4Lo

Why do you have to put the transmission in neutral to shift into 4 Lo?

 

Because the transfer case has to change to the low range gears

 

But how is it that you can change into 4 High at up to 60mph.
This truck is my first 4x4 since the early 90’s.

 

High range just engages the front hubs so you can do it on the fly, the transfer case doesn’t change.

 

Because if you're doing 60 and trying to shift to low range, you're doing it wrong.

 

The 2HI to 4HI shift just engages the 4wd gear in the transfer case. If you have ADD this shift uses a syncro to spin up the drive gear, chain, front output gear, and front driveshaft in order to match speeds in the transfer case and then the 4wd clutch can engage. At that point all speeds are matched and the ADD actuator can engage the remaining CV to the diff. Without ADD and with hubs locked the 4wd gear will already be spinning at ground speed so the gears are already speed matched and the 4wd shift collar is simply slid to engage the 4wd gear.

The 4LO shift is a speed mismatch and doesn't have syncros so that's why you need to be stopped in most cases. The reason you put an auto in neutral is the following. When you are in drive and sitting still the the transmission output isn't spinning because you are holding the brakes which connects all the way through the drive line to the torque converter output. When you shift to 4LO the transfer case goes from the high range, through neutral, and then engages the low range gears. When the transfer case goes into neutral during the shift the brakes of the truck no longer hold the output of the torque converter because the connection has been broken. When this happens the transmission begins spinning from engine output and therefore spins the input to the transfer case. The output shaft of the transfer case is not spinning as it's held stationary by the brakes. Now you have a mismatch between the transfer case input speed and the output speed and therefore the low range drive clutch cannot be engaged and will grind. Basically the low range clutch sleeve is attached to the output shaft (not spinning) and trying to mate with the low range planetary carrier (being spun through the transmission by the motor) and that's a no-go situation. The only way to easily match all speeds is to get everything in the transfer case stationary and the only way to do that is stop the truck and put the transmission in neutral.

 

Lol good question. Answer above is a great explanation.

Go 15 mph in low. Pay attention to the transmission shifts and RPM of the engine. Now go 15 in 4H. Do the same.

Now imagine trying to shift from high to low at 40 mph. BOOM!

Lol we can’t do it because we like our drivetrain attached to the truck. And rods in the engine block.

Look up “mechanical over rev” just for fun.