Rear diff over heating?

Got a perplexing situation on my hands fellas... Heres the short version.

Drove my dually to Washington DC last week dragging a 2200lb trailer to pick up a full size 4wd ext cab long bed truck with a dead engine. No probs on the way there to speak of (other than the road construction along the DC belt way) and I averaged ~75mph the whole way with two stops for gas.

On the way back with the other truck on the trailer the trip went about the same other than a slightly lower average mph due to traffic but uneventful none the less...until I got about 5 miles from my house. I was stopped at a light waiting to make a left turn and I had the window down enjoying the nice fall evening when I caught a whiff of 90w gear oil. I didnt think much of it since I was at a major intersection with lots of traffic and I just figured a semi truck had lost some lube or something. However, when I made the left turn at the light I got a pretty loud chattering sound from the rear diff which caught me by suprise. I immediately pulled into a parking lot to look things over thinking the trailer had fell off or something was dragging. After doing a quick walk around and not seeing anything I could still smell 90w gear oil slightly so I grabbed a flash light and looked up under the back of the truck at the diff, axle ends, brake drums, diff vent tube, etc but didnt see any gear oil anywhere. Scratched my head and decided to limp the truck home since I was only a few miles out but every time I made a sharp turn I got the same chattering sound from the rear diff but when driving straight ahead no noises or other issues. Since Id been on the road for over 12hrs and it was dark I figured Id check things out in the morning.

Got up the next morning to unhook the trailer so I could get the truck in the garage and tear into the rear diff to find out what the deal is. While menuevering(sp?) the trailer into its spot beside the garage I had to make several sharp turns and I noticed that the chattering was gone. After unhooking everything I drove to a large parking lot about 100 yards from the house and drove in circles in both directions with the steering at full lock trying to see if I could get the diff to make some racket. None, nada, zip....truck drives just as it did when I drove to DC unloaded and has been noise free since.

With all that rambling said my theory is I over heated the diff. My dually is lowered 6" in front and 8" in the rear which tucks the rear diff up pretty far and the stock front air deflector under the front bumper is only about 3" off the road so Im thinking with the added weight of the other truck on the trailer, the steady state ~70mph speeds, as well as little if any air flow under the truck the rear diff got hot and the posi started chattering when I turned sharply. IMO thats the only thing that would explain why the diff fixed itself, for lack of a better term, by the next morning since it cooled off sitting during the night.

Last thing is I had the rear diff rebuilt with ALL new GM ring/pinion, GM bearings (timkens), seals, etc except for the posi unit itself. I reused it since it was fine (or so I thought) but the original ring/pinion were worn and howling from the previous owner letting the diff run low of lube. Ive put ~5k miles on the diff since the rebuild with zero issues up until the other day when it started chattering when turning.

So, any thoughts? Anyone ever had a 14 bolts posi "chatter" when making sharp turns after some extending free way towing? If so what'd you find? Im pretty sure it just got a little too warm back there but Im open for suggestions too if anyone has any thoughts or should I just start saving pennies for a new posi unit? Truck has 106k miles on it and runs flawless otherwise and Id think the factory posi would be just a wee bit more durable than that since I was prolly only towing 6800lbs or so which should be a piece of cake for the tow rig (IMO).

TIA for suggestions.

 

I cant think of any other explanation. Maybe check the fluid level just to be safe. If it happens again you should look into a Mag-Hytec diff cover. They increase the fluid capacity and have fins for cooling.

 

Im inclined to agree with you on the over heating deal and Im deffinately going to check the lube level first chance I get just to be sure even though I couldnt find any signs of losing any lube. I'll prolly swap the standard gear oil with some synthetic stuff too but how to cool the diff with minimal hassle I'll have to work on. I thought about fabbing up a rear diff cooling unit/pump that runs off the diff yoke like the cup cars use but thats a lot of monkey work added on to possibly break/crap out in the middle of no where so I may see if I can find a place to pick up some air and duct it onto the diff. The alum cover would be my last resort since the truck is lowered there is only about a half inch between the spare tire and the diff cover so anything that is larger than the stocker will require the spare tire going into the bed which Id rather not do since Id have to constantly move it when hauling anything.

