Ok, can't seem to find the answer in the operators manual. On the HEMTT(M985) the dash control for the drive system, what are the meaning and what axles drive in each setting. In center at "Off", "8X8" to the right is self explanatory, and "Inter Axle Diff. Lock." And what is the difference between "8X8" and "Inter Axle Diff Lock" ? The manual says how to use and in what conditions but little else. Thanks.
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Axle interlock vs 8X8
- Thread starter Gunzy
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dk8019
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Ok, here's my experience using both, although I'm still not 100% convinced I have it correct.
8x8 - As you would imagine, all axles are powered, differentials are open
Inter Axle Diff Lock - I believe this should be simply differential lock, as that seems to be what it does, it's an air locker for each differential. - Tank mode in my mind, all 8 wheels moving together
So when you shift to low, 8x8 is directly engaged, then you can lock the differentials. In High, you can only choose one or the other, lock the rear differentials, or put her in 8x8. I have not been able to stick my truck in 8x8 Low, with the inter lock on.
8x8 - As you would imagine, all axles are powered, differentials are open
Inter Axle Diff Lock - I believe this should be simply differential lock, as that seems to be what it does, it's an air locker for each differential. - Tank mode in my mind, all 8 wheels moving together
So when you shift to low, 8x8 is directly engaged, then you can lock the differentials. In High, you can only choose one or the other, lock the rear differentials, or put her in 8x8. I have not been able to stick my truck in 8x8 Low, with the inter lock on.
dk8019
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You know, I say that, but then I looked at this:
It appears that the air line only runs to the first axle of each pair, simply driving the axles at the same speed. Maybe it doesn't have a true locker for cross differential locking.
It appears that the air line only runs to the first axle of each pair, simply driving the axles at the same speed. Maybe it doesn't have a true locker for cross differential locking.
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Check the -10.
dk8019
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The -10 is more than a bit confusing on this point, it basically tells you when to use each mode, but not entirely what it does. When looking at the maintenance manual schematics, it seems that the airline for interlock only runs the the two middle differentials, thus lock the inter-axle diffs together. I found another post here on SS that suggests that the differentials are LSDs, which explains the traction capabilities I've seen so far.
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"In left position (INTER-AXLE DIFF. LOCK) locks interaxle differentials in front and rear tandems. In right position (8X8 DRIVE), engages transfer case drive to front axle."
Putting the transfer case in Low range puts it in 8x8 mode as well.
I would suspect it to work like this -
Transfer high, neutral switch = high range, rear axles only powered, interaxle diff unlocked
Transfer high, 8x8 switch = high range, all axles powered, interaxle diff unlocked
Transfer high, interaxle switch = high range, rear axles only powered, interaxle diff locked
Transfer low, neutral switch = low range, all axles powered, interaxle diff unlocked
Transfer low, 8x8 switch = low range, all axles powered, interaxle diff unlocked
Transfer low, interaxle switch = low range, all axles powered, interaxle diff locked
Putting the transfer case in Low range puts it in 8x8 mode as well.
I would suspect it to work like this -
Transfer high, neutral switch = high range, rear axles only powered, interaxle diff unlocked
Transfer high, 8x8 switch = high range, all axles powered, interaxle diff unlocked
Transfer high, interaxle switch = high range, rear axles only powered, interaxle diff locked
Transfer low, neutral switch = low range, all axles powered, interaxle diff unlocked
Transfer low, 8x8 switch = low range, all axles powered, interaxle diff unlocked
Transfer low, interaxle switch = low range, all axles powered, interaxle diff locked
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All I know is I pulled into Andy's driveway and my tires dropped into drainage ditch and she was slinging dirt and would not move in high range I hit the the interlock and she took off like a scalded dog.
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See post #9
Well I'll be, for a change what I was thinking was right.
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Let me lead with saying that I don't have a HEMTT, but I have done plenty of research because I do like them.
I'll try to explain with some videos using what I've learned from various sources. I could be wrong but this is the way I understand the drivetrain.
Switch in the OFF position in HIGH. Only the inner rear axle is powered. I think the guy in the video was assuming the driver had the axle interlock engaged. It apparently wasn't.
https://youtu.be/KvqwGOcINzg
Switch in 8x8 position in HIGH. This engages the transfer case and powers the front and rear inner axles only.
https://youtu.be/fggZg4-REq0
Switch in the AXLE INTERLOCK position in HIGH. I couldn't find a video of this. In this position both rear axles are powered. This also engages both front axles, but the transfer case is not engaged, so the fronts aren't powered.
Switch in the AXLE INTERLOCK position in LOW. The transfer case engages the front inner axle when placed in LOW and both outer axles are then engaged when the switch is placed in AXLE INTERLOCK.
https://youtu.be/SK3lpK_rPE4
Both inner axles are constant input and air shift output, making them selectable. The outer axles are only constant. When AXLE INTERLOCK is selected with the switch, the outputs on the inner axles are engaged to power the outer axles.
With that switch in the OFF position, only the rear inner axle is actually powered. I have no clue on lockers or other traction devices in the axles but from everything I've seen, they are all open. So with everything locked in you are have power to all 4 axles, but only 4 wheels, due to open differentials.
That's my understanding of the drivetrain. I would enjoy being proven wrong with a video including all the selections and the results....in the mud of course. I like HEMTTS in the mud.
Thoughts?
I'll try to explain with some videos using what I've learned from various sources. I could be wrong but this is the way I understand the drivetrain.
Switch in the OFF position in HIGH. Only the inner rear axle is powered. I think the guy in the video was assuming the driver had the axle interlock engaged. It apparently wasn't.
https://youtu.be/KvqwGOcINzg
Switch in 8x8 position in HIGH. This engages the transfer case and powers the front and rear inner axles only.
https://youtu.be/fggZg4-REq0
Switch in the AXLE INTERLOCK position in HIGH. I couldn't find a video of this. In this position both rear axles are powered. This also engages both front axles, but the transfer case is not engaged, so the fronts aren't powered.
