Trying to Diagnose Bad Differential

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by TheBaron97, Aug 6, 2020.

  1. TheBaron97

    TheBaron97 Bobtail Member

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    Hello, I've been searching through old posts to see if I could find any information on how to diagnose a bad differential.
    I bought a 2004 Peterbilt 379 last month and have been working it steady for about three weeks, until a few days ago when I was pulling a full load of production water up one of the steepest dirt roads I've ever been on. I had my inter axle differentials and my full lockers engaged before heading up but somewhere near the middle I started hopping and spinning out because of washboards and potholes in the road and I heard a load snap. I was in third gear (13 speed) and I stayed in third after the loud noise and made it the rest of the way out, however when I got to flat ground again and disengaged my lockers my truck would go anywhere. When I engaged my inter axle diff the truck will start acting like it wants to go but it feels like it's going to kill the engine before I completely release the clutch, so I didn't try a full pull with just my interaxle engaged. But when I use my full lockers I go forward just fine, albeit with what seems like a little less power? And definitely some odd noises and vibrations. To make a long story short I ended up dropping off my load and driving back a half hour home going 55 mph. I spoke to a mechanic who rebuilds differentials and he told me that it was most likely one of my axles and to pull both of my axles and see if either of them are snapped. And if neither of my axles were snapped to pull the differential because it's probably bad.
    So I spent the whole day, I pulled both sides of my axles first and neither of them appeared to be damaged except for the splines were a little bit nicked and dinged in spots but not too badly (see photos attached) So then I moved on to pulling my differential and after I got it out I didn't notice any obvious damage at all.

    So here is my question, how do you diagnose a bad differential if there aren't any obvious broken gears or any metal in the oil? And also can I reuse my axles in their current state or do I need to buy new ones or get them machined or something? The differential is sitting in the back of my pickup truck now with the axles. Also I have a friend that bought a re-manned differential from Peterbilt for somewhere around 1500 but everyone has been telling me a new differential cost anywhere from $3,500 to $4,000 and I'm not sure how to get the best bang for my buck if I want to go kind of cheap on this one. I worked in the oil field and differentials brake constantly due to the amount of weight we haul and the abuse the drivers put them through. Let me know if I can get any photos or post any clarifying questions. Thanks in advance.
     

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  3. TheBaron97

    TheBaron97 Bobtail Member

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    I apologize for the spelling errors, when I tried to go back and change them the text goes all wacky. I don't know if anyone else encounters this problem.

    But something else I think I should add, I disengaged all of my lockers and put it in gear before I pulled my differential. And the driveline spins free and sounds like hell but the truck doesn't go anywhere and I can get out and look at it spin. However due to the fact that I was on the side of the road I didn't take the time to check whether the small drive line between my two axles was spinning or not. The differential I pulled was my front drive axle not my rear. And I'm 90% positive the front drive axle is the one that has power when I'm not driving with my lockers on. I don't know why I would go through all of the trouble of removing the differential before double-checking which axle was actually my drive axle, but here we are anyways.
     
    Rideandrepair and tommymonza Thank this.
  4. baha

    baha Road Train Member

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    You may have snapped a rear axle??
     
    Last edited: Aug 6, 2020
    Rideandrepair and BoxCarKidd Thank this.
  5. AModelCat

    AModelCat Road Train Member

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    Just some spline wear on the axles. Hard to tell with the pictures but it looks like it could go either way.

    Most cases you can pinpoint which diff had the failure simply by engaging the lockers 1 at a time. I'm guessing you either grenaded the interaxle diff or one of the side gears in the diffs.



    Just a heads up, BOTH axles are full time drive axles. They each do 50% of the work all the time. The interaxle is simply nothing more than a diff between the diffs, which allows for speed variation between the two drive axles. Lockers simply lock both axle shafts together within the same housing. The interaxle lock locks the thru shaft of the front drive together.

    Believe it or not, when you spin out and only one set of duals are spinning, the other 3 are getting exactly the same amount of torque to the ground as the spinning wheel. But because its not enough to propel the truck forward, it just sits there with one corner spinning.
     
  6. TheBaron97

    TheBaron97 Bobtail Member

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    If I snapped a rear axle though,wouldnt I still be able to drive with my lockers disengaged?
     
  7. AModelCat

    AModelCat Road Train Member

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    With everything unlocked, its a balancing act between all 4 wheel ends. Break ANYTHING between the input yoke of the forward diff and any of the hubs and that's it. Path of power is broken, driveshaft will just spin.
     
  8. BoxCarKidd

    BoxCarKidd Road Train Member

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    Been there and followed that kind of advice. Put a pencil magnet down in the bottom of the rear rear. What do you find?
    Kind of sounds like multiple problems.
     
  9. TheBaron97

    TheBaron97 Bobtail Member

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    I'm going to do further research tomorrow and get back with you guys. Thanks for the responses.
     
  10. Isafarmboy

    Isafarmboy Medium Load Member

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    Just the way he explained it happened....its got to be the power divider on the front axle. Heard it all before.
     
  11. TheBaron97

    TheBaron97 Bobtail Member

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    How would I know if it's the power divider? I don't know what is supposed to spin and what's not.
     
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