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Hummer Knowledge Base

The Hummer Knowledge Base

Transfer Case/Shifting:

Here is a direct quote from the AM General "Advanced Mobility Operation Guide", Section "Transfer Case Shift Lever" page 14:

"The High (H) position permits the front and rear axle to operate independently through the differential inside of the transfer case and still maintain 4-wheel drive. The High Lock (HL) and Low Lock (L) positions provide full-time 4-wheel drive with both axles locked together, bypassing the differential in the transfer case......"

Dave XLC2

When shifting from H to HL, there should not be any noise to speak of, unless some of the wheels are turning at different speeds than others.

My owner's manual implies that you can shift "on-the-fly" from H to HL. It states that you must slow to less than 2-3 MPH before shifting between H and L, however. In BOTH cases, the manual says to shift the tranny into N before shifting the t-case.

What often happens is that the t-case will be shifted into N for a moment, and then will not shift into H or L without grinding & noises. What happens is that when the t-case is in N, the innards of the tranny will begin to spin (even when in N) because the engine is still turning the torque converter and such. When this happens, the procedure is something like this (sequence is important!):

  1. Turn engine off
  2. Wait for engine to stop completely, plus about 2 seconds
  3. Shift t-case into desired range
  4. Shift tranny into N or P (if it is not already)
  5. Restart engine

To avoid this problem when shifting from H to L (or back), it is recommended (in the owners manual) that you shift "relatively quickly" to avoid the t-case spending much time in the N position.

Hope this helps...

Dave Breggin
'95 Diesel Wagon

The shifting procedure for the t-case is:

with vehicle stopped or moving very slowly
place transmission in N
firmly shift _thru_ neutral to L or H

If the t-case stops in N for more than a brief moment, the transmission will begin spinning the t-case input shaft.

Even in N, the transmission will still spin its output shaft due to viscous coupling of parts inside the tranny.

If the t-case gets spinning, the only treatment is to stop the engine and wait for every thing to spin down. (Around 10 seconds)

BTW, I have been experimenting with methods of making the shifting from L to H easier. It seems that if, after you shift the tranny to N, if you briefly shift it to R (wait for it to engage), and then shift it back to N, the t-case seems to shift more easily out of L.

Dave Breggin
'95 Diesel Wagon

Many times there is enough fluid coupling in the trans to allow the output shaft of an UNLOADED transmission to spin while it is in neutral. This is not uncommon. Where it becomes a problem is when you are shifting to Low range or anytime that you pass through Neutral in the Tcase. For that reason you should not have a problem when shifting from H to H lock and back. A lot of manuals state that when you make the shift through neutral in the Tcase to push it quickly through. Also make sure your engine is not idling above 600 or so RPM. The slower the engine is turning the better. Once you are in Neutral in the Tcase and want to shift out of neutral, it is always better to stop the engine before moving out of neutral. This problem does not appear as often these days in 4X4 trucks since most have a Park position which locks up the output shaft of the transmission.

Glenn M1043

I find that when shifting the t-case into low, if you back up a few feet first you should be able to put it into low with no problem. I do it every time and have never heard the gears grind. Just be sure to pull the lever firmly (in one continuous motion) not stopping in Neutral. I don't know why this works but it seems to work for me...

'97 HMCS

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