Engaging 4WD high at speed?

TJDave

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I'm well aware that 4 Low must be engaged when at a complete stop or barely rolling. However, I have heard some people say that while 4 High can be engaged at speed, you should never engage it at speeds over 35 mph.

Is this true? If so, what's the reasoning for this?
 
4Hi was designed to be 'Shift on the fly', you can shift back & forth between 2hi and 4hi at any safe speed. Though keep in mind that neither 4hi nor 4lo should be used on pavement unless it's covered with snow or ice. Nope, 4hi shouldn't even be used on pavement when it's raining... even heavily.
 
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Thanks Jerry, that's what I figured. I had just been reading around the Internet and some people were saying they don't advise shifting into 4hi above 35mph. Of course they had no logic as to what their reasoning for this was.

Good to know though!
 
Jeep says 55mph. And only use it if conditions result in wheel spin in 2WD. You can easily shift in and out if you are travelling straight as often as conditions require. In winter you might want 4WD on a street but might want 2WD as you turn into Walmart since 4WD will tend to push you straight, and then click back into 4WD. It becomes an art... just like shifting a manual transmission but you want to shift while travelling straight or the driveline bind might not let the transfer case shift when you want/expect it to shift.
 
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I've read that you shouldn't exceed 55 mph in 4 High as it can damage the transfer case.

I've also heard that you can go from 4 Low to 4 High at speed without having to stop to shift. Not sure if that's true as I've never tested it out.
 
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I've read that you shouldn't exceed 55 mph in 4 High as it can damage the transfer case.

Well, if you're going over 55mph in 4WD then I think it's probably safe to say you don't need 4WD since the road conditions must not be that bad to warrant speeds like that.
 
I've engaged 4hi going as fast as 35 and had it to about 50-60 in 4hi and that was sketchy at best defiantly shouldn't be going that fast if you need 4hi


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I've engaged 4hi going as fast as 35 and had it to about 50-60 in 4hi and that was sketchy at best defiantly shouldn't be going that fast if you need 4hi


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Good to know. Yeah, I figure if you're going faster than 55mpg it's not a 4wd situation.
 
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I'd agree that over 55 one would think 4WD would not be required but living in Canadas mountains and prairies I can say it is. I will shift into 4HI and leave it there on the highway during snow storms or just bad road conditions regardless of speed. 30 years of doing that at speeds way over 60mph and nothing has broken. We has a major storm yesterday. Pulling a trailer up a mountain grade at 70mph... I kept it in 4HI.

4WD cuts the power to each wheel in half. IF pulling a trailer up a hill you need 100 horsepower to maintain speed... 2WD means 50 horsepower to each tire. 4WD means 25 horsepower to each wheel. If there is snow or ice and the traction is lost at 35 horsepower you would spin out in 2WD where 4WD lets you continue.

Does speed make a difference in a Wranglers 4WD system? No. Turning sharp corners makes a difference where driveline bind will try to push you in a straight line. But at 70mph on the highway? We barely turn the steering wheel. 70mph, even on a curve you are really traveling straight. At least straight enough to avoid any driveline bind or cause a tire to skid, lose traction.
 
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I've also heard that you can go from 4 Low to 4 High at speed without having to stop to shift. Not sure if that's true as I've never tested it out.

4LO, N, 4HI. Technically possible. From N you need to precisely match engine rpms to driveline rpms. A transmission might with stand the gear clash if you fail to get it right. A transfer case is not designed to experience that clash and possibly a partial gear engagement. The risk to destroy the transfer case... and $$$$ involved... say NO don't try it.

Shifting an unsynchronized transmission is easiest in multi speed transmissions like semis with 15speed or 18speed transmissions. In those cases the perfect shift window is much wider than in a 5speed or 6speed. The transfer case is a 2speed. And not designed to change ratios at speed.
 
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I'm certainly not going to try it. Thanks for the explanation though. It makes perfect sense!
 
