Power divider/axle interlock
- Thread starter GAnthony
- Start date
He likes to burn up rear ends, I guess. One of the most FOS screeds I've ever seen on these boards...and we've had some doozies.
I've forgotten to switch the divider lock off in this 579 a couple of times. I get a dash alarm when I get up to 25 mph.
Obviously, Peterbilt has a strange idea that it shouldn't be run at highway speed for some reason. But the manufacturer and my training are wrong because some guy on the internet said so.
Ranger_375
Well-Known Member
I guess I can weigh in.
This applies to "my" '17 noobstar, your mileage may vary, if you already know all this then it's not intended to teach you a damn thing nor offend your intelligence. Also, dunno if it was just my trainer/company, but I'm lucky I had a new truck with the owners manuals because I didn't get taught a gordang thing regarding what-all everything is/does.
PDL != diff lock. Tandem functionality 101: Each axle has a differential to make up for tire size differences, but primarily for going around turns without dragging a tire. A drive tandem also has a differential between the two axles if you have a driven rear axle. If you don't, this whole discussion is pretty much moot as you're driving the big truck equivalent of a 2wd pickup....
So you flick this thing on. Now the drive power is going straight back off the transmission all the way to both axles equally. What happens in those differentials is a function of traction and tire mismatch (if any). It locks the inter-axle differential, so each axle is now driven at the same rate. It does not mean you're not going to get wheelspin! Backing under a heavy trailer in a nasty dirt lot I've still had independent tire spin on both axles (which irked me, because now I had to use momentum to get under it instead of finesse).
I get, yet don't get, why it accelerates axle wear. If you have mismatched tire diameters due to tread depth, the differential's going to be making up the difference regardless the whole time you're going down the road. That diff action isn't going to really change with power applied, and that intermediate driveline and power divider differential is going to be freewheeling, so you're spinning multiple sets of spider gears (axle diff and power divider) instead of confining it to each axle due to diameter differences. I suppose if you have a mismatch not only between sides but also front/rear in a tandem set, you'd have way more wear... but you'd also have way more wear and loss of economy as you'd be having traction wear on the larger set of tires and drag wear on the smaller set.. larger's going to be trying to cover more distance and all that.
I can support the concept that it would add to the ability to jackknife with un-augmented traction (not chained up) because freewheeling axles are less likely to break loose than ones with power applied. I am sure the one time that my drives have gotten squirrely, it would have been a horrible day if I had been running with the PDL on. As it was, the freewheeling tires were still back there to help maintain things where they were supposed to be, and throttle reduction + gentle trailer drag got things copacetic. I still have no clue what specifically contributed to that, as the road was dry and I was doing 5 under the truck recommended speed getting passed like I was standing still by other trucks. I just pay additional attention to that specific curve now because the pavement pirate wanted to hijack my day right there.
Traction control I just leave on, unless I'm in a position where I think wheelspin is going to be a boon. Finesse with an auto is a challenge, there's no feathering the clutch to milk just what you want. I also use it as an indicator just like a straight line brake check ABS check as to actual road condition performance versus the speed I'm at. If I feel it engage (and see the indicator light up) then guess what... slow the frick down, hot rod. You'll feel the chitter in the rear end when it engages, as it's functionality is typically provided through the air brakes as well as electronic throttle retarding.
I'd like to have diff locks as well as a PDL, but that's where a "my spec" versus "fleet spec" truck differs.
This applies to "my" '17 noobstar, your mileage may vary, if you already know all this then it's not intended to teach you a damn thing nor offend your intelligence. Also, dunno if it was just my trainer/company, but I'm lucky I had a new truck with the owners manuals because I didn't get taught a gordang thing regarding what-all everything is/does.
PDL != diff lock. Tandem functionality 101: Each axle has a differential to make up for tire size differences, but primarily for going around turns without dragging a tire. A drive tandem also has a differential between the two axles if you have a driven rear axle. If you don't, this whole discussion is pretty much moot as you're driving the big truck equivalent of a 2wd pickup....
So you flick this thing on. Now the drive power is going straight back off the transmission all the way to both axles equally. What happens in those differentials is a function of traction and tire mismatch (if any). It locks the inter-axle differential, so each axle is now driven at the same rate. It does not mean you're not going to get wheelspin! Backing under a heavy trailer in a nasty dirt lot I've still had independent tire spin on both axles (which irked me, because now I had to use momentum to get under it instead of finesse).
I get, yet don't get, why it accelerates axle wear. If you have mismatched tire diameters due to tread depth, the differential's going to be making up the difference regardless the whole time you're going down the road. That diff action isn't going to really change with power applied, and that intermediate driveline and power divider differential is going to be freewheeling, so you're spinning multiple sets of spider gears (axle diff and power divider) instead of confining it to each axle due to diameter differences. I suppose if you have a mismatch not only between sides but also front/rear in a tandem set, you'd have way more wear... but you'd also have way more wear and loss of economy as you'd be having traction wear on the larger set of tires and drag wear on the smaller set.. larger's going to be trying to cover more distance and all that.
