4 doors, 2000, 4 buttoms at the cluster (auto 4wd). Few days ago I put 2 new tires in the front axle. I did not have the cash for the whole set so I replaced only the front ones. At the rear axle I have just the same OEM tires but with 80,000 km on them and of course a smaller diameter. The 4WD department is like new in this truck as this Blazer rarely sees the dirt and everything used to work nicely (4 auto, 4H-4L). I engage the 4 auto to drive in a dirt road (not really slippery) and soon the 4wd service light was on, specifically after I did reverse and turning the wheel all the way. I tried few things and the light went off. I tried to fix the condition spinning the wheels a little on the dirt. After that spining the 4 auto and everything was fine. I naturally used 2H to drive home on pavement. The system is not fine anymore as I can only drive 2h and 4 auto. No way with 4 H and 4 L. I understand the bigger tires produced something (locking) that the 4 wd module identified as a service problem and 4 H is then not permitted. How can I (in the middle of paved roads for 50 km) produce the wheels "spinning" that following this analysis is then necessary to unlock the differential (front?). Am I correct to point in the larger tires as the culprits of this problem?.

 

If there is a significant enough difference in tire diameter, then yes, it can affect your 4wd system. Without reading the codes stored in the TCCM and reporting them to us, you/we really do not have an idea of what the TCCM is complaining about by illuminating the service 4wd light.

 

Here in Santiago the Blazers are not common. To get the codes I need to go to the official representative and their costs are usually expensive to my budget.
But the tires are the only difference, probably (1" in diameter). The 4wd system was perfect before and it practically have had no use or abuse.

 

Your options are to either get the codes read (usually dealer as you have stated) or put new tires on the back and hope that the difference in tire diameter was the problem.

 

Yes, I'm thinking in new rear tires too. Probably is cheaper than to intent the multiple different aspects that may be related with the service light and the lack of 4H and 4L. I have asked for a near dirt area to spin the wheels and see if this can be cured and I'll go there in a while.

 

After some few spinnings on gravel with 4 auto, I got 4H but can't the 4 L. Nevertheless the tires hypothesis seems to be initially correct. I prefer to spend in tires than in modules. Maybe I can be lucky in this.

 

As Kyle stated, it is possible to get an error with mismatched tires. You are also right that you should consider the tires before thinking about the modules, especially if you do not have access to a code reader.

If you have a 1 inch difference in diameter, you must either have very worn tires, or perhaps they are a different brand or make. (even though they are the same specified size)

The transfer case control module monitors the front and rear shaft speeds. It will set a C0376 Front/Rear Shaft Speed Mismatch if something is really off. Specifically off by 20%. If you have 235/75R15 tires, you have about 90.7 inch circumference. A one inch reduction in diameter only yields about a 4% change, hard to believe that is the cause all by itself. Note that the speed sensors themselves can have slight errors too, so that would add to the tire error.

Measure the circumference of the tire to compare, it is harder and less accurate to measure the diameter.

Then, ask around your friends who might work on cars if they have a good code reader that can access the TCCM. That will save you money. Or you can find one to buy for a little over $100 US and save next time you need a repair.

Lastly, be careful spinning tires aggressively, especially if the tcase has not completed a shift. You can shift on the fly, but a violent engagement is asking for broken stuff.

Good luck.

 

TKS for your guide. I tried to fix the problem putting one new tire per axle. That will produce the satellites to work but unfortunately it failed to gear in nothing but 4 auto and 2 H.
I call the Chevrolet official service and the computer reading only will cost me $70.
As I'm a mature driver I recall the times when the rough trucks used to have two handles for this 4WD stuff and not that fancy bottoms.

 

I never said that the tires were the cause of the problem, but good job on the diagnosis of that aspect of things.

These systems are quite complex. $70 should give you a clear picture of what the problem is. There are a few other things that you can check first though. You can remove all of the connections to the TCCM behind the passenger side kick panel, the encoder motor on the transfer case, and to the switch in the dash and check them all for corrosion and/or bent pins. You can also test the switch in the dash to make sure that it is putting out the proper resistance values for the mode selected. THIS POST details the resistance values you should see at the switch. It also has some links to other articles explaining the operation of the NV236/NV246 transfer cases.

 

Sorry, I didn't really phrase that properly, I typed fast. I meant to agree with you, if you were referring to my comment.