I'm glad you took it to a mechanic before posting the question. I'm off work today so I'll take the time to post.

So you're saying it went into 4L all by itself? No one turned the switch?
Does it drive properly in 4L? Shift through all the gears?

Can we assume you have gone through the extensive diagnostic procedure in the factory service manual? It's an absurdly complex system.

If the auto trans works, even in 4L, I find it hard to believe the TCM is faulty.

Looking at the circuit the only way I can see that it might possibly shift by itself is if the position feedback circuit in the transfer case actuator had dirty contacts. Is there any damage around that area or possible water ingress?

If no other solution is found I don't see why you couldn't shift it manually back into 4H, disconnect the actuator and ignore the error.

 

Thanks for the quick reply. Yes it shifted all on its own. I drove it around the block and it shifts through the gears just fine.

Yes, the mechanic said he want through the diagnostic procedure in the service manual. I can only assume he did everything that it calls for.

I checked the position sensor before I dropped it off and I didnt notice any damage.

I didn't realize I could switch it back into 4H manually. I will have to check that out.

I should note also that the Transfer Case, 4 LOW, Neutral and ESP lights are all blinking non stop at about twice a second.

The mechanic said he also called the Suzuki service line. According to them it could also be caused by a faulty 4WD control module, but that they couldn't tell for sure without and known functioning TCM.

 

Sounds like a trip to a wrecker would be the way to go.

Or before that, check all the connectors are in place.

I accidentally unplugged one of my connectors to the gearbox module, located in the kick panel in the foot area, and when I put it back in, I didn't get a locking clamp clicked properly. The car drove without errors, but shifted like a mad man.. I checked everything, and found the clamp slightly loose, got it locked in place, and the error went away.

I would be surprised if a module would just "die" out of the blue, it just doesn't seem right. Before spending any money, I would check everything from the BCM/ECM to the module in question.

 

If you place the switch in 4L is everything normal?

Is taking it to a dealer an option? Unless your mechanic has a Suzuki scan tool there are proprietary codes that should nail the problem down.

IMO, the fact that it changed itself points to the 4WD control module and/or actuator/wiring as the module is the only item that has closed loop control over the shift actuator motor. All other hardware inputs to the module are for mode selection and to report current status, and all other communications are serial (CAN,) none of which are going to override the dash selection switch. I would zero in on the actuator connector as any added resistance could fool the controller.

The actuator motor turns a cam to each of four positions. It has redundant resistive feedback at those four positions and two additional switches to verify H/L and unlock/lock. You could power the motor from 12V momentarily to see if that loosens it up, or even leave it at the 4H position if you can figure out where that is.

 

Are you sure that it actually IS in 4L?

Does it feel like the gears are 50% reduced, and can you feel the diff is locked?