 

I'd slow down. 70+ is too fast with a trailer IMHO. But, many people do it. Ever read the speed limit signs for trucks? It's the same for an "autos with trailers."

That will probably solve any overheating problems you are having. Then the second thing I would do is swap fluid for some Mobile 1 75/90. Slowing down and synthetic should surely take care of it, I would think.

 

Thanks for the info guys.

To hit the high spots.... Slowing down isnt really an option since Id get ran over from behind. Granted I wasnt breaking any land/air speed records just staying with the general flow of traffic with an average speed of 70-75 mph (sometimes more sometimes less depending on traffic flow).

Ive used some of those high zoot extra capacity alum finned pans for transmissions and all things being equal I didnt see even a one degree drop in fluid temps compared to the stock capacity stamped steel pan. It prolly didnt hurt anything but didnt help any measurable amount either other than lightening my wallet some. Im inclined to think the same thing about a rear diff cover but until I try one I really dunno.

Since Im going to have to pull the cover to drain the fluid anyways I might as well yank the carrier/ring gear/posi and look it as well as the bearings over for evidence of excessive heat. When the diff was rebuilt a lot of time was spent getting the preload on the bearings as well as the ring gear pinion tooth mesh within spec but thats not to say something hasnt changed and all that is out of whack now so I'll check that too.

My option code reads "G80" but that is really just GMs code for limited slip/posi/whatever you want to call it carrier and I dont think it denotes what kind of limited slip/posi it is. In my dealings with GM over the years Ive found the govlock in some cars with the G80 code while others have had the almost as weak cone type limited slip with the G80 option code. When I had the diff apart in this truck it didnt look like any govlock Ive seen with the typical weights/springs but then again it acts like a govlock on slick roads as well as this was my first tear down/rebuild of a 14 bolt so what Ive seen before in the car diffs may not apply to the 1 ton truck rears. Its pretty well an enclosed unit and I didnt take it apart so I cant say for sure if its a govlock but by outward appearances it isnt one. Ive heard mixed reviews on the 14 bolts G80 posi from some off road guys but their use tends to be on the extreme side considering what theyre doing with their rigs so I figured the stock posi was more than up to what Id ask of it. Not so sure now but before I drop coin on an aftermarket replacement posi Im gonna try the synth gear lube if everything looks ok and see how things go.

Still a little perplexing to me as to why it might have gottem hot considering the beef of the rear diff in my truck and that I really didnt have a whole heck of a lot of weight behind me even with minimal air flow to cool it. I just dont see it needing any air flow to stay well within acceptable temp ranges but then again whadda I know

 

Posi chatter

The noise you heard will occur after extended highway driving, when turning a corner because the posi has no lubrication. The fluid has all been thrown to the outside of the housing on the highway. If you continue to run standard gear lube, you need the GM additive for the posi unit. Another solution is synthetic so as not to lose viscosity from the heat.
You do not need to inspect the internals, the fact they are fine after a spin in the parking lot is enough.
When I drove my Chevy Super sport to shows, I would weave back and forth after exiting the Interstate to spash lube up into the posi. Usually a couple turns of the wheel would stop the clunking noise.
I don't know the year of your truck but all the Dana 80 rears use synthetic lube. The smell indicates the heat is too much for the 90 lube.
I know all you weekend haulers will bash me for this but I pull a diesel dually and 30' car hauler 9 months a year and the one thing most of you need to learn is slow down. Unless you have air brakes, pulling over 8,000 lbs at more than 65 mph is unsafe. The only reason for higher speeds out west is there usually isn't as much to hit or other people to run into.
With or without air brakes, your wheels will lock up and the more weight, the farther you slide. That crap about being run over is a lame excuse. You want to drive your new car or SUV, with 4 wheel disc, 75-80 mph fine. But these trailers don't have disc brakes.

 

Norm,

Im assuming your post is directed specifically at me so if Im mistaken and is more of a "blanket" type thing then I apologize before hand.

With that being said your post is full of inaccuracies.

Provided the diff lube is at the correct level the posi as well as all the other internal parts are getting forcefully bathed in fluid either from splash at high speeds or by submersion at lower speeds.

