How hot is too hot?? Rear diff temps??

[cttitan]

Just wondering what a normal temperature is for the rear differential.

I checked mine this evening after taking my 24' TT out for a spin to check it out. The rear diff was hot enough to keep you from putting your hand on it for more than a 5-10 seconds.

I have checked the fluid level and it is OK, just wondering if anyone has measured the temperature or has any experience in this area.

[titansurveyor]

Not me, hoefully someone else knows.

[RED BRAWN SE CC]

Mine is just warm to the touch.

[fish_head]

I was wondering the same thing cttitan. Mine is also too hot to keep your hand on after a trip down the highway. I was running Valvoline DuraBlend and now I'm running Mobile 1. The Mobile 1 seems to run slightly cooler, but still hot. I'd like to know the normal operating temperature of the rear diff too.

[CaliforniaTitan]

I dont know the normal temp...but I could NOT touch the diff after driving 86 miles home....that was too hot for me.

Chris

[Austin]

If you can leave your hand on the diff for any length of time, it's nowhere near too hot.

Gears get hot. That's why synthetic gear lube is a better option - it usually has a higher breakdown temperature.

Try crawling under and hugging your tranny sometime - it'll get good'n'hot too.

[WIS-key]

I had been reading the posts about low diff. oil levels, so I checked mine after driving ~25 miles. It was like cttitan's - too hot to leave your hand on for more than 5 seconds.

The fluid level was "OK" - not right at the bottom of the check hole's threads, but easy enough to bend your pinky slightly and check it.

I decided to drain front and rear and refill with Mobil 1 (75W-90). The rear fluid was dark - in direct light you could tell it was still honey colored, but was full of metal particles. The drain plug had a good deal of junk on it as well. I'm assuming this was normal for break-in. The front diff's oil was very clean with minimal junk on the drain plug. (Has anyone else noticed that both drain AND fill plugs in the front have magnets on them?)

I checked it again tonight after about 20 miles of driving, and the diff was noticably cooler - cool enough to leave your hand on it indefinitely. This is hardly a scientific test, but it was nice to see the diff running cool even after running it at 70 on the freeway.

I'll be curious to check it after I start hauling my 3,500-lb boat around.

-Rob

[Big Platinum]

just changed mine today after 4,000 miles. fluid level was ok (a little low but acceptable) but man did it look bad. it was dark gray and the drain plug had gunk all over it (not metal but looked like the grease you use to lube up suspension fittings). it looked like my diff had been getting hot too, paint starting to crack and peel, hope i got it in time and the new oil will help, have to keep a close eye.

[fish_head]

Maybe it's just me, but Mobil 1 75W-90 gives me more piece of mind than the Valvoline DuraBlend 80W-90 did.

[bestatchess]

http://www.oil-tech.com/temptest.htm

[Farmer Fred]

That's one of the most unscientific pieces of "data" I have ever seen:

"I took several trips hooked up to the trailer and loaded with gear. I sought the most severe conditions I could find, like the 7 to 8 percent grade on highway 395 headed to Dead-man Summit out of Bishop with the outside air temperature at l02º. I left the transmission in fifth gear and pulled the grade at 65 mph. I was never able to get the NV4500 transmission to hit the l90º mark. The Dana 70 rear differential never exceeded l50º. Numerous other trips included highway 50 to Carson City and Highway 5 traveling in both directions over the ‘Grapevine’ netted the same results."

AMSOIL Technical Services reports these temperatures are 20º to 50ºF cooler than temperatures normally found in comparable units under comparable conditions.

The only way the data could be "comparable" would be to record the temps with Amsoil and then drain, refill with another oil, and record those temps.

[bestatchess]

Yes, it would have been better to have before and after tests, but it does give a general indication of rear differential operating temp. Along the lines of a "scientific" approach, I would note that a calculation could be made to determine the amount of power (e.g. watts) that would need to be dissipated as heat from the rear differential based upon conversion efficiency of the differential and the power needed to tow a load. For example, assume the rear differential is 96% efficient, and the engine is applying 200 HP (after drive train losses) to the rear differential, then 4% of that 200 HP or 8 HP or about 6,000 Watts would need to be dissipated from the rear differential as heat. (I'm not saying these numbers are accurate, just an example of what is going on). Now based upon whatever heat needs to be dissipated, you need to do the heat transfer calculations to make sure that whatever is required will actually happen in order to avoid cooking the diff. Now the thing that bothers me is that these calculations have been done previously by automotive engineers and tweaked based upon billions of miles of experience with rear diffs in 1/2 ton trucks, and then Nissan has somehow apparently decided to go with a quart less differential fluid than other mfgs. of 1/2 ton trucks.

As far as testing with your hand, I think both 150 and 180 will feel pretty hot and my hand is not that precisely calibrated. Isn't 180F about the hot setting on a home water heater that will scald your hand? I'm not sure, but I think that gear oil is good for pretty good temps, maybe up to 250 F or so. I could do some testing, but I don't feel like spending $140 on a laser pyrometer when I could spend that on a bigger capacity aluminum finned diff cover instead. Anyway, I think that the paint burning off the differential is a good indication that something is wrong.

[Farmer Fred]

I agree about temps in general. Diffs that bake the paint off are probably too hot.

The Amsoil site clearly aims to imply that the Amsoil caused the diff and tranny to run cooler.

[lizardking]

I have Amsoil in my rear diff and no problems.. My diff never gets hot and I have no issues of the paint being baked off.. My paint is fully intact and I have 6200 miles on her so far..

[titanjbc]

Is there a correlation between diff problems and areas of the country? One exploded diff was in AZ, I think, the other southern CA, where temps have been higher. I live in MA and yesterday was the first day over 90. I would think the diff runs hotter if it's 90+ deg. vs. 40s-60s...