FMTV Highway Speed

Coach you dont know why Meritor or Tacom made the change. It could very well have been that by going to the 6x6 intermediate axle setup there are less parts to manufacture or inventory. The driveline issues with t he early LMTVs were fixed by 1998. This is just my opinion and dont take it personal but I would have less peace of mind in modifying and welding an axle than any pptential driveline issue.

scottmandu said:

Coach you dont know why Meritor or Tacom made the change. It could very well have been that by going to the 6x6 intermediate axle setup there are less parts to manufacture or inventory. The driveline issues with t he early LMTVs were fixed by 1998. This is just my opinion and dont take it personal but I would have less peace of mind in modifying and welding an axle than any pptential driveline issue.

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Not taking it personal at all and agree that who knows exactly why the switched. For sure. change over may have been for inventory reasons..... mentioned that in various threads long ago.

Will just have to go with my own peace of mind I guess. I trust my welder. He's not a backyard type. Works for Company... that gets big contracts, including military.

coachgeo said:

IMHO that is not the best route to go. Nothing wrong with having AWD and the existing Granny Gear (labeled as "First Gear")in the existing system. When in true 4x4 "mode" the ratios available (1st thru 5th) pretty much cover everything someone needs when off road. It covers from what would be equivalent to bottom gear of low range in a typical 4wd Tcase via granny/1st gear all the way to an equivalent to high gear in 4wd High; via the 1.2:1 in
5th gear. Added benefit... is when in the existing tranny's 4wd mode you don't have to do anything to reach those ranges... no Tcase buttons or levers to activate when you need a gear that would fall in low range or gears that would fall in high range..... it is all automatic.

What is needed is more alternative ratios above 5th gear in the existing tranny and this may very well be possible via programming changes to the existing tranny computer. Thus, what you really need is someone who has permission from Allison to rewrite the tranny operating software to program the existing tranny to use other physical gear combinations that are already possible via different combinations of the existing physical gears there now. Or someone who can build a second computer to hijack the existing computer and trick it to do these things and not touch Allison proprietary programming. (or a hacker?)

It is also very very possible other physical gears can be put in the tranny ifffffffff software could be written to handle them. Way I heard it there is several of physical gears available that would fit into this tranny cause it is based on a widely used Allison 3000 model tranny which is; via software only and/or physically geared different plus different software, set up for use in various applications) This is straight info from a Tranny rebuilder extremely familiar with these transmissions. He does not know of an Allison programmer though... I asked him. But he has never looked for one either!!!

Tranny ratios for 3700sp and probably for version just before that are

• First 6.93 : 1 .......... Granny only available in 4x4 mode
• Second 4.18 : 1
• Third 2.24 : 1
• Fourth 1.69 : 1
• Fifth 1.20 : 1
• Sixth 0.90 : 1 (not available in 4x4 mode)
• 7th 0.78 : 1 (not available in 4x4 mode)

NOTE: Italics is all gears available in highway AWD mode.

Example an 8th gear that is 0.5x would be sweet

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And here is the issue!! Assuming this info is correct and it looks right, This is a MD3070PT trans. There is a civi 2 WD trans, the MD3060 and 6th is .65 which would solve the speed issue with 3.9 R&P The 3070 trans is likely the same trans, 6 speed with a lower 1st added making it a 7 speed. So the 3070 needs to be converted with 7th made to be .65 with parts from a 3060. Then 68 mph would be 2400 rpm.

I believe the final gear (7th) is located in the transfer case. By swapping the 6th gears you would end up with a 6th gear lower than 7th.

scottmandu said:

I believe the final gear (7th) is located in the transfer case. By swapping the 6th gears you would end up with a 6th gear lower than 7th.

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Scott knows more than many so odds are in his favor on knowing the make up of the tranny. Trust but verify they say sooo..

m-35tom said:

And here is the issue!! Assuming this info is correct and it looks right, This is a MD3070PT trans. There is a civi 2 WD trans, the MD3060 and 6th is .65 ....

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Look at what all the gears are please in the 3060. If "first gear" is same ratio in each then we know it is as Scott suggest that 7th is in the T-case.... if 3060's first is same as the second gear of the MD3070PT then that may point to it being that first/Granny gear is the one added into the 3070's T-Case

The problem though is..... no matter whom is right...... there is nothing you can do about swapping...... WITHOUT rewriting the computer software that runs the transmission.... if you swap a gear out.... the computer will fault to an error cause it see's it as something going wackadoodle in the tranny and will go into limp mode or just shut down. Least that has what has been relayed to me by Allison Expert (Owns an exclusive Allison rebuild and parts company) I've had long discussions with on this topic. Have quoted his responses on this subject in a thread in here somewhere.

