Could anyone tell me how warm the rear end should get? Can't hold your hand on it after driving 30 miles. It's a 1994 5.2 auto 4x4 with 231000 miles on it. Bought it 14 years ago with 42000 on it. Been an excellent truck for me!

 

I can't say exactly what temperature is. But sure some long driving can make it too hot to touch. That is one reason why gear oil is so thick so it can heat up without needing an external cooler and still provide lubrication.

If you feel like it to hot, double check your fluid level and color. If needed drain and refill it.

 

+1 on checking the fluid level! Just pull the drain plug and look in there.

But the temperature is still safe over 200 degrees F... so that would be like sticking your hand in boiling water! If you have an optical thermometer, you can measure the temp.

 

I had an axle temp gauge in some of my work trucks and I've seen them hit the 200's properly lubed. And thats with a load on them.

 

Reread my last post... I corrected myself.

But anyway... yeah, most differential temp gauges go up to 250 degrees, so as long as the temp isn't going higher than that.

And if you think its getting too hot, you can always go to a different weight oil. IIRC, these axles take 75W-90, so you could go up to 75W-140 that will help it at the higher temps.

 

I checked the fluid level before I posted on here. What led me to ask the question was I just started hearing a new "noise". The rear end has always whined since I've owned it. So I started feeling around for heat, bearing play.....So anyway the axle tubes were warm & got warmer towards the differential. Can hold your hand on it for a good 5-10 seconds, so I don't think it's near 300 deg. Will check it with a therm. Great forum. Glad I stumbled on it.

 

Yes, but 140 is recommended in temps above 100 degrees. Only problem using 140 all rear round, Is that when you 1st start driving in cold weather. The axle runs real stiff from the fluid being thick.

 

Thats not how multi-weight oil works Az93...

75W-90 and 75W-140 will have the same "cold" viscosity, so there is no difference between the two when you first start driving. The difference is when it gets hot, the thicker oil will help keep things properly lubricated and do a slightly better job at cooling.

Keep in mind, they do viscosity tests at a specific temperature, so at a certain temp, the 140 will be thicker than the 90, but as the temp gets hotter than their test temps, the 140 will get thinner and thinner, so thats why its better in a situation where the temperature is hotter than normal.

 

So why does oil get thinner the hotter it gets?

 

Well, multiweight oil gets thicker as it gets hotter to a certain point, and then starts thinning again past that point.

But straight weight oil gets thinner as it gets hotter because the molecules become excited and spread out as heat is added... read a thermodynamics and fluid dynamics book for more info! :P