I would assume that anyone who read my post would take for granted that I am smart enough to know that the posi additive was put in along with the gear lube. I wouldnt think that there are more than a hand full of car/truck guys on the planet that dont know about the additive.

Considering the price of the replacement parts for this diff along with the time/labor involved to install them as well as the fact the cover has to come off to drain the fluid I think an inspection is warranted. I dont look forward to being stuck along side the freeway ~500 miles from home with a locked up/burned up rear diff in a dually as well as a trailer and whatever is on the trailer. The ~$1000 tow bill to get all that mess to a garage will make the time/effort saved from not inspecting the diff seem awfully unimportant then.

If you had a locker of some type then thats all good but any clutch type posi/limited slip that is operating correctly and has the correct type and amount of lube shouldnt make any noise at all no matter what.

I wont argue that slower speeds in any case whether it be on a skate board or a semi truck is always safer. Thats not always practical and is at times just as unsafe as running 75mph. Running 55mph and having an unattentive 80k lb truck drive into the back of you is not any safer period.

Exactly how did you come to this conclusion? Air brakes have absolutely no more stopping ability than hydraulic or electric and are only another form of applying the braking mechanism. Provided you arent over loaded or have worn/malfunctioning brakes you wont stop any faster/better/safer than any other type of braking application set up.

While that may be true to some extent if you actually tow using some common sense then its not an issue. If youre not watching whats going on two or three cars ahead or right on someones rear bumper then youre asking to have an accident no matter if youre doing 25mph or 95mph. The key to towing safely at ANY speed is to assume that every driver around you is blind and on a cel phone, watching whats going on at LEAST 3 cars ahead of you, and expecting/anticipating idiotic moves by other drivers. Something as simple as watching for brake/signal lights of the cars ahead of you either through the back glass of the car in front of you or over top of the car in front of you will pay off the first time a typical idiot driver pulls a bone head move and you see it quickly.

Not only is that something they teach you when you get your drivers lic but most anyone who drives for a living knows that if you lock the brakes your just along for the ride and have no control. Anti lock helps here for the ppl that dont have the sense to ease up on the brakes to unlock the wheels but none the less I would hope that anyone that is towing any signicant weight understands that they MUST keep the tires turning to have any hope of staying in control in a panic situation. Any truck driver worth weight in poop knows that "if you lock em you wreck em" period.

Ever had a trucker who was talking on the cel/CB, looking at his GPS, filling out his log book, or getting road head from a lot lizard drive his Peterbuilt into the back of you because you were doing 55 and he was doing 75 and he didnt notice how fast he was closing in on you? Call that lame all you want but it happens every single day multiple times. Been there done that got a t shirt and Id prefer to not get ass ended ever again by something that weighs 70k lbs more than me thank you.

Some do have disc brakes but thats not the point... The point is that as long as youre not over weight for your tow rig and its brakes as well as the trailer brakes and are just a little less brain dead than the typical driver you can safely tow at free way speeds in traffic or not. Being over loaded, having worn/malfunctioning brakes, or not paying attention will get you wrecked/dead just as easily at 30mph as 90mph so the lecture on what speeds you can and can not tow is what is lame and if someone who is towing doesnt know 99% of whats been mentioned then they will wreck eventually no matter what.

Thank you and back to your regularly scheduled thread.

 

Man, I didn't see that one coming.

I don''t know where to start, if it seemed like I was talking down to you or anyone, I'm sorry.
My experience was limited to a 12 bolt posi in a 69 Nova SS, I owned the car for 9 years and the rear-end always reacted the same way to highway driving. Nothing ever needed to be rebuilt or replaced and I accepted the explanation because it came from a mechanic and racer with many years of experience.
I'm sorry you felt my opinion on towing speeds was a lecture. As you can see I don't post often. I try to post only when I have something to contribute or an opinion to offer. I try to be open to everyone else's opinion as this is an open forum.
Any licensed driver can buy a truck, a horse trailer, a car hauler, a travel trailer, etc,etc. I think some of those people are on this forum and don't have the experience or training of a professional driver.
The only thing I can't except is the theory that we drive to stay ahead of drivers not paying attention to traffic. I drive 60-65 depending on road conditions, terrian and load. This keeps my diesel in a good rpm range, produces good mileage and keeps me in the right lane most of the time. I agree you do not want to be an obstacle.
I'm not sure but it seems your last comment was to stay off your thread so I will do that, I don't want to offend anyone and I don't need to be where I'm not welcome. Drive safely.

