question 4x4 on the fly engagement

[dhk coatings]

should the front drive shaft engage while in motion without the hubs locked?
I pulled her into 4x4 just to try it and it wouldn't engage until stopped. the hubs where unlocked

 

[dhk coatings]

also, did the rear dif come with a Detroit or a locking dif from the factory, this truck is definitely locked back there. I wonder if its been welded closed?

 

[Shadetreemechanic]

limited slip was factory option, not true locker and yes truck should go on the fly from 2high to 4 hi. Must be stopped to get into 4low

 

[mblaney]

should the front drive shaft engage while in motion without the hubs locked?
I pulled her into 4x4 just to try it and it wouldn't engage until stopped. the hubs where unlocked

Short answer, NO!

the front axle, diff, and drive shaft have to be running at (approximately) the same speed as the rest of the drive train to engage. If your hubs are locked then everything is in motion and you can engage the transfer case on the fly (unless you are spinning the rear wheels). The way you tried it you were attempting to bring the front gearing up to speed by jamming the gears together. I would expect some epic grinding, or a huge bang as gears meshed. You might even end up with some spare parts doing this. Since your hubs are unlocked there would be no point in doing this anyway.

 

[madpogue]

^^^^ Um, trucks with automatic hubs, the front driveline is stopped while in 2WD, and you can engage the 4x4 on the fly. No "epic grinding". I wouldn't do it under _power_, however.

 

[mblaney]

The question was "with the front hubs unlocked". My assumption would be manual hubs, unlocked.

 

[MontanaJack]

Some DLs turn in 2wd, some don't. As our trucks get older, so do the hubs. When that happens the grease inside gets thicker and sticky. This can cause some drag and turn the front DL a bit. It should not be spinning at full speed though.

It's also not unheard of to have to come to a stop to engage the 4x4. I had an old 79 f150 that I actually had to put it in reverse before the 4x4 would slip in.

If you're concerned that the hubs are not unlocking, try turning them in and out a few times. When they're unlocked TAP on the hub housing with a hammer (a dead blow hammer). Or you could lift the front axle onto some jack stands with both tires off the ground. Hubs unlocked. 2wd. Tranny in neutra. Now try to turn a wheel. If both wheels turn along with the front DL at the same speed, then your hubs are not working properly. It may be time to pull them apart, clean everything and repack with a quality hub grease.

Here is a video that has a basic review of what a hub does and how if you're interested:

http://youtu.be/F441XfFCPN0

 

[madpogue]

The question was "with the front hubs unlocked". My assumption would be manual hubs, unlocked.

The t-cases and driveline are the same with manual or auto hubs. If trucks with auto hubs can be shifted on the fly without the disastrous results you imply, why couldn't an identically-equipped (except for the hubs) truck with manual hubs be shifted on the fly?

 

[snicklas]

Not sure on the older auto hubs, but the way ESOF in the newer trucks (99+) work, is the hubs are vacuum operated and they are engaged before the transfer case shifts (and you can feel the computer "burp" the throttle during the transfer case shift)...... unless you have a vacuum leak and need to rebuild the hubs this summer, but then they still have a knob and can be locked in manually, which is what I did this winter after I found the vacuum leak. (Switch to 4x4 and the HVAC "switched" to defrost for about 60 seconds......)

Personally, on the older trucks I had that had manual lockouts, I would not shift the transfer case into 4x4 while I was moving unless the front lockouts were engaged. I would shift in 4x4 at a complete stop, without the lockouts in to exercise the 4x4 once a month to keep the lube moved around, but only while at a dead stop. I've also had trucks that would not engage 4x4 unless the transmission was in neutral, or would not disengage without shifting into reverse because of torque on the driveline......

 

[Greg5OH]

I kinda want to get the auto hubs on my chassis working but from everythign I have read, they will jsut fail in short order soon enough, right when you need em most (isnt this always the case with everything).
i think for winter, i will jsut leave them locked in for most o0f the time, so cruising along just bump the front axle lever into 4hi.
I know you shoudlnt really be going faster than 45mph or so in 4hi, but what if you are on snow covered freeway. I usually drive 60mph anyway because the truck is very stable, but can i be doing 60, hubs locked in, and bump it into 4hi at that speed?

 

[mblaney]

The t-cases and driveline are the same with manual or auto hubs. If trucks with auto hubs can be shifted on the fly without the disastrous results you imply, why couldn't an identically-equipped (except for the hubs) truck with manual hubs be shifted on the fly?

For the same reason you "can't" shift a manual transmission into gear without stopping the transmission using the clutch. Half of the gear train is rotating, half of it is stopped. I don't think the transfer case has a clutch type deal to allow the gears to come up to speed before engaging.

All of my responses have been with respect to the OP question - with hubs unlocked. I really have no idea how automatic hubs engage on-the-fly (hub spinning, diff stationary) but this is not relevant to the OP question anyway.

 

[MontanaJack]

For the same reason you "can't" shift a manual transmission into gear without stopping the transmission using the clutch. Half of the gear train is rotating, half of it is stopped. I don't think the transfer case has a clutch type deal to allow the gears to come up to speed before engaging.

You can shift into 4x4hi on the fly as long as the hubs are locked. You should not do so if you are going 45+. As for the transmission... Any manual can be shifted higher and lower without using the clutch at all. In fact this is primarily how I drive so that I save wear and tear on my clutch. Engine rpms simply need to match with your speed and gear you want to go into. This is accomplished with only the accelerator.

As for going 60 mph. DO NOT SHIFT into 4x4 going that fast, but, after shifting its acceptable to go that fast. Probably not a real good idea to do so for long distances though.

You must be stopped with the tranny in neutral to shift the t-case into 4lo.

 

[david85]

Not sure about the shift on the fly systems in the F-series of this era. However on the ford rangers/exploders/B2s used the "touch drive" system. In that setup, the transfercase is electric shift via control module, so there is no floor lever in the cab. Only push buttons to engage 2H, 4H or 4L (no neutral). Shifting into 4L still requires stopping.

This was accomplished by an electric clutch in the T-case that would allow the front drive train to accelerate to road speed before mechanically coupling the front differential. My Dad recently acquired a mint 1990 ranger that has this setup. AFAIK, the hubs are purely mechanical and do no require any outside energy source (vacuum or electric) to engage or disengage.

I didn't realize the F-series even offered shift on the fly back in 1990. Am I missing something?

 

[madpogue]

The manual t-cases are considered shift-on-the-fly (4HI ONLY). The auto hubs are centrifugal, and are engaged BY the driveline. That is, when in 2WD, the front driveline is not turning, the hubs freewheel. When you shift the t-case to 4HI, the t-case engages the front driveshaft, the front axles shafts start to turn, and this locks the auto hubs. When you shift back to 2WD, the auto hubs remain locked, so the front driveline continues turning. You have to reverse for a few feet (8 feet, IIRC) to unlock the hubs.

 

[hce]

Depends on the transfer case. Ford manual transmissions have no provision to synchronize the front driveshaft with the rest of the drivetrain when shifting on the fly. The electric shift models have a clutch that will spin the front driveshaft to speed with the rear allowing the shift collar to engage without hubs being locked