I can support the concept that it would add to the ability to jackknife with un-augmented traction (not chained up) because freewheeling axles are less likely to break loose than ones with power applied. I am sure the one time that my drives have gotten squirrely, it would have been a horrible day if I had been running with the PDL on. As it was, the freewheeling tires were still back there to help maintain things where they were supposed to be, and throttle reduction + gentle trailer drag got things copacetic. I still have no clue what specifically contributed to that, as the road was dry and I was doing 5 under the truck recommended speed getting passed like I was standing still by other trucks. I just pay additional attention to that specific curve now because the pavement pirate wanted to hijack my day right there.
Traction control I just leave on, unless I'm in a position where I think wheelspin is going to be a boon. Finesse with an auto is a challenge, there's no feathering the clutch to milk just what you want. I also use it as an indicator just like a straight line brake check ABS check as to actual road condition performance versus the speed I'm at. If I feel it engage (and see the indicator light up) then guess what... slow the frick down, hot rod. You'll feel the chitter in the rear end when it engages, as it's functionality is typically provided through the air brakes as well as electronic throttle retarding.
I'd like to have diff locks as well as a PDL, but that's where a "my spec" versus "fleet spec" truck differs.
Duck
Sarcastic remark goes here
Yeah.
Problem was I used the interaxle lock hooking up to a trailer on ice, then left it on until I got out to the street.
By then, there was a strain on the lock because one drive axle had more tread on the tires than the other. The strain prevented it from disengaging even though the switch was off.
By forcing both axles to turn at the same rate, but one axle with thinner tread trying to turn slightly faster, it just kept building up strain and heat.
Mechanics never diagnosed it though. I did.
Based on the fact that the mechanics couldn't....
Because to put it in the shop required I back into a parking spot (releasing the pressure by turning the axles in the opposite direction) then unhook, taking weight off the tires,... And the first thing they did in the shop was jack a hub off the ground.
So there was nothing for them to find except high axle temps.
After I finally figured it out, I told the boss and he had me test the theory...
Stop in a rest area, lock the interaxle, drive through the rest area and unlock the switch while on the ramp back onto the interstate, then watch the axle temp gauges.
Sure enough they started climbing.
Next exit ramp, stop, wait for no traffic to see me, then back up about two truck lengths, interaxle lock switch still off,... Then get back on. Axle temps fell and stayed normal.
Having axles with two different tread depths on them was the problem. Either that, or the spring that disengages the lock was weak.
The fact that the axles were not DESTROYED by this was probably because of the extremely cold weather at the time.
No you didn't, I did
Duck
Sarcastic remark goes here
Find and quote it.No you didn't, I did
I diagnosed it while driving.
I remember you and like one other person agreed with the diagnosis but everyone else either had their own theory or simply didn't understand the explanation as it was written.
Can't quote a phone conversation, LOL.
If you remember, I told you that you needed to rotate those drives before you had the problem because you had racing slicks on front, and thick rubber on the rear. Can't remember if it was actually measured, but it was a definite noticeable difference in tire size on that fine piece of machinery you were driving, LOL
Duck
Sarcastic remark goes here
Well in that case you probably did predict it but all I heard was "blah blah blah"...
And yes they were measured.
One axle had almost new tires, the other was getting close, but still legal.
and different brands. I remember thinking you had different tire sizes when I glanced at them.
I remember it was shortly after that when the axles heated up, and you safely parked and called me on the phone because it would be unsafe to talk while driving........
Duck
Sarcastic remark goes here
OK now you're sabotaging your own credibility.
ready to roll
I ain't got no panties
I forgot to flip the switch back once on a loner truck and drove 100 miles on dry pavement at 65 mph untill i could smell something like burnt axle grease and then i flipped the switch off. 
I forgot to flip the switch back once on a loner truck and drove 100 miles on dry pavement at 65 mph untill i could smell something like burnt axle grease and then i flipped the switch off.
When I was shuttling trailers overnight for US Foodservice, one of the delivery drivers (the one who drove the same truck I drove overnight) would always have the axles locked. I told him, he acted dumb. I told the company, they did nothing about it. Wasn't long until he burned up an axle.
I had the joy of getting a loaner truck from MHC while the truck was down, and that night, I picked out a nice long KW, complete with a sleeper. It was a nice ride for me, since all I had to do was pull a set of doubles to Little Rock and back. It sucked for him because he had to pull one of those pups around all day long behind that truck making deliveries. Took about three weeks to get the POS daycab back...
r3gulator3
IMA KEEP IT REDNECK
I wouldn't say it's hard. It's labor intensive. Have to pull driveline and axles. Drain diff and remove power divider. There are people who say you want to align the input yoke with the output yoke on the back of the axle but that's an old wives tale. You will need a puller for the yoke. And seals for the axles. Rtv to seal the power divider. And a LOT of brake clean. Make sure you get all the metal out of the axle housing.
r3gulator3
IMA KEEP IT REDNECK
Oh I'm sorry, I made the assumption they you were going to do a full carrier replacement. Yes the older versions of power divider you can just pull that section. To do it that way you will have to shim bearings and adjust endplay ECT. If you have a manual and a dial indicator that will read thousands of an inch. It's not super complicated just take your time and make sure it's right. You will still want to pull the full carrier to ensure you get all the metal cleaned out.
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