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So I am new to older jeeps...or just older cars in general.I have a 1990 yj and im not sure if there is the right way to put it into 4wd...do I have to be parked? Driving? Idk! Also is it bad to drive on the highway or for a long time on 4wd?
 

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you can shift into 4 hi @ low speeds and 4 lo you should be stopped or a crawl. you don't want to be in FWD on a hard surface or stuff can tear up. ice or snow would be ok, don't have to worry about that here.
 

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if its the np231 transfer case, you can shift into 4hi while moving, just let off the gas to expedite the shift. To get into 4Lo, make sure you go into neutral on the t case first as well as neutral on your transmission. its also better to be stopped to go into 4Lo because sometimes it can take a harder pull to get into 4Lo
 

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I was told it's best to throw it in 4wd with the rear tires spinning and then go onto dry pavement and repeatedly do tight u-turns, is this true? ;)

Seriously...the above mentioned advice is sound. Let off the gas and ensure all wheels are spinning the same speed, engage it into 4hi...don't run 4wd on pavement unless there's enough snow or ice on the ground for your wheels to slip while turning. If you feel it hop around turns, you shouldn't be in 4wd. 4lo...engage in neutral at either a dead stop or moving 2-3mph tops...4lo is for really low speeds. If you run too fast in 4lo, I think it's possible to grenade your transmission. To disengage 4hi and 4lo...reverse the process used to engage. 4lo may grind some...that's normal. If it grinds a lot...that's not so normal...either you're moving too fast or if you're at a stop, you need to be rolling just a little to get the gears to mesh right. If you still can't get it to engage...shut the engine off, engage it and restart the engine...I hear that works. Never tried it myself.
 

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Definitely don't engage 4wd on high traction surfaces.

Reason being is you don't have an open or limited slip center differential, it's a solid differential that applies even drive speeds to the front and rear axles at all times. Your front wheels physically travel a farther distance than your rear wheels in a turn so that can cause binding on high traction surfaces (tarmac, concrete, etc). The front end is also geared just ever so slightly faster than the rear as well so that the front end can pull the rear (since the front has the weight of the engine to aid in traction).

On low traction surfaces (dirt, mud, ice, etc) this isn't a problem as whichever wheel has the lightest load on it can slip easily and most of the time without notice to remove any binding issues from the driveline.
 

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Just FYI

On a stock YJ, when you pull the transfer case into 4HI the transfer case manually shifts into 4WD causing the front and rear drive shafts to spin at the same speed. If you are going straight ahead without any specific wheel spinning (slipping) then both differentials turn the same and both front axles spin the same.

Now the inside axle shaft on the passenger side is spinning at the same speed as the shaft solidly attached to the passenger wheel.

When the transfer case shifts, there is a pneumatic switch on the side that connects the vacuum from the engine through that switch to a vacuum motor on the front axle. That pulls a fork in the axle that slides a collar over the two halves of the passenger side axle shaft, thereby locking them together. You are now in 4WD high.

There is another pneumatic switch on the axle housing that mechanically closes when the vacuum motor has engaged the fork. That closes the electric circuit to the 4WD indicated on the dash and causes the 4WD light to come on.

If you are under heavy load on the engine, accelerating or pulling, then there is weak vacuum to operate the vacuum motor. That is why sometimes you have to let up on the gas to get the vacuum to spike and get enough pull to engage.

If you are turning a tight radius and trying to put it in 4wd, the front axle halves won't be turning the same and there will be a problem. If you are spinning any of the wheels, it will be a problem. All 4 wheels have to be turning at the same speed to properly engage.

Hope this helps understand some.
 

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RTFM - Read The F***** Manual, as they say. :whistling:

Have fun and don't go getting stuck in the middle of nowhere! Bring another vehicle and progressively do harder stuff. Don't go driving in to a river. Also, do not hook any tow stuff to a bumper hitch ball, they can easily break.
 
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