Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Rear differential vs. rear bearings

Rear differential fixed by local Subaru dealer. Next day, after 350 miles, white smoke and flames underneath vehicle. Car towed to Subaru Dealer in Bay City, MI. Michigan Subaru Expert said the rear differential froze up. SIA (Subaru of Indiana) engineers looked at car and said rear bearings heated up causing rear differential to fail. Which explanation is correct?

Is a lack of fluid involved here? Were bearings inspected at time of diff. repair?

Both explanations are correct; just phrased differently.
This was likely caused by a lack of lubricant and maybe, just maybe, someone forgot that last critical element in this repair.

If the bearings are blue or purple from heat then no lube is the cause.

The car never made it over to the lube rack…350 miles, not bad…

I agree; and one wonders if there was any whining turning to howling (car, not owner) before the disaster struck.

The last couple of miles, as the smoke and flames developed, maybe the kids screaming in the back seat covered up the death-rattle from the rear-end…:slight_smile: eeeeeeEEEEEEEEEEEAAAAAAAHHHH

I heard no noise, just a soft “pop…pop” like the car was going into another gear. Then I saw the smoke so pulled over to the shoulder of the Interstate.
The whining and howling happened BEFORE I had the rear differential fixed. (Got it fixed the next day.)

Love your explanation but I was alone.

Addendum: Local car mechanic stated that rear wheel bearings wouldn’t heat up enough to cause that much heat to seize up the rear differential. A wheel would come off first. He’s never heard of SIA’s explanation in all his years of working on cars.

Well they somehow neglected to refill the gearbox with oil or they completely botched the repair. Rebuilding differentials is seldom done in the field anymore, it’s a tricky repair…

A better solution is a factory assembled unit from a salvage yard…In your case, the dealership should replace the entire differential with a new factory assembled part…

A rear wheel bearing (single) could fail enough to make it very difficult to turn the wheel. The differential would compensate by sending power to the other wheel. However this would cause your differential fluid to heat up in short order as differentials are designed to send power to both wheels 99% of drive time. Running with power to a single wheel would overheat the diff.

Not sure if this would actually happened but plausible explanation from SIA.

Is this car under warranty?

I’m wondering if what they’re saying is that a rear wheel bearing froze and this led to a destroyed differential? A wheel bearing freezing would be so unusual as to hardly be worth considering. Wheel bearing would have nothing to do with differential oil but carrier bearings would.

And I’m also curious as to why people from Subaru corporate are involved in this. They seldom get involved in anything unless it’s a warranty issue or things have hit the fan.

Is this a case of we don’t know the entire story? what does lube the outer bearings on a Subaru?

The outer wheel bearings are sealed units. The cars have independent rear suspensions and the rear hubs are connected to the differential by halfshafts.

OK,as I suspected, even more unlikely that both bearings overheated an damaged the diff. Same setup on BMW, I have replaced them due too noise only and as preventive measures when roughness was felt while rotating.

I am going to assume that the dealer did fill the rear differential and that the rear bearings involved are the pinion shaft and differential carrier bearings. There was a caller to the Cartalk show that experienced the same confligration. C&C were unable to come to a definitive answer.

I wonder if the problem is a bind between the front axle and the rear axle in this AWD vehicle. I know that the manual transmission uses a viscous couple to transfer power to the rear wheels and the automatic transmission uses a hydraulic actuated clutch to do the same. If the viscous coupler is locked or the PCM is commanding full hydraulic clutch engagement, a bind between the front and rear axles would overload the rear differential causing the lubricant to overheat, vaporize (white smoke), and combust (fire). So have the mechanic check that the AWD coupling is indeed able to slip. BTW the same overload would occur on the front differential but it can get some cooling from the transmission oil cooler. After this is repaired, take the Subaru on a 50 mile trip and feel the temperature of the rear differential (caution it could be blazing hot).

Also make sure that all of your tires are the same diameter and circumference.

Give us feedback if this was/was not the contributing factor.

Unfortunately not. It’s a 98 Subaru Outback which I also have serviced before going on trips to make sure everything is okay. My trusty mechanic sent me to the Subaru dealer to have the rear differential replaced to the tune of mucho bucks.

Since there’s a ton of the story not known at this point let me pose these questions.

The differential was replaced to the tune of, quote, mucho bucks.

Was the differential a new, reman, or used one?
Was the differential the same gear ratio?

A Michigan Subaru expert (where my car is now) examined the car and told the local dealer that the rear differential had frozen up. Apparantly, the local dealer asked SIA (five miles from the dealership) to pay for the repairs. Instead, they sent their Subaru engineers to inspect the car. They said it was the real wheel bearings that heated up thus heating the rear differential enough to cause the problem.

The rear differential was replaced by the local Subaru dealer. I’m just assuming that they used a new one and that the gear ratio was the same. The cost was $1,645 if that helps.