OK, thought I understood that when you put a car in park, the transmission precludes the rear wheels from moving & the park brake was just an added safety feature.

However, I'm questioning my understanding after doing a brake job on my xjr.

Prior to starting the job, I stopped the engine with the park brake switched off. When I raised the rear of the car, the wheels did not turn at all. when I removed park brake calibers, the rear wheels turned easily. I drove the car prior to calibrating the park brake calipers & it seemed to rock back & forth in park, indicating that the drive wheels had some play in them.

Why did the wheels not turn with the park brake off and yet they moved when the calipers were removed(I would guess that the park brake was not actually off; however, the switch was held in the off position as I turned the ignition off.)? There seemed to be more wear than I expected on the disc. Could it be that my caliper was putting the park brake on when it was not needed? It never smoked or gave any other indication of being on when it shouldn't have been. Why did the wheels spin freely when suspended & yet allowed only a rocking motion of the car when it was lowered? Why did the transmission allow the rocking?

Hope this makes some sense & thanks.

 

Park lock the trans, so the rocking is play in the driveline

From the Owners manual

The parkbrake is automatically applied when the key is removed
from the ignition switch

The parkbrake is automatically released when the gear selector
is moved from the Park ‘P’ position

If the vehicle is stationary and the gear selector is in ‘D’ or ‘R’
and the parkbrake is applied, then the parkbrake will
automatically release when the accelerator pedal is depressed

To disengage the parkbrake, with the ignition switch in
position ‘II’ or with the engine running, apply the footbrake and
press the parkbrake switch down


Cheers
34by151

 

Suspect I must have not turned the park brake off as I had thought.

Still wonder why the rear wheels can be spun freely while in park & when the car is lowered, only yields to the play in the drive train. I am assuming that the park break was not on before calibration.

 

Both wheels will spin with an open differential if they are both jacked in the air. Unless you installed a quafe rearend (locker). The diff will only allow them to spin in opposite directions. That's the short way of describing how a differential works. If you need more info Google "open differential"

 

Mark, thanks. I've heard the terms for years. Now I see said the blind man.

 

Ah well, actually ... even a Quaife will let you spin the wheels if the car is jacked up because, technically, it isn't a 'locker', it's a torque biasing differential and when there's zero torque on a wheel it will run free like an 'open' diff.

 

So the parking brake can be applied in D or R if the car is not moving, so is the parking brake an electrically operated (rather than mechanically operated) parking pawl and not an actual brake?

Or does it have a braking function other than locking the transmission?

 

Yes you can apply/release the brake with the lever at any time including when moving

The caliper is mechanical but the operation is eclectic
The cable is pulled by a motor which in turn is controlled by the EPB module

The trans will lock the output shaft in Park. You need to put your foot on the brake to release the Park position though

Cheers
34by151

 

EPB is also the emergency brake. It applies in a (workshop manual) specified manner that depends on car speed.

Here, it is tested annually with the wheel(s) moving because it is the emergency brake and must work (with a specified minimum force). No reason not to test it there too

 

JAG V8, how are they checked? What are some of the common issues with the parking/emergency brakes other than pad & rotor wear? Are the issues apparent or are more sophisticated tests required to detect them?

 

They put the (rear) wheels onto a large powered roller and the test person operates the EPB (lifts paddle). The roller measures the braking force.

It is essentially the same as driving along and applying the EPB but the machine measures each side to check:
1. reasonably balanced side to side
2. sufficient force

(The main brakes are similarly checked.)

Things such as pad wear are also checked.

This is our annual inspection (MoT test) - the entire manual is free online if you want to see the detailed list of the whole car test.

In terms of common failures of the EPB I think a fair few are due to there typically being no maintenance done by owners. That and just age - things fail...

 

I recall, when I was a child, my dad would get the car coasting down a hill in neutral and apply the e brake. We're atop a hill with a 30 - 45 degree grade; in memory of the old boy, think I will give it a try.

Thanks again, Jag V8, for bringing this to our attention. Next time i have the alignment checked, will have them put on the brake checking machine you mentioned.

 

The workshop manual does a fair job of describing what the EPB does at various speeds (so as not to lock the wheels or damage itself, I suppose).

If you're moving very slowly it applies fully but otherwise only a bit per paddle lift.