how much "slop" is acceptable in the driveline?

[DZink24]

i just had my rear driveshaft rear u joint changed again about 1,000 mi. ago. currently sitting @ about 65,000 miles. i have some pretty significant slack, or slop in the driveline, resulting in very significant thud and clunking noises, esp. at low speeds and uneven pavement. with the truck in N, and the pressure off the transmission, i can twist the driveshaft by hand and it almost feels as if there is a broken tooth in the rear diff. there is that much play. both u joints are tight. i cannot move the shaft up and down, as i could when the u joint was shot. "clunk clunk, clunk" is the sound that i hear as i drive extremely slow over diagonal speedbumps.

every Dodge i have ever owned has had driveline issues, but this may be the worst i have seen yet.

 

[SRay]

DZink24: How far does the shaft turn?

 

[MChrist]

With parking brake set and truck in neutral, how many degrees (roughly) can you rotate driveshaft by hand from max left to max right?

 

[DZink24]

i can't really judge the degrees, but i will say i can move it at least 1/2 inch. but probably not more than 1 inch (2-3) cm. i understand some play is "normal" but the thing that gets me is at both ends of the shaft- at the diff, there is a noticeable "clunk" inside, almost like the gear teeth are not tight to each other. and at the back of the transmission/ x case, where the front of the drive shaft slides into the case, there too is slop. i will check it tomorrow, when i have the truck on level ground.

 

[MChrist]

Mine goes from 2:00 to 4:00 by hand, 1:00 to 5:00 loading up drivetrain forward and reverse. I have the Eaton Tru-Trac, so maybe it's the locking mechanism play??? Don't know.

 

[SRay]

DZink24: I think there is supposed to be . 010"-. 014" of backlash between gears with dial indicator on the ring gear.



I noticed your dyno numbers, the torque seems pretty low.

 

[SRay]

Dzink24: Sorry, that was the Dana 80 backlash setting, please disregard that message.

 

[cerberusiam]

i can't really judge the degrees, but i will say i can move it at least 1/2 inch. but probably not more than 1 inch (2-3) cm.

Mine goes from 2:00 to 4:00 by hand, 1:00 to 5:00 loading up drivetrain forward and reverse.

Either one of those is too much slack and indicates wear in the diff. The drive shaft flange should move less that 1/4 inch with the emergency brake set.



If the diff has too much slack it will show in the t-case also as those gears have a lot more end play in them.



The excess slack could be pinion bearings or more likely the spiders and side gears in the diff. You will need to pull the cover and figure out which one is causing it. The correct lash in the R&P will not generate anywhere near a 1/4 of slack in the drive line, but, worn spiders and side gears will generate a lot of slack and noise.

 

[MChrist]

Thanks Cerb, I'll get her on the lift tomorrow. I had the Tru Trac installed at around 30,000 and have 78,000 now. I'll keep people posted. Not totally sure what I'm looking for, but if it's obvious, I'll find it.

 

[SRay]

MChrist: How do you like the Tru-Trac? I had one installed in my ranger and loved it but no idea how well it performs in a Dodge.

 

[DZink24]

now i wonder if i should leave it in park and try. i dont have an emergency brake, and the more i think about it, there actually should be some slack in the driveline with the truck in N and all tension off of the gears.

that torque reading is off the motor in stock form. is that a bit low for a 555 motor? i was surprised at the HP #, only losing about 16 horses from stock.

 

[cerberusiam]

There will be more slack in neutral but that does not matter as long as you eliminate the wheels turning. Block the wheels so they won't turn and leave it in neutral, if you still have a lot of slop it is in the diff.



Your TQ is low but that is dependent on how you ran the truck on the dyno. If you didn't brake boost to over 12 psi, use a lockup switch in OD the TQ always comes in a little low.

 

[SRay]

DZink24, Cerberusiam: That could be the reason the tq is low, depending how the truck and dyno operated. Usually the Cummins has always been underated, producing more than advertised. My friend's 2010 Ram pulled 291 rwhp/740 rwtq stock.

 

[MChrist]

MChrist: How do you like the Tru-Trac? I had one installed in my ranger and loved it but no idea how well it performs in a Dodge.

It works great, much better than stock limited slip. Quite often I have to pull 8,000 lb trailers through customers' yards, and can't afford to spin my tires. One would spin before, and not now.

But if it's causing a lot of slop after 50,000 miles, that may not speak too well of longevity. I'll know more in a few days.

 

[cerberusiam]

Usually the Cummins has always been underated, producing more than advertised. My friend's 2010 Ram pulled 291 rwhp/740 rwtq stock.

Nope, always less than advertised on a chassis dyno. Never ever saw one pull more than rated unless there was problem with the dyno connections.



The advertised numbers are engine dyno numbers NOT chassis. Not to mention the engine dyno numbers and curves are with all the management thrown out. It just doesn't operate that way as installed.



Even the 291 is suspect. I would guess that is not SAE numbers and the TQ is way off even for an 800 tow special. The chassis dynos are going to loose 12% on a good day and if you did the run right, 15-20% normally. It will look much better with corrected numbers but normally that only really applies to aspirated engines not inducted.

 

[SRay]

At 291 hp with 20% loss came to 349 hp flywheel. The torque seems way off but the dyno operator says it's what it says.



We could've tried it on the hydraulic continuous load dyno but the dyno day was on a sunday and the people that owned the truck are church going people. Power numbers likely would've been different.

 

[cerberusiam]

At 291 hp with 20% loss came to 349 hp flywheel. The torque seems way off but the dyno operator says it's what it says.

I am thinking 325 HP and thats a 350 HP rated engine, correct? The HP is about right but the TQ is still off. He can say "it is what it is" but it ain't. :-laf



Doesn't matter a lot becuase the corrrect way to use the dyno is make changes then see if the numbers go in the right direction. Trying to base exact numbers of an accelerometer is simply not going to work with these trucks.

 

[SRay]

Yup it's a 2010 with 350/650 cummins. Maybe if it was on my favourite dyno, it should be more closer or under 650 tq at the wheels.

 

[MChrist]

Ok, getting back to the "slop" topic... yesterday, I got my truck back on the lift. This time, I lifted the rear a few inches, put rubber chocks fore and aft of the rear tires and lowered the truck back down. With truck in neutral, I only get about 1/2" of play measured at the pinion seal. This was by moving driveshaft by hand both ways. Last time I did this, I was getting movement from 2:00 to 4:00 position, but the rear tires must've been moving slightly.
I changed diff fluid, nothing special noted there, and the Eaton Tru Trac is enclosed, so nothing to see there, either.

I took pictures, but nothing looked wrong. I guess I'll just let her go. By the way, I couldn't find 75w90 NON synthetic ( which is what Eaton recommends) so I used 80w90 mineral.