Shifting into 4 hi while moving will cause more wear on the T-case. While jeep says you can do it, it will wear it out more. I do not know specifically the 241 or TJ case but I have seen rebuilds of GM t cases who also use new venture gear T cases and shifting while moving does chip the gear a bit overtime that is needed for activation smoothly. Not enough to break the T case at all or even damage any expensive parts like the planetary gears but slight wear over time. T cases commonly have it worn down but never are the point of t case failure just rough shifting between 2/4 4/2. Shifting with the trans in neutral is the best way. That said I'm sure 60mph from 2wd to 4wd would shred the edges of the activating gear.

How old are those casses?
How many miles on them?
Mechanical or electronic shift?
 
I have shifted from 2wd to 4Hi a few times when driving in the winter. I try to keep it as low of speed as possible but sometimes you need 4wd and can't stop. I try to keep it under 35mph and when I shift it, I make sure I am off the gas and all tires are hopefully spinning the same speed. I have not had any issues.

I had a buddy shift his from 2wd to 4hi when the rears were spinning faster then the front and it did not sound good at all. I am sure the driveline takes a beating when that happens.
 
250K miles on my '05 TJR, I never shift the transfer case stopped I often shift it at highway speeds as the dry pavement transitions to ice, slush or compact snow. Same with my YJ which I drove for 20 years and 330K miles. I now have a n '06 TJR with under 60K miles and it likely never saw 4WD used, there is zero difference in how the '05, '06 feel when shifting in or out of 4H. Clutch in is always a good idea.

The theory is flawed. Yes, both axles must be turning identical speeds, yes you should shift travelling in a straight line and yes you should lift off the throttle so there wull be no tires slipping. But in a straight line, power off there is no wear even at 70mph even on dry pavement.

We are also talking about vehicles like Wranglers with the same front and rear axle ratios. Old school pickups often had different front and rear ratios. F250s, I've had 3, all came with 4.10 Dana 44 front axles 41/10, and 4.11 Dana 60 rear axles, 35/9. Those shifted into 4WD best at 15mph. And they only shifted out of 4WD after reversing for 15'. Pretty sure GM and Dodge did the same thing back then. The speed difference kept a tension in the power train rliminating any play or slack. If you pulled onto dry pavement in 4WD you would hear a howl from the tires or transfer case, likely the tires.

I change transfer case oil every 2 or 3 years, I've never seen these chips of metal you speak of.
 
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250K miles on my '05 TJR, I never shift the transfer case stopped I often shift it at highway speeds as the dry pavement transitions to ice, slush or compact snow. Same with my YJ which I drove for 20 years and 330K miles. I now have a n '06 TJR with under 60K miles and it likely never saw 4WD used, there is zero difference in how the '05, '06 feel when shifting in or out of 4H. Clutch in is always a good idea.

The theory is flawed. Yes, both axles must be turning identical speeds, yes you should shift travelling in a straight line and yes you should lift off the throttle so there wull be no tires slipping. But in a straight line, power off there is no wear even at 70mph even on dry pavement.

We are also talking about vehicles like Wranglers with the same front and rear axle ratios. Old school pickups often had different front and rear ratios. F250s, I've had 3, all came with 4.10 Dana 44 front axles 41/10, and 4.11 Dana 60 rear axles, 35/9. Those shifted into 4WD best at 15mph. And they only shifted out of 4WD after reversing for 15'. Pretty sure GM and Dodge did the same thing back then. The speed difference kept a tension in the power train rliminating any play or slack. If you pulled onto dry pavement in 4WD you would hear a howl from the transfer case.

I change transfer case oil every 2 or 3 years, I've never seen these chips of metal you speak of.
I hope you would never see chips of metal. It's such a slight griding overtime again nothing that would damage the Tcase enough to kill it. Honestly, I made such a point about something that NO ONES tcase will ever get damaged from so there no point in even talking about it.
 
As mentioned much earlier, the TJ's transfer case was designed to be a "shift on the fly" where going between 4Hi and 2wd is concerned. Shift back & forth while driving to your heart's content. Just know too that the TJ's transfer case was not designed to be in 4x4 when on dry pavement, don't use it when on the pavement unless it's covered in snow or ice.