Hello everyone!

I have this weird problem that if I put my car in reverse, then I get the SERVICE 4WD. If I don't use reverse, then everything works fine, all wheels turning, low-gear, all good. As soon as I pop in reverse, it's only 2WD.

Any guesses? What does the car do differently in reverse? Will it open some locks? Or close them?

Thank you!

 

The 4WD itself works fine, no issues with that. Only the message comes up with the reverse gear.

 

Would anybody have a wiring diagram to share for the 4WD system?

 

No diagram (simple google search provided a few different ones), but could be a number of things, the 4WD actuator could have some corroded contacts, the diff locking ring wires could be broken or connector is corroded, the switch itself could be shorting. First thing you need to do is get a scanner/tech2 to see what the truck is reporting exactly. I had a similar issue and mine ended up being the diff locking ring wires corroded and broke form the inside. I had to remove the diff cover and rewire from inside the diff out, huge pain in the ***, but doable without removing the internals

 

The thing is, everything works really well. Except when I put it in the reverse, SERVICE 4WD.

But actually, I think it fixed itself now. I wanted to really understand the exact moment when the light comes up - do I need to move or just when selecting gear or... Anyhow, I can't get it appear anymore. The more I tried, the harder it was to get the message. I still don't have the diagram, but I believe the car does something differently in reverse. So now, just like the mode switching toggle trick does its magic, I think there's also a toggle reverse gear trick.

But a new problem has risen. Everything works well, except I've lost the LED lights on the buttons. I did many things at once. I had the dashboard and everything open for some work on the HVAC, and I also checked the wires underneath. I don't know what it was, but the LEDs are not working. Buttons themselves work fine. So I guess I still need the diagram.

 

Forgot to mention, the neutral LED works.

 

The TBC 2A or sth fuse was out.

Anyway, the SERVICE 4WD still persists. I finally got a DTC. C0388 - Rear Axle Differential Lock Circuit.
I have checked the following:
The diff lock coil resistance is around 3.6 Ohms.
When first measured, I got 3.5V coming from the transfer control module to the coil. When I measured it some hours later and while driving, I get 0V. This is without the rear diff lock.
With the diff locked, I get 12V.

I only get the error in reverse gear, sometimes switching, sometimes when driving, but only in reverse.And the diff lock needs to be not engaged. When in low gear and lock enageged, there is no error.
If I use a resistor instead of the coil, I don't get any errors.
While driving, with my not so expensive multimeter, I get around 2.3-3.8 Ohms on the coil (with peaks 1.5 to 4.8 Ohms).

I still don't get it. The service manual asks to change the transfer case module, but it seems wrong when everything works fine going forward. I don't understand what happens with the car in reverse anyway? I don't see it engaging the rear diff lock.

While getting the SERVICE 4WD, the rear diff lock button starts flashing also.

 

Would anybody know, if I put 12V on the coil, should I hear a sound? I don't.

 

A coil itself is just an electromagnet and is silent. What should make a click will be whatever it is that the coil is actuating which is generally a rod within the center of a coil (some are not a rod like in a relay the coil pulls a small plate to one side). If there is some resistance than in the most basic sense a coil is not totally broken. If totally broken a coil will have infinite resistance (but a coil can be shorted as well).

 

I agree with MixMan, if you are getting proper resistance you are making circuit; at least during that time you checked it. If it's not happening 100% of time then it sounds like a temporary circuit failure most likely due to a corroded contact somewhere within the axle diff lock circuit or diff lock itself.

The truck does do different things in reverse i.e. i've seen the truck not disengage 4hi-lock (front diff lock) until I've put the truck in reverse; so it could be the computer is probably re-checking results when you shift from drive to reverse.