Aluminum Differential Cover Thermal Results

[Blacklane]

I recently installed a finned aluminum differential cover from PML Covers on my 2012 Chevrolet Avalanche 1500 that I intend to use for towing a travel trailer. In my previous truck, I had to replace the differential bearings, which is not a small task, so I wanted an easy way to drain and fill the fluid regularly in the hope of avoiding the same problem in my newer truck. So I wanted a good differential cover with drain and fill plugs.

I powder-coated the cover in gloss black to match everything else under my truck. Installation was simple. I replaced the differential fluid with AMSOIL Severe Gear 75W90. I used a paper gasket and used medium loc-tite on the screws. (I once had some diff screws loosen causing a loss of fluid).

I have a thermal camera and I had a chance to take thermal images of the differential before and after installing the PML cover. I took both images after towing a trailer for about an hour on a highway, followed by about fifteen minutes of slower, steady driving. I could not perfectly duplicate the ambient temperature, so one image is at 70F degrees ambient and the other is at 80F degrees. In the thermal images, you can determine the ambient temperature by looking at the frame temperature in the background, which is pink/purple/blue.

I assumed that the stock differential and axle had plenty of heat-dissipating area, since the oil can flow all the way down both axle tubes. A few square inches of aluminum fins would not add much. The thermal images revealed a different story. The stock differential temperature was mostly 170-200 degrees F while the PML differential was mostly 140-150 degrees F. The axle tubes are green colored, indicating that little heat is transferred down the axle tubes.



Click For Full-Size Image.


Click For Full-Size Image.


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Full disclosure: I also sent this info to PML Covers who posted it on their website and sent me a tee shirt, so I guess you could say I have a bias. However, I think these results would be applicable to any similar differential cover.

[jerem0621]

Thank you for the information. I have been considering adding one of these covers for my Ford 8.8 that is under my F150. I like the idea of easier fluid changes.

Thanks!

Jeremiah

[PDX_Zs]

D@mmit. This thread just cost me a bunch of time, but may have also saved my bacon.

I was researching my rear axle, AAM 14-bolt 10.5", and found that my model year bearing cap may have been assembled wrong. Common on 2010+ GM trucks.

So now I've got to pull the whole thing apart to check it out. And while I am there, I will of course do new synth fluid and a new fancy cover.

🙂

[THenne1713]

It is also possible that (some/ part) of the lower temp is because of the AMSOIL synthetic.

Blacklane wrote:
I recently installed a finned aluminum differential cover from PML Covers on my 2012 Chevrolet Avalanche 1500 that I intend to use for towing a travel trailer. In my previous truck, I had to replace the differential bearings, which is not a small task, so I wanted an easy way to drain and fill the fluid regularly in the hope of avoiding the same problem in my newer truck. So I wanted a good differential cover with drain and fill plugs.

I powder-coated the cover in gloss black to match everything else under my truck. Installation was simple. I replaced the differential fluid with AMSOIL Severe Gear 75W90. I used a paper gasket and used medium loc-tite on the screws. (I once had some diff screws loosen causing a loss of fluid).

I have a thermal camera and I had a chance to take thermal images of the differential before and after installing the PML cover. I took both images after towing a trailer for about an hour on a highway, followed by about fifteen minutes of slower, steady driving. I could not perfectly duplicate the ambient temperature, so one image is at 70F degrees ambient and the other is at 80F degrees.

I assumed that the stock differential and axle had plenty of heat-dissipating area, since the oil can flow all the way down both axle tubes. A few square inches of aluminum fins would not add much. The thermal images revealed a different story. The stock differential temperature was mostly 170-200 degrees F while the PML differential was mostly 140-150 degrees F.


Click For Full-Size Image.


Click For Full-Size Image.


Click For Full-Size Image.


Full disclosure: I also sent this info to PML Covers who posted it on their website and sent me a tee shirt, so I guess you could say I have a bias. However, I think these results would be applicable to any similar differential cover.

[Blacklane]

It's only a sample of one, so we can't draw any conclusions about the effect of the fluid change. Maybe it would have not failed, maybe it would have failed sooner, or maybe it made no difference. We have no way to know.

[mike-s]

Blacklane wrote:
IAt mid-life, the fluid was replaced with AMSOIL 75w90 and it was full of oil when disassembled.

Well, there's the problem. Actual testing against real world specifications (no, the "three ball test" isn't one) beats multi-level marketing fluff every time.

[Blacklane]

I don't think it was corrosion that killed my old differential. The fluid that came out was not milky brown like you find when there is water in the oil. It was clear grey with lots of silver flakes. The differential had never been under water and the vent hose was just fine. At mid-life, the fluid was replaced with AMSOIL 75w90 and it was full of oil when disassembled.

My GM Service Manual has a close illustration of this wear and calls it "Fatigue Wear."

I assume there was one small flake that caused another flake, etc., and over several years, all of the bearings were damaged. The gears were just fine.

The truck lived a pretty normal life of mostly twice-daily half-hour commutes with a few weekends and vacations towing a trailer.

Since my new truck is also an Avalanche with virtually the same differential, I want to take better care of this one.

[BenK]

Agree with Turtle...it looks like corrosion damage...NOT extreme pressure. They do their dirty work differently.