I've found reference to the C6 clutch pack being in the transfer case, and first gear is obtained by activating C1 and C6 clutches. It would appear that you are more correct stating that first gear is in the transfer case. 7th gear is obtained by activating C2 and C4 clutches which are in the main body of the transmission.

scottmandu said:

I've found reference to the C6 clutch pack being in the transfer case, and first gear is obtained by activating C1 and C6 clutches. It would appear that you are more correct stating that first gear is in the transfer case. 7th gear is obtained by activating C2 and C4 clutches which are in the main body of the transmission.

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Actually found reference to this today in TM9-2320-365-34-1 page 1-9(page 45 in the .pdf).

“c. Transfer Case. The transfer case (3, Figure 1-5) provides the transmission (2) with the seventh gear (low gear, or 1st gear) and delivers power from the transmission to the front and rear driveshafts. In normal driving conditions, the transfer case splits the output torque of the transmission, providing 70 percent of the torque to the rear output drive yoke and 30 percent to the front output drive yoke. In low gear the output torque of the transmission is split evenly, with 50 percent going to the front output yoke and 50 percent going to the rear.”

So if I am interpreting it correctly normal drive(2nd thru 7th) uses all 6 speeds of the Allison with the 70/30 AWD torque split provided by the transfer case(center diff)with no gear reduction? “Mode” drops to low range in the transfer case and locks in the 50/50 4WD torque split(no center diff) and the Allison is now limited to its first 5 speeds...

Might be possible to alter the allisons 6th speed to get a little faster or efficient cruise, but how the transfer case would feel about say an extra 10% in RPM increase is outside my paygrade it might also be very complex, or it may be easy depending on who we could find to hack the beast... interesting idea if diff gears are becoming unobtanium though...

the t-case 70/30 torque split only happens if the rear is lighter than the front load the rear down with say 3 yards of dirt than the torque split will be more to the front than the rear. there is no magic in the t- case has an open differential in it between the front and rear driveline it will deliver power to the axle with the least traction, just like the old np 203' in the m880's

OK, picture this. Take a hub apart, remove all 4 spider gears, remove both big gears, put the cross back is just to prevent oil loss or make 4 dummy plugs with 'o' rings and a flange on the inside to keep them in. Make a hub to mate with axle spline and mount it on the outer cover. Or make a cross with the spline to mate with the axle. Now you have a 1:1 at the hub. Now do not use 6th or 7th speed since they will be to high. This may be good for hgwy travel and no serious off-roading. Not really that hard to swap one way or the other. Eventual design could be a select-able hub with axle locked to cover or locked to the outer gear.

snowtrac nome said:

the t-case 70/30 torque split only happens if the rear is lighter than the front load the rear down with say 3 yards of dirt than the torque split will be more to the front than the rear. there is no magic in the t- case has an open differential in it between the front and rear driveline it will deliver power to the axle with the least traction, just like the old np 203' in the m880's

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Yep, it only really comes into play once you start to loose traction. More weight in back = more traction... They did the same thing on ford aerostars and expeditions(maybe others also but those are the limit of my experience) with an electric locker controlled by the driveshaft RPM sensors and a little ECU. It made the auto 4wd lockup a little smoother since the front wheels were already receiving some torque...

I was not feeling well on friday so I dug into the workings of the transmission and transfer case because I didnt understand how the parts in the exploded views could make that low first gear. The upshot of this is I Dont think a gearchange between a 3060 and 3070 Is a possibility to increase highway speed as they are the same transmission gear sets. The LMTV tech/op manual description I posted earlier is a little misleading stating that the transfer gives a 7th gear(more on this later) Basically the 3000 series is configured for the same 6 forward speeds, 3 reduction, 1 straight thru(1:1) and 2 OD ratios with 6th being a .65:1 ratio. It does this using 3 planetaries(9 gears total) and 5 clutches(C1-C5), and ours does exactly this same thing in 2nd thru 7th with the same output ratios from the transmission as a 3060 would have. So why is our final ratio .78:1 in 7th(2600 RPM and 7.8 axle ratio = around 60ish MPH with 46.6” tires and 3375ish shaft RPM so 2600 RPM = 3375 driveshaft RPM =.78:1)?

I found a cool supplier that gave parts descriptions(and tooth counts) for allison parts so I ran the part numbers for the gears in the transfer case. The trans output shaft turns a 30 tooth gear, which drives a 37 tooth idler gear below it in the case, which in turn drives a 36 tooth gear down in the lower driveshaft housing where the planetary diff and its lockup clutch(C7) are located. These 3 gears are fixed, non shiftable and deliver an additional 1.2:1 gear reduction between transmission output and transfer output. When you add that 1.2:1 to the .65:1 out of the 3000 series transmission, you get our 7th gear .78:1 final ratio.

