The hubs will not function correctly or at all if there is any grease compressed in the end. If you or your mechanic have a habit of generously coating the splines on the stub axle with grease, give it up. The only thing the grease is for is to keep the splines from rusting. In normal service, the only time anything slides on the splines is when you assemble the hub to the rotor.
Grease or accumulated crud in the hub will pack in around the coil spring that preloads the engagement mechanism towards the disengaged position. More than a tiny bit will prevent the hub from engaging completely, since it hydraulic locks the sliding part under the spring. That prevents the engagement cam from idling all the way up on the flat part where it is completely engaged and 'latched'. If it doesn't engage completely, it will go in and out depending on the load on the axle. Your "don't back up at all" observation is probably based on this problem.
| So what to do?
It is possible to disassemble the hubs, but there's a coil spring in the bottom that needs to be compressed while a wire retaining ring is reinstalled. Easy if you have an arbor press or even a drill press. Really tough if you don't. Once disassembled, the unit can be cleaned and inspected, but the only service part is that bearing in the end. Even that will need to come from a bearing store, as it's not available separately from Ford. It's also quite practical to lubricate the hub with a good ATF or synthetic gear oil. I use a 75W-90 oil that I keep for other things. If you live in a very cold (-20f and below) climate, use ATF. Anyway, clean out all the grease that you can with a paper towel and maybe a little paint thinner. Don't flush the lube out of that bearing-- just get as much of the stray grease and crud out of the thing as you can, Then pour a few inches of the oil or ATF into the hub, and let it soak for a while. I do this while the brakes are off, so maybe 2 hours if I'm working really slow. The idea is to make sure the whole thing is exposed inside to the oil. After the suitable period, drain off the oil and reinstall the hub. Whatever is left in there is plenty to keep it happy.. Drain off as much as you can conveniently, and don't be tempted to leave a bit in the hub-- it will spill out on the face of the rotor while you get the speed nuts and the wheel back on.
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The ATF will help flush out any stray crud that might be in there. It will also flow up around and into the sliding piece inside, the one that is engaged by the cam. When this works freely and there's no hydraulic or crud pressure trying to disengage, the hub works great. If the teeth on the sliding part, the inside of the hub housing, or the outside of the drive spline are damaged, the thing will stick.
You can do a little creative filing on the teeth if you have the unit completely disassembled, but not much. If the cam is broken from repeated partial engagement and disengagement, you need a new hub.
When everything is perfect, the only wearing parts are the bearing in the nose of the hub housing when it's not engaged, and the pads on the two-piece cam on the bearing nut when it -is- engaged.
Other related things:
The front U-joints (on pre-95 cars) will contribute to premature hub failure. Similarly, the inner needle bearing inside the spindle will allow the stub axle to move around, perhaps causing the hub bearing and spline to be misaligned. Both of these are critical maintenance items on your 4WD cars.
Also:
Remember that pushing the button on the dash only engages the transfer case. Repeatedly engaging/disengaging the transfer case doesn't help the hubs unless you insert a "reverse" in the middle of the sequence to try to disengage the hubs.
If one hub works more quickly than the other, you will further damage the slow hub by repeatedly doing the transfer case button bounce. Just so you know...
With everything OK, my hubs disengage after backing less that 3ft.
When I pull the wheel bearing and cam assembly on my automatic locking hubs (1992) I have trouble pushing the cam back on. It seems so fragile I worry about breaking it and getting a good grip is awkward. I have found that a short piece of 1-1/2" PVC pipe makes it easy. The inner dimension nearly matches the cam diameter, and the PVC pipe makes it possible to push evenly around the cam without deforming it. An easy push with my palm and the spring loaded cam pops onto the shaft.
I have a '96 4DR 4WD XLT with 17,000 miles. About six weeks ago I would intermittently find the 4HI and 4LO dash lights flashing when I was in 4WD Auto mode. Normally the 4HI light should be lit steady. The flashing pattern was six times every two minutes. The light would be out between the six short duration flashes.
The first two trips to the dealer for warranty service were useless. On the first trip the service counter girl wrote the problem down as a 4LO problem and I didn't catch her error. After having the truck for a full day they did nothing. They recommended that I bring the truck in while it is showing the condition so they could hook up the diagnostic readout unit. On the second visit I arrived with the truck in this error condition. I showed the symptom to two mechanics and they refused to hook up the unit and insisted I make an appointment to leave it another day. I reluctantly did so after explaining that I was hoping they'd at least try something. I found out later that these two are good guys but they're afraid of new technology.
Now the story gets much better. On my third trip I decided to stay and wait for the repair. I insisted on talking to the mechanic who would be working on it so I'd be sure he got accurate information from me. I just didn't trust anyone to relay my observations anymore. Well, I got to talk to a young fellow who listened intently, discussed some possible ideas with me, and then went to work. I ended up in the service bay with him and watched as he connected Ford's service bay diagnostic system (SBDS) to the data port on my Explorer. I was amazed how the truck came to life after the computer said it was "Initiating communications" and "Comm link established" - I swear that's what the screen said. I felt I was in a Star Trek episode! As the system put the truck through several diagnostics I looked in amazement at the windows went up and down, the windshield washers squirted, wipers wiped, lights flashed (including the under hood light) and so on. The there was a message on the screen indicating "4WD Initialization error" and "Possible faulty front drive shaft speed sensor". There are Hall Effect sensors in the transfer case on both the front and rear drive shaft outputs. These signals are sent to the GEM (Generic Electronic Module) and the GEM, using additional input signals, drives a magnetic clutch in the transfer case to engage FWD power. I had just received my new shop manuals and I've studied the systems and I'm really impressed. The mechanic cleaned the sensor and ordered a new one. It was installed on the fourth trip and the problem is FIXED!
Well, I'm an electrical engineer, not a mechanic, but it seems to me that these days the electronic support systems in a vehicle can be a great aid to a mechanic in diagnosing problems. The only problem is that the mechanic needs to know how to use them. I had always preferred an older mechanic in the past because I thought that experience was key. Now, I think my best shot is a mechanic in his 30's who's got some mechanical experience but isn't intimidated by the electronics.
I asked why the error codes weren't stored to be read on my previous visits. This fellow very diplomatically explained how easy it was for a mechanic to accidentally clear the error codes before reading them if he wasn't familiar with the operation of the SBDS. Boy, if I ever owned a car dealership I'd hire this guy! I took the time to find the owner of the dealership to tell him how satisfied I was that this fellow worked there, and how lucky he was to have him as an employee. He seemed shocked that anyone would say something nice and then said, "Hey, there's free coffee over here". I don't think the owner has a clue what's important.
BTW, I thought the flashing pattern was indicating a specific error code, but I've since learned that it's just a generic error code and you need the Ford SBDS computer to find out what the actual fault is.
Now I'm ready to lay down some cash so I can connect my laptop running Ford software to diagnose my own vehicle! For business reasons I'm sure that day will never come :o(
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