Diffs - how hot should they get?

Just got back from a camping trip. My XJ doesn't see highway driving normally, so I decided to put my hand on the diffs and see how hot they get. I can keep my hand on the front diff for 5 seconds and the rear for 3 seconds. I realize this isn't a great way to test, but I was wondering if anyone else has tried this and knows what "normal" is. I wasn't towing, but I was loaded down with gear.

Specs: 99XJ, 4.0, 231, HP30, Chr 8.25, 2" BB.

If you can keep you hands on the diffs that long, then they are fine.

By the way, when was the last time you changed the diff oil?

I have recently been using an infrared laser temp gauge and my rear dif gets up to about 55* C after some local driving. I'm running royal purple 75W140 synthetic. front diff is about 10-15* cooler all the time.

rstarch345 said:

If you can keep you hands on the diffs that long, then they are fine.

By the way, when was the last time you changed the diff oil?

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The diffs were changed two years ago, about 10k miles ago. They haven't been submersed in that time, but I plan on switching to a synthetic this summer. I only drive 7 miles to work and stay out of the deep water when I'm off road.

Thanks for the reply.

DaveD912 said:

The diffs were changed two years ago, about 10k miles ago. They haven't been submersed in that time, but I plan on switching to a synthetic this summer. I only drive 7 miles to work and stay out of the deep water when I'm off road.

Thanks for the reply.

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It doesn't matter if they are submirsed. EVERYONE seems to think diff oil lasts forever. They are subject to heat, like the engine, they are subject to wear, like the engine, the oil breaks down, like engine oil.

While diff oil will last longer than engine oil, it needs to be changed at regular intervals. Same goes with the transfer case and power steering fluid.

rstarch345 said:

While diff oil will last longer than engine oil, it needs to be changed at regular intervals. Same goes with the transfer case and power steering fluid.

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Right now I change the diffs at 12k and the t-case yearly. Yearly is about 5k miles. Do you have any recommendations for the intervals?

DaveD912 said:

Right now I change the diffs at 12k and the t-case yearly. Yearly is about 5k miles. Do you have any recommendations for the intervals?

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Excellent, sounds like you're on top of it better than the average Joe.

Good intervals, but always consider usage. If you do any towing, carrying heavy loads, do it sooner based on the amount of towing. If not, then the intervals that you have are good.

Any thoughts on dead dinosaurs vs synthetic?

Ya really what are the intervals.

I changed all my fluids last year because I wasn't sure when they were done last (got it in Q4 of 2005)

But I want to keep on a good schedule from now on, that it's MY XJ now and all

Are there any tricks to changing diff/ transfer case oil? and what weight should i use? or should I just call the dealer to see what I need? 99/4.0/3.55/np231/d30/8.25 first jeep ever

A couple things I learned recently

You have to remove the diff cover to drain it - but leave a couple of the upper bolts attached - it helps to control the flow of the old oil out of the diff and into a BIG catch basin.

Leave it to drip for a good long time. the last bits flowing off the gears and out the tubes are real slow because of the viscosity. I've left one for over 1/2 an hour and it was still dripping.

wear rubber gloves, old clothes and have lots of disposable rags. It will get everywhere and it stinks real badly.

to seal the diff cover back up, use "The Right Stuff" black gasket maker (after cleaning the gasket surfaces with carb cleaner or alcohol based cleaner for no oil residue). This stuff is an incredible sealant. I don't use anything else anymore.

80w90 for differentials for normal use.

I believe 75w140 for towing in the rear axle?

Transfer case - just remove the upper fill thing first (it's like a bix hex bolt) and then remove the lower one, let it flow into a catch basin.

rstarch345 said:

It doesn't matter if they are submirsed.

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Depends, if you have a cool diff, probably no big deal. A hot diff on the other hand, when it comes in contact with the cold water acts like a vacuum and will suck water in any and every nook and cranny it can.

I have front and rear AutoMeter diff temp gauges. The rear on the highway @ 70mph will hit about 150F with 85/140 and the front about 100F with 80/90. I can also definitely say that synthetic fluids run cooler by about 20 degs. HTH -B

The factory Owners manuals gives good intervals based on an average user. Since I don't know what an average user is, I base ALL my fluid changes on three parameters. Time between changes. Milage between changes and what use I have put my vehicle through.

For example. I will be taking a 2 week holiday towing a 18' fiberglass boat for a 1000 mile round trip. I will also be spending 2 days wheelin' in various terrain while on holiday. After the wheelin' I will inspect the fluids for water. Before and after the trip, change engine oil, change transfer case and transmission fluid. After the trip, both diffs and power steering fluid.

Depending on where I wheel, I may change diff fluid and power steering fluid after each event. Engine oil every 3 - 5 thousand miles. Transfer case and transmission every 10 thousand or so.

Also, even though there is more expense, I use all synthetic fluids. I buy everything by the gallon or 5 quart jugs.

I've welded drain plugs in my diff covers to facillitate changing.

I don't consider my XJ a beater. It gets worked hard for play (and yes it gets damaged) but it is also a proud DD that gets driven all over the country.

I don't understand why the C8.25 and D35's use rubber drain plugs.

Sounds like it's just asking to suck in water.

Vs. the D30 with a screw in plug.

Blaine B. said:

I don't understand why the C8.25 and D35's use rubber drain plugs.
Sounds like it's just asking to suck in water.
Vs. the D30 with a screw in plug.

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Cheep Cheep Cheap !:spin1:
It's a fill plug, BTW

early D35 had a screw in plug (mine rusted in in old Xj I bought with 500,000km of winter salt driving). Least of that xj troubles!!

Right, fill plug, not drain lol.

I put a bead of RTV around my 8.25 plug lip. I dunno if that helps, but it was submerged last september. I haven't checked the oil. Whoops.