Hello everyone! Quick back story on this truck, since I’m sure I’ll be a regular on here moving forward. I was always into cars instead of trucks, so this is all new to me. My brother Shaun passed away last October due to heart failure. He was truly the most amazing big brother anyone could ask for. He was 42 and taken way too soon, leaving behind a 14 year old girl and a family that misses him dearly. When he passed, I found an insurance card to a 2006 Sierra Denali crew cab that he loved so much he used it as the front cover on most of his profiles, his detailing business, etc. I decided to do a CARFAX to see if I could find the truck. I noticed a shop in PA was servicing it regularly and called them immediately. The guy luckily gave the owner my info and he called me later that day. He said it failed PA inspection for rocker rust and after hearing the story, he said make him an offer. My father and I drove up, gave him 5k for it (pretty rough shape) and drove it home.

his daughter was in shock when I showed her it and tries to learn as much as she can watching my dad and I restore this thing. So in memory of Shaun I want to make this thing like new again, plus make it my “hot rod” since this is now taking all my free time and money.

Sorry, onto the issue. I’ve stripped the front frame completely (every part and took off the GM wax). I got mostly moog replacements parts (literally everything you can think of - I wasn’t messing around). But, my issue now is I’ve gone through three front differentials (GT5 - 4.10). It’s got a new front driveshaft (balanced true) and two new CV axles. So everything touching the front is new. But again, I’m eating front diffs like water. Sharp rights I hear some faint grinding, and vibrations from 35mph and up. When letting off the gas at 60-65mph, I hear and feel a grinding feel. I just don’t know what’s causing it. Every time I drain the fluid on a new front diff (from powertrain products in MD) it’s pure silver. Drain plug shows no chunks, just tons of shavings. I get about 50 miles out of a diff before it’s toast.

details on the truck.
-lowered 2” under factory height.
-210,000 miles
-new transfer case installed a month before I got it
-on third rebuilt front diff
-trans is probably original, fluid looked fine
-rear diff fluid fine
-uncertain on rear driveshaft balance - physically no damage noted

Has anyone else had this issue?

 

Hey, thanks for the intro and sorry about your loss. 🙏

Welcome to the forum. Lots of very knowledgeable folks here.

And that's a sweet looking truck! Is that front axle the NP149 AWD? Front and rear axle ratios match? I can't think of anything that will cause.

 

thank you. I really appreciate it. It was very sudden, so not even having the chance to say goodbye is heart wrenching some days. You have good ones and bad ones.

I hope to get some contacts on some custom work (hood, interior, etc.) eventually. I wanna really make this thing turn heads one day. But, yes NP149. And funny you say that, the previous owner used it for a work truck, so I’m wondering if he may have downgraded to 3.73s possibly not knowing to do the same in the front. Just out of curiosity, do you think it would move smoothly at all (driveway speeds) if the front is 4.10 and the rear 3.73? In my head I was thinking that would be instantly immobilize it? Or am I wrong? And thank you for the reply.

 

Pop the rear diff cover off and count the teeth, then determine what ratio is in the rear.

You should really be hounding the place that rebuilt the front diff. They might have royally screwed something up.

 

Being AWD I would imagine if the rear and front axle ratios didn't match you have tire screeching going down the highway. Unless the open differentials allow for one axle to spin at a different rate on the front and rear? 🤔

Surely the shop would have checked after the second rebuild. Are you getting this done under warranty?

Even if the ratios didn't match, the wear should be on the tires, not on the gears.

Maybe the shop is just not doing a good job on the overhaul as far as setting the clearances and backlash. You'd have whine and overheating. I can't imagine what's going on to create so much metal in that short distance of driving. 😬

Something doesn't sound right.

 

that’s my next step, just to make sure. And also, to your other post, I’m not sure what’s exactly failing. The company I bought it from just swaps it with another one and where the old ones goes, I have no clue. They said they ship it off to be rebuilt, but don’t track specifics with what fails. (Go figure) but thank you for the reply, I’ll check the rear next just to make sure.

 

yeah, I figured if they didn’t match it would immediately lock the teeth up and start crunching through them. Maybe I’m wrong, maybe the way the AWD is set up, it just allows slippage in the transfer case (which might mean this new transfer case is toast too).

but yes, all under warranty. I’m on the third differential now, so it’s not costing me anything other than my time pulling it in and out, well minus 25-30 bucks in fluid each time.

who knows, maybe they build crap diffs. I heard a ton of good stuff about them, so it’s hard to believe they jacked up building three differentials. But I guess anything is possible.

 

My first thought is gear ratios are off. Physically count teeth or carefully track rotations or driveshaft and tire to verify. Second step is get a untouched front diff takeout from a salvage yard with the right ratio from what you measured.

 

I’d bet also on different diffs, I actually had a dealer install the wrong ring and pinion on a new Tahoe way back in 96. It ate rear carriers every time I put it in 4x4, the dealer put a 3.42 in it when the factory ring was a 3.73. I replaced entire rear end 3 times before my buddy figured it out.