Diff lock vs inter axle lock

Discussion in 'Experienced Truckers' Advice' started by Air Cooled, Jan 13, 2017.

  1. Air Cooled

    Air Cooled Road Train Member

    Jul 17, 2011
    Howdy folks. So as the title says I'm looking for the biggest difference between the two and why and when you would use either.

    Most trucks I've driven have had a diff lock. I'd often switch it on when going across mud, loose and uneven terrain, and if a wheel spins. I know what it does.

    The other night I drove a dump truck. It was a nice change of pace. Well, as you may guess, it had both switches. I read the manual and both are pretty much used the same way in off road situations mostly. So on each load I tried one or the other. There was a steep loose incline that I had some wheel spin without any switch on but after each load I tried one or the other and had no problems climbing the loose rock hill.

    I believe in the manual it stated that the inter axle transfers even torque to all the drive axles and the diff lock did something similar by having the drives not run independently. I also read do NOT run both at the same time as steering will be heavily compromised.

    I'll be running a dump truck more often in the summer and I'd like to know under which circumstances one would favor the other and why. Thanks :)
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  3. AModelCat

    AModelCat Road Train Member

    Jul 7, 2015
    Inter axle locks the front and rear drive axles together. Diff lock will lock the diff together so that it behaves like a solid axle.
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  4. x1Heavy

    x1Heavy Road Train Member

    Mar 5, 2016
    White County, Arkansas
    That pair of drive axles will pull together with the inter lock.

    With diff lock both sides of the primary drive, usually the first one will pull together. But if you throw both on... that thing will forget how to turn right or left worth anything.

    I prefer the interlock with independant power to all 4 wheels on both axles. So if they have to spin a little let em. Big doggies gotta eat.
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  5. cnsper

    cnsper Road Train Member

    Feb 28, 2014
    Interaxle is like limited slip on both axles. Diff lock is like having positraction on both axles. I always start with interaxle the lock the diffs if that is not enough.

    Never ever ever flip one when a wheel is spinning. Bad things happen when you do that.
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  6. Accidental Trucker

    Accidental Trucker Road Train Member

    Jun 4, 2015
    Think of it this way. You have an open differential on both axles, and an open differential between the axles.

    Interlock locks the differential between the axles. You will still spin the tires with the least traction on each axle. Differential locks lock the differentials in each axle. Both wheels on one axle will turn the same speed, but the power will go to the axle with the least traction.

    Interlock leaves the truck pretty driveable, but you will have slightly less traction than with locked axles. Both together works fine to get out of a sticky situation, but as mentioned, steering will be less responsive.
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  7. Raezzor

    Raezzor Quis custodiet ipsos custodes?

    Aug 1, 2009
    Columbus, OH
    Also, you can use the interaxle lock at higher speeds but it's recommended not to use the diff lock above something like 25mph. I know our Ryder daycabs automatically kick it off after 20mph or so. Kind of a pita when you are sitting there spinning even though you are fully locked and your #### diff lock keeps turning off because the truck thinks its moving 25mph.
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  8. Lepton1

    Lepton1 Road Train Member

    Nov 23, 2012
    Yukon, OK
    Good question.

    Most OTR drivers are never in a truck with both. Almost always you only have an Interaxle Differential Lock (IAD Lock), which is also called a power divider. A tractor with two drive axles has three differentials. This locks the differential BETWEEN the axles. It distributes power to both axles. You can run with engaged all day long at highway speeds, just never engage it while you are actively in a spinout.

    The differential lock, which is on trucks that are specced for off road driving, will lock the front and back differentials. Only use this for short periods of time at slow speeds (below 25 mph). I only use it for powering through mud in low range. It WILL cause your steering to be less responsive. The locked axles will tend to push you straight when you want to try. Overcome this by easing off the throttle in turns.

    This is a picture that shows a truck with both options.

    The middle switch, with the "X" between the axles is the power divider. The right switch, with the "X" on each axle differential, is the Differential Lock.

    When off road or in slick conditions of any kind I engage the power divider. Once I get in really bad traction conditions I engage the differential lock.
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  9. Roberts450

    Roberts450 Road Train Member

    Feb 6, 2016
    This is a go straight recipe when its slick out. Interaxle and 2 diff locks.

    Attached Files:

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  10. Aradrox

    Aradrox Heavy Load Member

    Jan 21, 2016
    The PDL switch on International would be the interaxle lock right?
  11. not4hire

    not4hire Road Train Member

    May 16, 2012
    Yes, PDL = Power Divider Lock
    Lepton1 Thanks this.
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