I was reading through a new driver's account, and he said something about the "pros and cons of locking your differential." That reminded me of a question I had been meaning to ask.

I've been driving a single axle for most of my career, and whatever I knew about having two drives is musty and old.

I think I lock that thing when the vehicle is stationary, and use it to get out of a dock at the bottom of a hill or something, or mud, etc., but shouldn't flip it while in motion, and shouldn't exceed a certain road speed with the thing locked, and power going through both differentials.

I guess this thing engages the short back shaft somehow or other, so power can go through both differentials, and all of it will be going to one of the four wheels in a slip condition, unless I have positraction rears. (No idea if I do.)

I'm a little confused by the time I spent in a cement mixer. That thing had two switches. I think one of them locked the differentials completely, so both wheels would turn at the same speed, and the other one performed the same function that I'm talking about in this thread, where both differentials have power, but it's some 60/40 split or whatever between the two wheels out of four that are getting it. (With only one of the four breaking all the way loose and spinning if you get in goopy mud. Or would two of them break loose and spin? Diagonals on different differentials?)

Where I'm running now, I can't imagine needing this except to deal with w*nt*r, or maybe having trouble getting under a trailer in a gravel lot. I'm not hauling sticks (furniture, I remind everyone, not logs) anymore, and I don't get on any skinny roads these days, so hopefully I don't have to worry about sinking up to the axle in mud in a ditch trying to turn around in a car sized cul-de-sac.

 

The standard differential lock allows even power to be distributed to both drive wheels on an axle. When it is off most if not all the power goes to the wheel with the least rolling resistance.

Never lock while you have a drive wheel breaking traction. You should be stopped or at a very low speed (15 mph or less) when you throw the switch.

Max safe road speed is about 40 mph with the lock engaged.

You can unlock them while moving.

 

Most truck have a power divider that you can lock. Very few have actual differential lock.

A unlocked power divider allows power to go to any 1 of the 4 wheel sets. When the power divider is locked power goes to 1 set on the front and 1 set on the back, usually the sets with the least amount of traction (if they spin).

Differntial lock does exactly that, it locks the differential which doesn't allow each side wheel set to move at different speeds, like when taking a corner.

A farmer I work for has a day cab that's must have the power divider, and differential lock on one switch. Light on dash has an X in the middle of the differential, and I spin all 4 when sliding the fifth. Also saw a heavy haul Kenworth spin 2 sets 1 front and 1 back when he has suspension problems.

 

The dump trucks at work have both the conventional inter axle power divider lock, and a separate control for the differential lock, which locks each differential so there is no differential action from side to side.

When both systems are activated all 4 sets of drive wheels are receiving full power.

This helps greatly to get the truck unstuck without external help, such as pulling with heavy equipment or a wrecker, however, if 3 of the wheels are on a slippery surface and 1 wheel has good traction, that can result in a situation where one axle shaft is taking 4 times the normal amount of torque, so caution must be used.

Also, once you get unstuck, if you forget to disengage the differential lock, it will be just about impossible to steer the truck, since all 4 drive wheels are locked and cannot turn at different speeds in order to make a turn.

 

I think things have changed since the old days and misbeliefs..... when locked in I still have only 2 wheel drive, I was under a trailer in the dirt and one wheel spinnin, so locked it in , the oppisite wheel on the different axle and the first spinin wheel were throwing dirt.... left 2 holes not four.... and while driving I can lock in the drives, as long as its not slippin or spinnin out they lock in, I can be driving 35 and flip the switch and its fine, I have drove at 65 with them locked in and its fine, i have locked them in at 50 mph and its fine, I looked everywhere in the owners manual from freightliner and it does not say to drive slower than 40 or any speed, it just says to not engage while spinnin, plus with this freightlinner, if it is spinnin the red light will not come on and it won't engage until its okay and it will lock in when it thinks its good to.... so its really hard to mess up with a new freightliner, I tried everything with that switch.... I lock it in when in snow, ice, dirt, or bobtail.....

 

on the tandem drives you have what is called a power divider you can flip it to engage or in at any speed you wish and you can run any speed you wish (but why would you) the purpose of this is to aid in traction as in an
uneven surface such as a drive way with a dip or a ramp where one axle will lose traction or on an icy road or a muddy area. NEVER FLIP THIS SWITCH IF A WHEEL IS SPINNING!!!! you will tear out the diff or twist drive shaft or ???= damage!! You still have two open differential rear end meaning that all four sets will not locked or pulling equally, that where the diff lock comes in most look like a park brake button but not in same location as the brake buttons this will lock all four sets of duals on the drives this is only for off road ONLY, IT WILL NOT LET YOU TURN it will cause a push like they say in nascar, lol You dont have a choice as to keep your speed down with this set up as when flip to the high side it pops the button out and disables this. sorry about being long but wanted to help

 

Yes, you are referring to the traditional power divider lock, which locks both axles together, front and back.

The trucks at work have that plus the differential lock which locks both rear ends side to side so that both wheels are getting full power.

These are all late model (2000+) Internationals.

As the previous poster said, you cannot steer with all those drive wheels pushing straight ahead with no differential action.

If you have the power divider lock and diff lock engaged, all 4 drive wheels will move, or something will break, whichever comes first.

 

Our petes have three switches on the dash, The first is the power divider, the second locks the front axel only, the third locks the back axel only. I would not advise using all at same time, truck tends to go more sideways than straight ahead. We usually only lock which axel has the tire chains on.

 

Here you go gentlemen.

Links to the manufacturer's tech literature.

Now you don't have to take my word for it. Enjoy.

http://www.meritorhvs.com/MeritorHVS...nts/tp9579.pdf

http://www.meritorhvs.com/MeritorHVS...nts/tp9238.pdf

http://www.meritorhvs.com/MeritorHVS...nts/tp9781.pdf

http://www.meritorhvs.com/MeritorHVS...tp9579ALBL.pdf

 

I don't drive no Meritor man. I drive a KENWORTH. Ain't you got no sense boy?

This has been an interesting thread. I can't remember what my dash says. It's not the standard OEM dash, but something chromed up, so it's impossible to tell how accurate it is. (The shift pattern plate shows it as a super 10 with an automatic top two, but it don't automatic nothin', and the plate LIES.) It calls this thing a "Highway" something "Differential" something.

It's got to be the "interaxle" thingie, because it's not a differential lock. I threw that on while backing up a gravel hill, and I had no problem steering.

I just had some idea the thing was like 4WD on a fourwheeler. I've never had a 4WD fourwheeler, but I understand you don't want to engage 4WD mode above a certain road speed for some reason or other. (Yes, I'm a truck driver, and my fourwheeler is a four door SEDAN. Sorry.)