1999 FLD 120 Rear end keeps going

Hey all. Got a question to see what you guys think about this. I have a 1999 Freightliner FLD120. It was chopped down to 1 rear axle many moons ago from what I heard. It was chopped and turned into a mobile home toter truck. I dont have any specific measurements I can give right now if you wanted to know as it is at the shop right now. So I purchased the truck about 10 months ago. It was from a guy I run with. He pulled the 10 speed out and put an 18 speed in it. I will have to double check with him as he is not reachable for a couple weeks. But after he put the 18 speed in, I believe, he started blowing rear ends in it. 3 times with him. Then he said the guy he had it at figured it out. Did not have any issue for like 5 months. Then I purchased it from him and a month later the rear end went. Had a guy put one in, and then it went almost 4 months to the day and went again. That was this Tuesday. What do you guys think could be causing this? I have thought of the drive shaft is not balanced correctly or the pinion angle. The other owner said the guy shimmed the rear and that is what he thought solved the issue. But if he did, it did not solve the issue obviously. He also said that when he had the 18 speed put in, they custom made the drive shaft so I just assumed if they did, the guy would know how to balance it also. I cant say what happened when the other guy had it but with it going 2 times with me, I am wondering why I have no issues at all with the U joints but the rear is going. Any thoughts?

 

gear ratio in rears not matching transmission, causing it to overheat, if you’ve changed everything else from driveline (which had to be swapped for 18) and make sure putting in synthetic gear oil, or it’ll get hot and burn them out.

 

What rear is in it? What HP and What kinda weights are you pulling?

if it had standard 40,000lb rear and they removed the rear axle that one drive axle is taking a lot more torque rather than spreading it between 2 axles.
I'd want a 46k rear if I was running a single axle

 

how does the transmission have any affect on the rear end? all the rear knows is that the driveshaft is spinning it and non synthetic gear oil was the norm not all that long ago

 

My first thought as well. That diff is taking twice the torque that it would in a tandem setup.

Curious what the actual failures are? "Blown up" is nothing to go off of.

 

By being “blown up” who knows, but i do know one who changed a 10 to 18, which had to change drive line, but now the gears didn’t match the rears and in high gear at 65 was turning 2200 rpm so by not knowing everything, someone just guessed, but at least it didn’t cost him, as the shops have been doing.

 

I don't understand your logic, that diff only knows what's being fed into it regardless if it's a single/twin/triple. Parasitic losses would occur downstream on the output side, but not the input. At Yellow, our single axles were 23k#, but ODFL we had 20k# rearends. I agree that we need to know what failed before making an educated guess. Most mobile home toters have a wheelbase and turning radius of a grocery cart, could that be "over-working" the diff?

 

In a tandem axle setup the input power is split roughly 50/50 between the two diffs via the inter-axle differential. Remove one of the axles and now the one remaining axle takes 100% of the input power.

 

Wonder if the driveshaft slip yoke has about 2 1/2+ inches room before bottoming out?

 

I understand that, but a diff in a single axle isn't built to handle twice the load of a diff in a tandem.