Went to change the differential oil on my 92 XJ6 4.0 and noticed a plastic cover that protects some type of round knob on the rear part of the differential, above the fluid filler plug.

I normally have the car serviced at my mechanic's, and this time decided to do it myself. That's why I never knew it was there.

Now I'm wandering what's it for?

 

Not a knob. That's the plastic cap for the differential breather.

Cheers
DD

 

Hi Dough,

That makes sense and explains why it's at the top of the differential. As the differential fluid gets hot it expands and creates positive pressure. It would be benefitial to vent the air pressure, but not the fluid itself.

I'm not a professional auto mechanic, but I have been tinkering and maintaining cars for a long time. Never seen a differential outfitted with a breather before. Learn something new everyday!

Out of curiosity I pulled the plastic cap and seems there is a clear passage to the inside of the diffential. I would have expected some type of spring or valve to let pressure out and not fluid. I guess it's just a simple air vent.

Thanks for the explanation.