M917 driveline knocking?

[Abrant23]

I have a M917 dump that has developed an odd clunking or knocking sound. The truck only makes the noise when accelerating or under load and only in 14-16 gears. Zero noise in 1-13 gears. Any ideas what might be the cause? Engine runs fine and transmission shifts and engages perfectly. No power loss or vibrations can be felt. I've searched and searched but cannot figure out where the noise is coming from! Any help is appreciated.

 

[NDT]

I have a 915 and 916 with the 7155 but neither has this issue. Maybe I can help with narrowing down which component. So you can be in 13th at say 30 mph, go from float to full throttle, no noise. Shift to 14th, say 35 mph, full throttle, "knock-knock-knock", release throttle, no noise? Is it a fast knock like a rod a sounds like, or slow? My first thought is that is issue is inside the 7155, because an axle, driveshaft, tire or transfer case related noise would be present at 30 mph as well as 35 mph. I would drain some oil from the 7155 and see if there are excessive filings at the bottom. I would also study the power flow through the 7155 and see if a gear group is in use at speeds 14-16 that is not in use at 1-13. Let us know how this turns out!

 

[Abrant23]

The knock gets progressively faster the faster you go in each of the high gears. When you shift into 13 there is no knock until you get almost to the shift point. Once you shift to 14 and get back on the throttle it starts again. Same for shifting to 15 and 16. Once in 16 it gets progressively faster till you're at cruising speed. If you lift on the throttle it goes away. If you ease into the throttle to just maintain speed on flat land it is there but no where near as loud. Any time you stand in the throttle it gets very loud. I have noticed a small leak on the pinion seal on the front drive axle but nothing major. I also checked fluid levels in both rear axles. They are fine. It has me boggled.

 

[NDT]

Now I am thinking the knock is related to the effort the engine is putting out. I have a boost gauge on mine and little if any boost is developed until gears 13-16. Sounds like you have an engine rod? knock whenever the 400 is putting out 200+ horses. Drive the truck up a very steep hill loaded with dirt and see if it knocks in 1st-2nd gear.

 

[Abrant23]

I'm in the lowcountry of SC, there aren't any hills anywhere near me! I do know when I'm pulling out of the pit with 20 tons of dirt on it there is no knocking. That's usually in 2nd or 3rd gear, but the dirt ramp is only like 40 yards long. I don't know if that is enough of a run for it to build up boost?

Thanks for the help btw. Hopefully I can get this figured out.

 

[NDT]

Next time coming out of the pit get on the throttle 100% at the highest gear you can and see what happens. Should be at max HP at that point. I don't need to tell you that the cause of this knock will soon make itself known if you keep driving the truck.

 

[Abrant23]

I heard it immediately yesterday evening on my first trip out to the pit with the window down. I heard it as a car passed by me and the sound was reflected off the car. My boss and I have been driving it for months, windows up, headphones on so I have no idea how long it's been doing it.

I don't want to find out the hard way what's causing it, that's for sure. I parked it soon as I got back to the farm and started googling everything I could think of. I suppose it'll be headed to a diesel mechanic tomorrow.

 

[Floridianson]

I get a knock out of the air system when the compressor kicks.

 

[BKubu]

PM RRACING; he repaired an M917 that was making a "clunking" noise. I am sure he can fill you in...or you may be able to search this site. I am sorry I can't be of more help.

 

[Abrant23]

Well, after $1800 at the diesel mechanic they threw up their hands and said the noise was a mystery to them. So I took it back to our shop and started going over what I thought could be the cause. Drained transmission. Only a flake or two on the magnet. So I drained the forward rear axle. Bingo. I think I found the culprit. Looks like the rear eve had some sort of failure. Now to try and fix it!!

 

[Abrant23]

Pics of the carnage

 

[Floridianson]

Ouch. You need a Detroit locker while you got it apart.

 

[doghead]

Noise crumbs...

 

[Abrant23]

There's the clunking sound....

 

[M813A1]

That is what I was going to tell you to check it sounded like you had a chipped tooth or 2 on the ring and pinion gear