I've got an 87 GTA with a 9 bolt posi (confirmed by inspection) with 35k miles on it. With both wheels suspended off the ground and the car in park, I can put all my strength into rotating each wheel in each direction but all I hear is the driveshaft turn a little and stop due to the transmission being in park. I cannot get the opposite wheel to turn.

When I drive the car and turn sharply, there is no evidence of the differential not working - and I have driven a car with a locked diff before.

Not really sure what the expected behaviour is here, can someone confirm?

 

Sounds correct for being in Park. Put the trans in Neutral and spin a tire, the other tire should spin the same direction.

 

What you describe seems more or less correct.

In perfect, like-new, as-designed, wear-free condition, it should take around 80 - 90 ft-lbs to turn one axle with the drive shaft locked (trans in Park or whatever). The other axle should of course turn the opposite way at the same time when the drive shaft can't turn. If the drive shaft is free, the axles should turn the same way, spinning the drive shaft as well.

 

Thanks all!

 

Traditional "posi-traction" differentials have pressure loaded against the friction surfaces. So the two axles are attached to each other and turn at the same rpm. That is, until force able to break that friction comes into play. Turning a corner does that and it's able to act like an open differential. A more aggressive pressure upon the friction surfaces can be double stock and still be smoothly drivable. It sounds like yours is working for now.