Corrosion will pit by eating and pressure will flake the surface metal by work hardening

Also ask if that diff ever got underwater when it was HOT...and check your vent valve...it is supposed to prevent drawing in water (one metric...other is to vent when it gets hot).

OR you have somehow gotten water into the diff



There are other things going on that the thermal images shows

The axle tubes are also part of the heat rejection system. Heat sink in mass to absorb, hold and reject via surface area. The why it isn't that hot as you move farther away from the heat source...the gear teeth

Since ALU is a better conductor, it will reject more heat. Thickness in this case not really a component, as the lube is in direct contact with the cover. Therefore will show hotter. It is also insulated in part from the pumpkin by the gasket (reduced thermal transfer surface area) and heat will be wicked away to the pumpkin/tube material, which is larger in mass

There are anti-foaming additives in all diff lube, but it still foams a bit. The hotter it gets, the more it will foam. Foam has lots of air bubbles, which is NOT as good a thermal conductor as the lube itself. This and the fact that the flow of diff lube isn't very good going down each axle tube has the tube able to cool it way faster than the input (new, hot lube going down the axle tube)

[Durb]

My extra capacity differential cover came black powder coated except the manufacturer's logo is buffed out to expose bare aluminum (appearance). I bought it for the drain plug and the o-ring seal (no seeping). If it is easy to swap out fluid I will change it more often. Got to be honest with myself. Nice report.

[73guna]

More then anything, that cover be lookin' good!

[Blacklane]

mike-s wrote:

Blacklane wrote:
I think it's helpful to look at the big chunk of iron near the axle tube. That should be a good place to compare the two configurations since it's the same for both. For the baseline case, that's showing "white" or around 199F. For the aluminum cover case, it shows "deep red" or 151F."

I'm not buying - that's confirmation bias. Explain why the area where the axle tubes enter the diff (which has the most thermal mass) is essentially the same. Comparing the pictures, by far the biggest difference is the cover itself. And, if you think a cover is lowering the temperature by 48 degrees, want to buy a bridge?

I figure that the differential is a big ball mostly filled with oil. The oil is heated by the friction of the gears and that heated oil uniformly heats everything it touches. (I used to think that oil flowed out to the axles where the heat could be dissipated, but the thermal images showed that to not be the case, so all of the heat must be dissipated in the differential housing, or "pumpkin" as it is often called.)

In the baseline "stock" case, the thermal image of the pumpkin is dark red and white, and using the scale on the left side of the image, must be around 160-199 degrees F.

When we change the diff cover and fluid and re-run the test, the thermal image is medium red to yellow, and again using the scale on the left side of that image, must be around 100-150F.

As you point out, the aluminum cover has a very different thermal profile: it conducts differently, it has different thermal mass, etc. So let's look for something that has not changed between the two cases. It turns out that we can see the iron part of the differential under the axle tube. This part has not been changed between the two measurements, so must reflect the heat of the oil in both cases.

Of course, there is the additional adjustment for ambient temperature of -10 degrees F for the second case.

I tried to keep the other variables constant, so we can only assume that there is a reduction in temperature of about 40 degrees F (50F minus the 10F ambient change) between the stock set-up and the new set-up with an aluminum diff cover and synthetic oil.

[Turtle_n_Peeps]

Looks like very typical corrosion damage. It could also be high load spalling. But I would go with corrosion damage.

You have a boat?

[mike-s]

Blacklane wrote:
I think it's helpful to look at the big chunk of iron near the axle tube. That should be a good place to compare the two configurations since it's the same for both. For the baseline case, that's showing "white" or around 199F. For the aluminum cover case, it shows "deep red" or 151F."

I'm not buying - that's confirmation bias. Explain why the area where the axle tubes enter the diff (which has the most thermal mass) is essentially the same. Comparing the pictures, by far the biggest difference is the cover itself. And, if you think a cover is lowering the temperature by 48 degrees, want to buy a bridge?

[fj12ryder]

Blacklane wrote:

fj12ryder wrote:
Yeah, painting the cover was a bad idea, why use something to help dissipate heat and then use paint to help insulate it?

Actually, painting the surface black improved the thermal performance.

The layer of paint has negligible insulating properties, however the black coating greatly improves the thermal radiation away from the aluminum. This was proven in 1804 using a Leslie cube filled with hot water. The 4 sides of the cube were painted with different coatings, including copper and aluminum, but the black side radiated the most heat. The experiment is often repeated in grade-school science classes using different materials with similar results.

In general this is true, but you painted a rough aluminum housing with lots of surface area to radiate heat and now you have a nice smooth surface that in addition has another layer for the heat to penetrate. That's why most of these aluminum housings are not painted.

[Blacklane]

Completely agree. That's why I said:

"I tried to control for these by taking the measurements after a good, long tow. After about an hour, I figure the temperature is at steady-state. Additionally, the larger thermal mass of the aluminum cover makes it equally slower to cool down, but I tried to keep those the same times as well.

I think it's helpful to look at the big chunk of iron near the axle tube. That should be a good place to compare the two configurations since it's the same for both. For the baseline case, that's showing "white" or around 199F. For the aluminum cover case, it shows "deep red" or 151F."