Wheel Lock

[FMS88]

Hopefully you're only missing the cap. If so, you should see six allen wrench bolts that affix the lock hub to the wheel hub (circled in red in the illustration below). Also, the drive shaft end is recessed about 3/4" within the lock hub housing.
Auto Hubs.jpg
If there's no lock hub, the drive shaft will protrude outside the wheel hub and you'll see a ring with 12 holes, four with phillips head screws, surrounding the drive shaft.
Auto Hubs_1.jpg
If the hub is not unlocking, the Wheel-Lock light in the gauge cluster will not turn off. You can also put it in 2wd, use the unlock the hubs procedure, and then raise the front wheel in question. When you rotate the wheel, it's drive shaft will not turn.

If you're just missing the cover and it doesn't look filthy inside, flush it with WD40, push new grease in as best you can and screw on a replacement cover. This will probably be fine, especially if you found it engaged/disengaged per above. If there's a lot of grit or contamination, removal, disassembly and cleaning is recommended. If you're not comfortable doing that, best to take it to a reputable shop.

The Aisin hub is the only manual one I know of that's still available. But look at the 4x4wire.com and pirate4x4.com forums. Some members have raiders and monteros, and may discuss hubs other than the stock and Aisin brands.

[MrPaco]

If the hub is not unlocking, the Wheel-Lock light in the gauge cluster will not turn off. .

The green light on the gauge cluster does go off when I put it back on 2H. So does that mean that the hub is functioning correctly?
What exactly controls the light on the cluster? Is it the transfer case? the front axle? Is it possible that it is getting "wrong information"? In other words, how reliable is the light on/off status as an indication of whether or not the 4x4 is working and engaged (or disengaged)?

And thank you for that diagram, very helpful. I will take a look at the wheel today when I get the truck back, and see how it compares.

[FMS88]

The green light on the gauge cluster does go off when I put it back on 2H. So does that mean that the hub is functioning correctly?
What exactly controls the light on the cluster? Is it the transfer case? the front axle? Is it possible that it is getting "wrong information"? In other words, how reliable is the light on/off status as an indication of whether or not the 4x4 is working and engaged (or disengaged)?

In short a control unit illuminates the green "Wheel-Lock" light when the transfer case is in 4H or 4L, a sensor on the transfer case detects rotation of the front drive line, and forward speed exceeds 3mph. Since there are no sensors on the locking hubs, the control unit "assumes" that if these three conditions are met, the hubs are locked. But this assumption can be wrong when a hub is faulty, so you're concern for wrong information is legitimate.

Your earlier test with the truck on a lift and noting the front tires rotation status before and after engaging and disengaging 4wd is the best and easiest test. If everything is as expected, the light and the locking/unlocking hubs are fine. I don't use 4wd much, but in 29 years and 160k miles, I've had no problems with the hubs. (The light has flickered but that was an electrical connection issue.) While doing other front end repairs at 130k, I disassembled, cleaned and lubed them. Internally they looked fine with lubricant that wasn't caked or dried.

If what you can see with the covers removed looks clean and properly lubricated, I don't think you have much to worry about. But if you're still concerned and plan to use 4wd a lot, have them disassembled, cleaned and lubed or replaced with manual hubs, if you want to go manual.

[MrPaco]

In short a control unit illuminates the green "Wheel-Lock" light when the transfer case is in 4H or 4L, a sensor on the transfer case detects rotation of the front drive line, and forward speed exceeds 3mph. Since there are no sensors on the locking hubs, the control unit "assumes" that if these three conditions are met, the hubs are locked. But this assumption can be wrong when a hub is faulty, so you're concern for wrong information is legitimate.

Your earlier test with the truck on a lift and noting the front tires rotation status before and after engaging and disengaging 4wd is the best and easiest test. If everything is as expected, the light and the locking/unlocking hubs are fine. I don't use 4wd much, but in 29 years and 160k miles, I've had no problems with the hubs. (The light has flickered but that was an electrical connection issue.) While doing other front end repairs at 130k, I disassembled, cleaned and lubed them. Internally they looked fine with lubricant that wasn't caked or dried.

If what you can see with the covers removed looks clean and properly lubricated, I don't think you have much to worry about. But if you're still concerned and plan to use 4wd a lot, have them disassembled, cleaned and lubed or replaced with manual hubs, if you want to go manual.

