Broken 4WD engagement - what's likely broken?

stm82, in my first post, yesterday morning, right after the problem happened, the 4wd light was stuck on. After sitting overnight and checking it tonight, the light is off and toggling the switch does nothing. I drove it and don't feel the 4WD engaged (usually I feel tightness when turning on dry pavement, it steered normally tonight).

I don't have a shop manual, otherwise I would check the schematic.

 

Well pull the actuator on the tcase and go from there.

 

Possible that since you parked it enough pressure was relieved so it could shift back into 2wd, unlikely but possible.

 

@Stm82: The light was stuck on for an hour after the 'event', but after sitting for a day, the light became 'stuck off' (no amount of button pushing causes the 4WD light to come on). Your logic (t-case first, then diff) makes sense. I am trying to get a wiring diagram today.

@JasonLee: You said to me "..... you just pushed the button and didn't shift the transfer case to/from LO to HIGH. The only mechanical movement in this case is that the front differential "ADD" Auto-disconnecting differential engages the sleeve that connects the CVs to the differential.....". Are you sure that the t-case actuator does not engage/disengage the front driveshaft when the button is pushed? It seems that the button might actually engage both actuators (t-case actuator to engage front driveshaft and then the diff actuator to engage axle shafts). If the button did not engage the t-case actuator, then the front driveshaft would be spinning all the time. That doesn't seem right.

It will be too rainy/snowy to be under the truck in the dark tonight, so my under-truck diagnostic work will need to wait until later in the week.

 

Look into gray wire mod it's pinned on main page and has all the 4x4 wiring diagrams

 

How's it going, @dcubed ?

 

@JasonLee, Despite my lack of activity on this thread, it is not evidence that I died over the winter. Rather, I found a super-inexpensive Subie Forester last December, so I bought that to drive in the snow throughout the winter and wait until this spring/summer to fix the Taco. However, the Subie only got a sticker valid through March because it had an evap leak. Once I fix the evap leak in the Subie, it will be legal to drive again, and I can park the Taco to work on it. Stay tuned, it may be a few weeks before I get back to the Taco. With no snow in the forecast, I am in no hurry.

 

I am finally getting back to the 4wd repair after driving it as 2wd vehicle for more than a year. The 4wd selector button still does nothing (no lights, no sounds). I did as Stm82 suggested, jacking, spinning and holding wheels.

When spinning either rear wheel, the front prop shaft does NOT turn. This tells me that the transfer case is free and not stuck in 4wd. It may not be actuating into 4wd, but at least I know it's not stuck in 4wd.

When I hold the left front wheel and spin the right front wheel, the front prop shaft DOES turn. This tells me that the truck is stuck (actuator fork in the right/passenger position, engaging the right/passenger wheel with the front prop shaft).

Also as Stm82 suggested, the next step seems to be to pull the front actuator. I can easily get to the bottom two bolts. The top two bolts seem difficult to get to. I also am concerned that even after removing the bolts, that there is not enough space to remove the actuator. I am going to do more research before proceeding. Any tips would be appreciated.

 

I have removed the front ADD actuator. When removed, the fork was in the driver side position (passenger wheel engaged) and the sleeve was appropriately aligned in the driver side position (passenger wheel engaged). The ADD sleeve in the diff tube slides freely left and right. When the sleeve is in the engaged position, both wheels seem coupled to the diff. When in the disengaged position, the passenger wheel is disengaged from the diff.

I don't know why the ADD actuator was in the 4WD position when I removed it, but perhaps that's an effect of my problem, and not the cause. Since I have the actuator off, I want to test it. Does anyone know the pin-out to drive the actuator motor to the two positions? It has the standard 6-pin connector.

If the ADD actuator is good, I am thinking of setting it to the 2WD (disengaged) fork position when I re-assemble it.

I also assume that it's going to test fine and that I will move to the transfer case and find a problem there.

 

ADD actuator is bad. It has trouble moving to the disengaged position.

TECH TIP: pins 2 and 6 operate the ADD actuator motor. The two different polarities applied across these two pins engage and disengage the actuator (2+/6- = engaged, 2-/6+ = disengaged. Pins 3 and 4 are the actuation position switch poles: open (disengaged) and closed (engaged).

I think that with the ADD stuck in the engaged position and with the transfer disengaged, the 4WD ecu is precluding any 4WD operation, hence no light or action when I hit the 2/4 button.

Next step is to replace the ADD actuator.

 

Mission accomplished. Bought a new diff actuator and replaced it. 4WD works properly.

Best deal on the OEM actuator was from Magengines.com from the UAE. Just over $200 including shipping, it arrived in less than four days. I was a little hesitant to order from them, but I am very glad I did. I am a little confused as to why the actuator arrived in the 4WD position. I would assume that the default install should be in the 2WD position; I certainly needed it to be in the 2WD position. Luckily I only needed to apply battery voltage to two pins to move the fork to the 2WD position.

Here's a summary of my ordeal and its mitigation:
- I broke my 4WD, hearing a loud clunk, when hitting the selector button with front and rear wheels at different speeds (rear wheels spinning as I was trying to climb a steep icy driveway).
- After the clunk, the 4WD light was on, though I was not in 4WD. I was able to drive normally in 2WD, even though the 4WD light was stuck on. The light went off the next day after being parked overnight, never to come on again until the repair was complete.
- jack stand investigation showed that the front diff ADD was stuck in the engaged position (passenger wheel half shaft engaged to the diff). In this condition, the truck was effectively in 2WD because the transfer case was in 2WD (not driving the front prop shaft) and the open front diff prevented engagement of the front diff to the prop shaft.
- I removed the front diff ADD actuator and applying (+) and (-) voltage to pins 2 and 6 showed that the actuator would not reliably move to the disengaged (2WD) position.
- I disassembled the actuator to find a broken tooth on the rack gear that drives the fork. The broken tooth was loose in the gear-train and caused binding as the actuator was trying to move to the 2WD position.
- I hooked up a new actuator, in the disengaged (2WD) position, without installing it in the diff. Hitting the 4WD selector button showed proper actuation to both 2WD and 4WD positions and lighted the 4WD indicator on the IP.
- I installed the actuator in the diff and tested the system to find that it was back to normal.

Clearly, the clunk I heard was the broken tooth in the front diff actuator. I am not sure why the transfer case did not engage or why it did not stay engaged, but clearly the 4WD ecu detected a bad situation and disabled 4WD. My guess is that the truck tried to put the transfer case actuator and the front diff actuator into 2WD after detecting a problem. Only the transfer case actuator was able to execute the command, and with conflicting sensor inputs (transfer case = 2WD, front diff = 4WD), the 4WD ecu prevented 4WD actuation. Only after replacing the front diff actuator, in the 2WD position, did the ecu see a safe condition and allow the actuators to be engaged.

I hope this helps you if you have a similar problem. The big tip from my situation is not to assume that you have a transfer case actuator problem.