 

Norm,

To be honest no hard feelings here and by no means did I imply that youre not welcome in this/my thread (its a public forum so its not really "my" thread anyways ). The last sentence of "back to your regularly scheduled thread" is just internet speak for "lets get back to the subject" or something similar and not meant in any way shape or form to try to exclude an individual(s). BTW, I dont post much here as Ive been around towing, hauling, semi trucks (I used to drive one) for many years so I usually only jump in a conversation if someone has a problem/question that has gone unanswered as well as the fact I have two other forums that Im an admin on that requires quite a bit of my time to keep the natives there happy.

Being that this is a written text form of communication its hard to truly tell what someones intentions are since we are not having the conversation face to face and written text may come off in a completely different way than its meant to. I take that into consideration but to be honest the way I read your post it came off as sort of a lecture but obviously by your above statement that wasnt what you intended and I apologize for the innacurate perception of your post on my part.

I agree 100% with you that there are ppl out there that even though they have the means to purchase a tow rig of some type they have no practical knowledge of the dynamics or the physics involved since they have never been instructed and/or exposed to anything beyond your average passenger car. Those ppl do indeed scare the pants off of me and I do what I can to avoid them or get away from them while on the road. Until those people get some windshield time under their belts while driving their rigs the only way for them to gain the needed knowledge until they become old hands at operating a tow rig is information provided by ppl such as yourself and/or this forum.


Anyways.... I drove the truck to work tonight (empty/unloaded) which is ~25 miles of which 10 miles are rural two lane road (45-50mph) and ~15 miles of stop and go traffic. Once Id parked in the lot here at the plant I got an ever so slight whiff of gear oil again so I crawled under the rear of the truck and put my hand on the diff cover....sigh.....it was HOT. Hot enough I couldnt hold my hand on more than a second or two it so theres deffinately something needing attention inside even though it didnt make any racket during the commute to work.

Like I needed another hole in my wallet right now .

 

I think its time for you to get a different rear axle. If the previous owner ran it low and overheated it, needed new gears, etc...then they had a hard time getting the preload/lash into spec,...my thought is maybe something is warped or installed a little off and long hauls with a load is causing too much friction...somethings funny with your rear...

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But we did have a detroit locking up occasionally in the back of a 3/4 ton suburban...hmm....let us know what you find?

 

i have the g80 gov lok in my 14ff crewcab tow rig. no problems at all. i even swaped it from my old mud truck when i did the detroit locker install and it was working great before the swap with 38" tires and built 400sbc. sounds like the gov lok was engaged at time of turn and stayed locked till you parked it . i would switch to synthetic also and try that. if any thing on the covers you have more oil so better cooling from even just the more oil.

 

I dont think anything is warped/bent and I should have clarified what I meant by a lot of time was spent getting things in spec and right on the money not that it took a lot of time due to problems. Not that Id completely rule out the diff being warped as stranger things have happened LOL.


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Im gonna tear into to it this week if I have time or if not then next week and I'll let ya know what I find.

 

I just lunched my rear in my 96 Suburban, the pinion seal was leaking and I didn't know (no apparent leaks, no drips, just an occaisional smell), and it started to howl, not even towing. Got it home and pulled the cover and only about 2 cups of fluid came out.

The pinion seems to have taken the hit, with it scuffed at the gear at the middle towards the center axis of the pinion gear shaft. And it is doing short trips now, it is getting very noisy very fast.

I have a 9.5" lined up to go in this weekend to replace the weak 8.5".

 

Just got done with the tear down/inspection/reassembly today (woulda tore into it sooner but other projects had to be finished...see wife for that list ).

Rechecked contact pattern/back lash on ring/pinion...perfect.
Bearings looked new.
Fluid looked fine.
Posi clutches...toast.

Rebuilt the posi and threw all the parts back in and everything seems ok for now but the test will be next week when Im scheduled to move a suburban about 300mi one way.