I got the truck back last night, and it's running well. The guy who worked on it said the locking hub (which is missing the cap) didn't look too bad and he improvised a cap out of cardboard and foil tape, until I can find a replacement. I haven't taken a closer look behind the cap he made, but will as soon as some of this snow/ice we have around here melts.
I was able to test the 4x4 on an icy parking lot, this is what I observed: when I stepped on the gas in 2H the back wheels were spinning and the truck wasn't really moving forward much, then I switched to 4H (green light comes on) and when I stepped on the gas the truck moved forward like a champ and seemed to be gripping the icy pavement with no problems. When switching back to 2H it was back to spinning wheels. So I guess at least this tells me that when in 2H only two wheels are engaged, and when in 4H all four are, so that's good.

The one thing I'm not able to tell is whether or not the hub is disengaging after returning to 2H (and backing up a short distance), so I guess I don't know if the front axle is still rotating.

FMS88, is this assertion correct? Based on your explanation above one of the three factors that turns the green light on is "rotation of the front drive line", is this different from axle rotation? As soon as I returned to 2H the green light goes off, so I'm guessing the sensor is no longer detecting rotation of the front drive line, but it's not telling me anything about the front axle rotation, right? (if these two are in fact two different things, which I don't know, and I guess is what I'm trying to ask).

Thank you for time helping a noobie figure this pretty basic (I'm sure) function.

[FMS88]

From your description, it looks like the hubs are engaging and disengaging properly. Other than finding a cap, I don’t think you have anything to worry about. Post a thread in the parts Wanted section to see if anyone has an extra or oneleft over from a switch to manual hubs.

Regarding your question about axle rotation, thanks to the front differential, any rotation of the driveline will result in axle rotation. Basically, you can’t have one turn without the other turning. So from the light’s perspective, driveline and axle rotation are one and the same.

I hope this was of help. Enjoy the snow (in 4wd).

[MrPaco]

From your description, it looks like the hubs are engaging and disengaging properly. Other than finding a cap, I don’t think you have anything to worry about. Post a thread in the parts Wanted section to see if anyone has an extra or oneleft over from a switch to manual hubs.

Regarding your question about axle rotation, thanks to the front differential, any rotation of the driveline will result in axle rotation. Basically, you can’t have one turn without the other turning. So from the light’s perspective, driveline and axle rotation are one and the same.

I hope this was of help. Enjoy the snow (in 4wd).

Yes, this has been immensely helpful!
I posted a thread looking for a cap, hopefully someone has one laying around... otherwise I'll have to get lucky at a junkyard. Thanks for the tip

One last follow up question on this, and I promise to move on...
On your comment about the axle and the driveline rotating together, I understand what you mean. But I read somewhere, cant remember if it was in this forum or not, that if the hub wasn't properly disengaging then the front axle could still rotate with the wheel, even when the 4x4 is not engaged, thus making the truck work harder and use more gas. Is that something that can happen in other vehicles, but not on this one because the axle and driveline always rotate together on these?
I'm overthinking it, I know...
Anyway, I trust your opinion, you sound like you know what you're talking about, if you think it's operating as it should (based on what I've been able to describe), then I feel good about it.

Thanks again, and yes, I'm enjoying the snow!

[FMS88]

One last follow up question on this, and I promise to move on...
On your comment about the axle and the driveline rotating together, I understand what you mean. But I read somewhere, cant remember if it was in this forum or not, that if the hub wasn't properly disengaging then the front axle could still rotate with the wheel, even when the 4x4 is not engaged, thus making the truck work harder and use more gas. Is that something that can happen in other vehicles, but not on this one because the axle and driveline always rotate together on these?

I understand what you're saying and you're right about, how do you really know. A hub that failed to disengage or, if manual, left in the locked position would cause the wheel to rotate the axle, differential and drive line. Since I've never had an auto-hub fail, I can't speak from experience, but I suspect the auto-hub control unit would not turn off the Wheel-Lock light if a stuck hub caused axle rotation after shifting to 2wd. If my suspicion is unfounded, then the only way to know for sure that a hub failed to disengage would be to shift to 2wd, execute the procedure to disengage the auto-hubs, raise the front wheels, rotate each wheel by hand and observe if the axle turns or not. If the axle doesn't turn, the hub is definitely disengaged.
I think you're fine, but it never hurts to question and explore the possibilities when your goal is to understand. Enjoy!