I recently purchased a mack dump. I have noticed that the back rear is getting pretty hot. So hot that you can't touch it. It is just the back rear. Our other macks get hot but not like this. Our eatons on the peterbilts just get warm. I am not sure where to look. At first we thought it was the power divider hanging up but it wasn't that (air powered). Tire size is about the same on both axles (less than 1/2in difference). I think if the rears were different ratios it would be in pieces by now. There is a normal amount of backlash by feel. Do mack rears just run hot? And does the back rear usually build more heat than the front. Is it possible that a bearing is failing?
I think you should be draining that diff into a clean pan to see what you have, Is it single or double reduction? usually a rear diff will run a few degrees cooler than the front due to fewer gears and bearings. I think theres a problem thats about to show up soon.:Banghead
It is a single reduction. I have drained the fluid and there was a good amount of suspended metal in it. We put new fluid in it and put about 30 miles on it. Haven't checked it yet. I thought the same thing about the front rear typically running hotter than the rear but our other macks are holding more heat in the back rear to. I should mention that this is at 55-65 mph. It doesn't make any noise or whining.
The rear axle turns the truck until you lock in power divider, so the rear should run hotter as its doing the work.
No, power is divided more or less equally to all four drive wheel sets as long as the divider lock is not engaged. If you had a major difference in ratios or tire diameters front to back the difference would be absorbed in the differential gears of the divider head so that would be the most likely place to overheat. If your tires on one side of the rear axle were markedly different in diameter than the other side that would create a constant turning of the differential gears (spider gears to the unenlightened) which would cause excessive wear and possibly heat. The most likely cause given the metal in the gear oil is simply bearings that are desperately trying to hang on until you replace them. Might have been set up too tight to start with, no way to know without pulling it.