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How hot should the rear differential get?

pabloconrad

NAXJA Forum User
Yes, I've searched and no hits.

My rear diff (D35, non C-clip, full of fluid) seems to be getting pretty hot. I was checking something and spit on it (I know, that's sick):puke: when the drool sizzled away pretty quickly. It gets quite hot to the touch. I was in 4WD doing Lead King Basin (a moderate trail) when I had to get under the chassis to check for a gas leak. :explosion

But, how hot is too hot? :flame:

Any help would be great.

Thanks in advance
 
Spit/water sizzleing is probably too hot, I would say. Those gears may be blue by now. I don't know actual running temps for diffs though. Any abnormal noises while driving after it cools? Could be too much backlash, or too much bearing preload. Either way you might wanna pop the cover off and inspect the gears for blueing and improper contact patterns:
http://hometown.aol.com/docboy209/80011506.gif
If you're not already, try running 75W-140 Synthetic gear oil.
 
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docboy209 said:
Spit/water sizzleing is probably too hot, I would say. Those gears may be blue by now. I don't know actual running temps for diffs though. Any abnormal noises while driving after it cools? Could be too much backlash, or too much bearing preload. Either way you might wanna pop the cover off and inspect the gears for blueing and improper contact patterns:
http://hometown.aol.com/docboy209/80011506.gif
If you're not already, try running 75W-140 Synthetic gear oil.
FOUR WHEELER justdid an article along these lines anything over 200* i belive is bad mag is at the shop in my toolbox so i cant check for sure
 
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Apparently according to the article, the additives in gear lube start breaking down seriously at 200*. It also states that the rate of oxidation increases dramiticly starting around 140. Definitely below the boiling point of water (or spit).

Pat
 
THe only time I run a diff that hot is the first run out on a new gearset. and I gauge it by how long you can hold your hand on the cover. if it burns your hand right as you touch it then its too hot. the maximum safe temp is if you can hold your hand on the cover for for more than 2-3 seconds. not highly acurate but a good general rule of thumb.
 
Rawbrown said:
THe only time I run a diff that hot is the first run out on a new gearset. and I gauge it by how long you can hold your hand on the cover. if it burns your hand right as you touch it then its too hot. the maximum safe temp is if you can hold your hand on the cover for for more than 2-3 seconds. not highly acurate but a good general rule of thumb.

Incase you are wondering, you can't hold your hand to 140 F. That is a rule of thumb where I work. If you can't hold your hand to it, it's over 140. If it liquifies the fat out of your skin, its over 400F! Spit sizzles around 214. Hope that helps:laugh:
 
every diff I have ever messed with has been hot/warm. My brand new rubicon with synthetic fluid put it at 5k gets hot hot hot. It bugs the crap outa me.

Rawbrown... what you think? Its been almost 7k like this. should i keep on truckin' or get it to the stealership?
 
how long can you hold your hand on the diff?
 
maybe a second or two. after a long trip to the coast or somehing... cant hardly touch it.

Do you think it could be the viscosity? i have 80w90syn in it. that is what my dealership (no longer working there) installed into all jeeps. I have really been thinking that i need to bump it up.
 
up there in fresno... what synth did you try?
 
Its BGsynthetic.

I think after my trip to the con' this weekend, I will change the fluid and check the gears and bearings. Go with a much heavier syn oil this time too.
 
Never heard of it. I would recomend Royal Purple. or maybe amsoil. stick with the top names. Check your pattern when the cover is off.
 
This is a pretty good topic...

Is there a difference between how hot it gets on the trail vs how hot it gets on the freeway in the middle of summer? I know mine gets hot on the freeway, but not as much on the trail but I don't really think to check the temps then.

Seems to me if its that hot on the trail, something just aint right.
 
here is how I see it and this is what I tell my customers. when I setup a rear axle I do the initial break in myself. its about a 20min drive and I goal is to work harden the gears. This creates alot of heat, can't hold my hand on the diff for even a second. after that it cools and a normal driving style takes over. Most customers of mine actually give a rats ass about properly breaking in the gears.
as you make the same drive, youll notice the diff to get progresivly not as hot till finally it stops getting progresivly not as hot.

if after an oil change and 500miles your diff is still getting too hot to hold your hand on the cover for more than a 3 seconds then I would double check the pattern.

I had a customer take his newly geared TJ with a super 35 out to Glamis dunes only 2 weeks after. The axle developed a gear whine on the way back. after much confusion I finaly figured out what had happened. on the carrier bearing, the end of the bearings themselves ground down about 1/16". at first glance the bearings looked fine. I found that he was running in 100 degree desert in 3rd gear low range with nothing less than 1/2 throttle all day.
voided his warranty.
 
Glad I asked. Not a topic most people even consider. I never thought about how hot they get until this past weekend. But it seems most every one doesn't think aobut this either.

The gears whine, and have been doing it for about a year. So I need to pull the cover and check everything out. It has about 265K miles (original) so I guess it's time for a rebuild.

Thanks for all the info.
 
I thought you were only supposed to run 75w-140 if you had the factory tow package. My diffs get warm at most and I use 75w-90 synthetic blend.
 
boomhauer said:
I have front and and rear diff temp gauges. The 8.25/29 runs about 100-110, the D30 about 90-100. Running Lucas Synthetic 75/90 front and rear.

Which gages, and how did you set them up so they don't get killed at flex/stuff?

I'm an "information junkie" while I'm driving, and this is something I'd been kicking around as a fun idea - no sense in reinventing the wheel, is there? Also, where did you put the sensors?

5-90
 
I'm guessing electric gauges, so you'd just have to route wires, and you could follow the brake lines, or the control arms up front. Interested in where he put the guages so they were in the oil though, diff cover?
 
5-90 said:
Which gages, and how did you set them up so they don't get killed at flex/stuff?

I'm an "information junkie" while I'm driving, and this is something I'd been kicking around as a fun idea - no sense in reinventing the wheel, is there? Also, where did you put the sensors?

5-90

The magazine article listed earlier in this thread has all the info you want on dif sensors.
 
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