the Auto4WD light suddenly worked again, but would just go out after a few minutes, followed by the "service 4WD light" lighting up. All lights would illuminate at start as they should and the other ones would not go out. Now the "service 4wd light" would also sometimes come on while 4hi is engaged. I took it to my chevy dealer and they ran some tests. In the end they told me that they could just find a general tccm fault code, but the when engaging the 4wd options from underneath the car (with a scanner or something) everything would work fine. They advised me to change the dashswitch and come back for further investigation if that wouldn't do the trick. So I replaced the switch yesterday and it seemed like it helped (Auto4WD light was steady, felt like it switched faster to Auto4WD), but then the Service light would still come up while being in Auto4WD as well as in 4Hi sometimes. I could feel both modes working as it's quite slippery in Winnipeg right now .

Does somebody have experienced something similar or knows something to do? I heard about the possibility of hooking up an OBD2 scanner and send certain commands via computer to get a more exact fault code than just by grounding pin 13, but I don't know if that will work with my version of the truck / tccm.
Best regards
Vince
P.S. Very nice board!

 

Grounding pin 13 doesn't work on 4 button 4WD (with auto). NV236 requires advanced scanner capability to do 4WD diagnostics. Dealer has one of the few scanners that will communicate with the TCCM. I have done it with Scan XL Pro in terminal mode but you have to know the hieroglyphics to send and how to interpret them - just to read the codes. Scanner at the dealer will command different 4WD modes, so it does a lot more.

Sounds like an intermittent problem with the TCCM as the TCCM is is what drives the lights in the display. Check for corrosion where the connectors plug in as this is a common problem. If corroded you will have to clean and reapply dielectric grease. Possibly TCCM could be bad internally at solder joints and require replacement. Sometimes bad encoder motor can damage the TCCM as well.

So if cleaning corrosion doesn't fix your problem, I would get a Cardone TCCM from Summit Racing and try it. It will drop ship to you from Cardone and Summit has a VERY good return policy. They also will price match with anyone on the Internet, so take advantage of this and call them.

I don't generally like trying parts, but this is a case where you can spend a lot of money at a dealer diagnosing, and dealer isn't going to take the parts back off after they try one! Without a Tech 2 scanner you are working at a definite disadvantage.

Hope this helps. Best wishes for a successful repair.

 

Hey,
Thanks for the answer! Do you have the right hieroglyphics by any chance? I thought of replacing the TCCM but what still bugs me is the fact that it works fine for a while after starting (switching modes, putting out torque to the front wheels when needed in Auto4WD) and also that the light just disappears after restarting. BTW I just had the dealer to a 4x4 checkup that was all, I replaced the switch myself (it fixed the issue with the Auto4WD light in the switch just going out randomly).
Best Regards
Vince

 

https://blazerforum.com/forum/2nd-ge...-blazer-89044/ see post#4

 

i swear most s10 4x4 issues lately ive repaird by just cleaning the connections at the control module in the kick panel of the passenger compartment. water and crud always seems to make its way in there.

worked on a trailblazer yesterday at work with inoperative 4wd, found a lot of people were able to repair the system light up but still no 4wd so found out the TCCM itself was bad

 

Hey,
first of all thanks for the answers and sorry for not answering myself. I'm currently travelling in the canadian north and it was just way too cold to do any work! I checked out your tutorial, but for some reason I wasn't able to get it to work. However, I found another method (I'm not sure if I am allowed to link it) and was able to pull three different codes: C0315, C0310 and C0550. Unfortunately, a google search for them doesn't really give me too much information so maybe you know what they mean.
Best regards and thanks in Advance
Vince

Edit: Doing further research I found out that I had the wrong message sent to the tccm so it send me all possible codes apparently. So today I did it again and deleted all codes and drove till the light went on again. I requested the codes and it only gave me C0374 and then the proper end code 00 00. Google tells me that this is the rear speed sensor circuit, but ai couldn't really find a picture showing me where the wires run, so I hope somebody can tell me.

 

These diagrams should work for location , ill see what else i have

 

Let me know if other cartoons are required.

 

Here is the link to C0374 OBD2 Code info.https://blazerforum.com/forum/diagno...plained-38960/

I found a TSB about speed sensor circuit giving C0374. This TSB is not named for Blazers but it is named for NP8 transfer case. Here it is anyway.
---------------------------------

TECHNICAL

Bulletin No.: 05-04-21-003C

Date: April 15, 2008

Subject:
Service 4WD Light Illuminated, DTC C0374 Set (Inspect Wiring Harness to Transfer Case Speed Sensors,, Replace Wiring Harness)

Models:
2004-2007 Buick Rainier
2003-2006 Chevrolet Avalanche, Suburban, Tahoe
2003-2007 Chevrolet Silverado Classic, TrailBlazer, TrailBlazer EXT
2003-2006 GMC Yukon, Yukon XL
2003-2007 GMC Envoy, Envoy XL, Sierra Classic
2003-2004 Oldsmobile Bravada
2005-2007 Saab 9-7X

With Four Wheel Drive or All-Wheel Drive

and Active All-Wheel Drive (RPO NP4) or Active Two-Speed (RPO NP8) Transfer Case

Supercede:

This bulletin is being revised to update the Model and Warranty Information. Please discard Corporate Bulletin Number 05-04-21-003B (Section 04 - Driveline/Axle).

Condition

Some customers may comment on intermittent illumination of the Service 4WD light. Upon investigation, the technician may find DTC C0374 set. The customer may also comment on intermittent erratic operation of the 4WD or AWD system after driving through rain/snow or simply going through a car wash.

Cause

The speed sensor signal may have become corrupted. Possible openings in the speed sensor wire insulation (twisted pairs) can allow water intrusion. Also wire connections contaminated by water may result in short circuits and erroneous speed sensor readings. This most often occurs on the rear speed sensor circuit.

Correction

Inspect the wiring harness to the transfer case speed sensors. On Rainier, TrailBlazer, Envoy and 9-7X models, fabricate a replacement speed sensor harness between the C101 connector and the speed sensors. Completely inspect and test all wiring. Refer to Speed Signal Front Axle Actuator and Indicators schematic in SI.

Replace the affected twisted pairs. Do not over-twist the two wires in the replacement harness. Wires should be twisted at a rate of 9 revolutions per foot. Use service connector pack, P/N 88987993 at the speed sensor end and terminal, P/N 15326267, at C101.

Terminal testing tools and service terminals can be found in Terminal Repair Kit J 38125. Terminals are available from SPX/Kent-Moore.

The smaller transfer case harness splices into the larger chassis harness a few inches in front of the crossmember. The chassis harness routes along the left side of the frame under the driver door area. Use nylon tie straps to secure the fabricated harness to the main chassis harness between the transfer case and C101.

On the full-size pickup and full-size utility models, replace the 2.2 m (88 in) pigtail harness that runs from the C151 connector under the hood to the transfer case. Use either harness P/N 15832722 or 15224663 depending on vehicle equipment. Refer to Propshaft Speed Sensors Front Axle Actuator and Transfer Case Shift Control Switch schematic in SI.

Important: Technicians should verify the integrity of the splice joints after the repair. All splice joints and connections should seal properly against water or a repeat condition can occur.

 

Can you please tell me more about "another method". I'm always interested in this stuff. You can PM if you like to send the link.