Switch in the AXLE INTERLOCK position in LOW. The transfer case engages the front inner axle when placed in LOW and both outer axles are then engaged when the switch is placed in AXLE INTERLOCK.
https://youtu.be/SK3lpK_rPE4
Both inner axles are constant input and air shift output, making them selectable. The outer axles are only constant. When AXLE INTERLOCK is selected with the switch, the outputs on the inner axles are engaged to power the outer axles.
With that switch in the OFF position, only the rear inner axle is actually powered. I have no clue on lockers or other traction devices in the axles but from everything I've seen, they are all open. So with everything locked in you are have power to all 4 axles, but only 4 wheels, due to open differentials.
That's my understanding of the drivetrain. I would enjoy being proven wrong with a video including all the selections and the results....in the mud of course. I like HEMTTS in the mud.
Thoughts?
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When you say "inner axles" are you meaning the #2 and #3 axles?
I don't have any real HEMTT experience, and I can't say for sure in so far as how this applies to the HEMTT, but I think you misunderstand interaxle differentials. A truck like a deuce or 5 ton have no interaxle differential, which is effectively the same as having the interaxle differential locked on a truck that has one. The interaxle differential does not prevent power from going to one axle or another in the tandem, but allows a differential action to take place between the axles in the tandem. This is much like the way an all AWD vehicle works and how it varies from a 4WD vehicle. A tandem setup with an interaxle differential with open differentials in each axle (like what I believe the HEMTT to be) can have 1 tire of the 4 in the tandem (or 2 of the 8 if it has dual wheels) in the air and spinning with the 3 other tires stationary. This tire also spins 4x the speed it otherwise would.
A vehicle with a tandem axle, interaxle differential, and open differentials can be immobile if any of the four rear tires break traction. If the interaxle differential is engaged, at least one tire on each rear axle will be powered. This is like a 5 ton or a deuce. If the truck is like a M916/M920 and has a tandem setup with an locked interaxle differential and Detroit lockers in the rear axles, all 4 tires are powered regardless of what any of the others are doing.
I don't have any real HEMTT experience, and I can't say for sure in so far as how this applies to the HEMTT, but I think you misunderstand interaxle differentials. A truck like a deuce or 5 ton have no interaxle differential, which is effectively the same as having the interaxle differential locked on a truck that has one. The interaxle differential does not prevent power from going to one axle or another in the tandem, but allows a differential action to take place between the axles in the tandem. This is much like the way an all AWD vehicle works and how it varies from a 4WD vehicle. A tandem setup with an interaxle differential with open differentials in each axle (like what I believe the HEMTT to be) can have 1 tire of the 4 in the tandem (or 2 of the 8 if it has dual wheels) in the air and spinning with the 3 other tires stationary. This tire also spins 4x the speed it otherwise would.
A vehicle with a tandem axle, interaxle differential, and open differentials can be immobile if any of the four rear tires break traction. If the interaxle differential is engaged, at least one tire on each rear axle will be powered. This is like a 5 ton or a deuce. If the truck is like a M916/M920 and has a tandem setup with an locked interaxle differential and Detroit lockers in the rear axles, all 4 tires are powered regardless of what any of the others are doing.
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I think that wrecker burnout was done using caging bolts on the #3 axle and setting the parking brake.
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Yes, those being the inner axles.
I'm following your logic though. I'm familiar with power dividers and differentials, but mostly on civilian trucks. It didn't seem a very rugged design to have a stand alone unit, let alone two, in the driveline given the conditions these vehicles were designed to operate in.
Post #2 in the thread below seems to bring it all together pretty well though.
http://www.steelsoldiers.com/showthread.php?64921-5-ton-sprag-transfer-case-a-power-divider
I think I am going to investigate putting in another switch to engage the dividers in 8X8 high range. That way you can truly engage all 4 axles in Hi. As I see this working you are either 2wd, or 4wd in high range and can only truly get 8X8 when in low range.
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We have a M915A3 at work, during wet conditions it wont pull the trailer up the hill. The first axle will spin out and it's a open diff. So when we flip the INTER AXLE LOCK it engages the rear most axle also. It also happens to be a open diff, but between the 2 of them it will pull a loaded trash trailer up the slick hill.
JDToumanian
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Castle Bravo is completely correct in his explanation... I like his analogy of thinking of each pair of HEMTT axles as a separate 4x4 pickup truck. In high range, with the air toggle in neutral, the front pickup truck is in neutral and the rear pickup truck is in 2wd. Flip the toggle to lock and the front is still in neutral and the rear is in 4wd. Flip the toggle the other way to 8x8, and both pickup trucks are in 2wd. With the transfer case in low range and the toggle in neutral, both trucks are in 2wd. Flip it to 8x8 and it does nothing different... Both trucks are already in 2wd since 8x8 is automatic in low range. Flip it to lock in low range and both trucks are in 4wd, which is one wheel on each axle being driven. The HEMTT doesn't have true lockers where both wheels of any axle are driven at the same time.
Gunzy, I'm not sure you need to change anything... The HEMTT can already do what you describe.
Jon
Gunzy, I'm not sure you need to change anything... The HEMTT can already do what you describe.
Jon
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I am talking about getting power to all 4 axles in High range which cannot be done unless there is another axle interlock lever . Thus, transfer case in high range, dash lever in 8X8 and an over ride lever for axle interlock. Then all 4 axles will be powered in high range, not just low range. That is what I mean by installing a switch to over ride the axle interlock.