So where does the very low 1st gear come from? The reason the LMTV manual is misleading is although the clutch pack(C6) that gives us that low first gear is located under that round rear cover at the back end of the upper transfer case, the actual gear change occurs in the main transmission gearbox by altering one of those 9 gearsets. The teeth on the outside edge of the C6 friction discs are anchored to grooves inside that rear cover. The only thing that clutch configuration can do is stop something from rotating and there is nothing in the transfer case that needs stopping. The transmission output shaft that drives the 30T gear is hollow and that c6 clutch pack bolts right behind it. The transmission has another shaft that runs inside of the main output shaft between the C6 clutchpack and the rearmost sun gear in the transmission. This change delivers a 7th creeper ratio out of the main gearbox...

so dont think a speed increase out of this trans is really an option, unless we can get a redisigned transfer with a 1:1 ratio.

Anyway, FYI...

Keep hunting those high speed diff gears fellas...

Hi, I was told by Allison that the difference was planetary P1. Please consider that they may have been right and reassess?
Tom

m-35tom said:

Hi, I was told by Allison that the difference was planetary P1. Please consider that they may have been right and reassess?
Tom

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Reassess what? There may be other subtle differences in the gearbox programming, but I think the biggest difference between the 3060 and 3070 is how that P6 clutch is applied to the main gearbox to get a 7th speed...

In the end the math doesn't lie... The 1.2:1 gear reduction in the transfer case is fixed(non-shifting) as there is no hardware or room in the transfer case to allow those gears to change orientation or ratio. That is what started me thinking about this in the first place as reading the manual description and looking at the exploded views the hardware shown didn't look capable of providing a shiftable low range gear like the manual alluded to. The final ratio provided by that fixed gearing adds up mathematically with tires and axle to deliver 60MPH at around 2600 engine RPM with the .65:1 transmission output that the 3000 series transmission normally delivers...

IMO, Unless they make a 3000 series gear set with a taller final overdrive ratio than .65:1(unlikley), I think increasing LMTV highway speed thru the trans is a dead end without a different transfer case(way harder to find than 3.7R&P gearsets).

my point is that the P1 planetary in the 3060 is a different ratio than the P1 planetary in the 3070. as far as programming that is entirely different issue. I still think speed change is best done in the hubs and I have a design for 1:1 and 2:1 selectable hubs

m-35tom said:

my point is that the P1 planetary in the 3060 is a different ratio than the P1 planetary in the 3070. as far as programming that is entirely different issue. I still think speed change is best done in the hubs and I have a design for 1:1 and 2:1 selectable hubs

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question with a selectable hub is...... will the ring and pinon be strong enough to run at 1:1. If you look at trucks of this size w/out hub reduction or a hub planetary there 3rd member is a HUGE pumpkin... which leads one to think the R&P inside is HUGE too.

What I was lead to believe is axles with either hub reduction or a Planetary hub, can afford to have smaller pumpkin/3rd member and smaller gears inside because part of the forces are shared in the hub by the gears they contain...... That seems to point that without gears active in the hub..... there needs to be larger innards in the pumpkins. With out that.... the pumpkin's R&P becomes a weak point. Very possibly too weak

that may be, but 1:1 would only be for hgy use and you would still have 2:1 for off road or high power situations.

I am very interested in your idea! Hopefully we will learn more about it in the near future.. Thanks!

m-35tom said:

that may be, but 1:1 would only be for hgy use and you would still have 2:1 for off road or high power situations.

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I know this is an old thread, but I wanted to ask if this idea has moved forward? It seems like an excellent idea to solve some problems.

Stratosurfer said:

I know this is an old thread, but I wanted to ask if this idea has moved forward? It seems like an excellent idea to solve some problems.

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I won't say it's impossible, but changing the ratio THAT much would not be a good thing. At 2:1 the overall ratio is 6.xx-7.xx something, I don't remember exactly. 1:1 it would be 3.90. you would not have any low end power, and nobody needs a LMTV that will run 120mph or whatever it figures out to.

I have a set of high speed gear for sale (3.07) if interested. A switchable hub design would be OK if it works, but just not a 1:1 ratio.

TacMac2012 said:

I won't say it's impossible, but changing the ratio THAT much would not be a good thing. At 2:1 the overall ratio is 6.xx-7.xx something, I don't remember exactly. 1:1 it would be 3.90. you would not have any low end power, and nobody needs a LMTV that will run 120mph or whatever it figures out to.

I have a set of high speed gear for sale (3.07) if interested. A switchable hub design would be OK if it works, but just not a 1:1 ratio.

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Feel free to PM me a price on those 3.07 gears.... it's one of the next things I need to get done.