I was taking my daughter to school this morning in my fairly new XJR, and she wanted to know what the chrome lever on the console was. I explained it was the electrically operated parking brake. She asked, "What would happen if you pulled the lever while we were driving?"

Hmmm. I assume the brake would not apply, but I didn't want to test it to find out. I haven't done a bootleg turn since high school! Anyone know what logic they used here? I can see a need to have the brake on with the car in gear, so you wouldn't want the car to have to be in park. Maybe it's referenced to the VSS?

 

According to the owners manual:

Inadvertent operation of the parkbrake whilst the vehicle is in motion will cause the message "PARKBRAKE ON" to be displayed on the message centre, the warning light in the instrument cluster will be illuminated and a warning chime will sound.


And according to the training guide, if the car is travelling > 20mph:

Pulling the Electronic Parking Brake switch once and releasing will result in the control module responding with an actuator apply time of 500mS.

Each subsequent pull on the EPB switch adds another 250mS of motor run time until full force has been achieved (approximately 3-4 pulls).

If the driver pulls and holds the switch in the up direction the control module will result in a ramp-up sequence as follows:

Ramp-on for 500mS
Stop for 500mS
Apply for 250mS
Stop for 500mS
Apply for 25OmS
Repeat until full loading registered

It looks like if you are travelling between 2 and 20mph a quick apply has the same affect as above but pulling and holding will just put the handbrake on.

 

I know everybody in the US calls it a parking brake, but it is actually an independent handbrake, mostly used when the car is parked. This brake does, however, provide the only remaining braking facility if the foot brake fails for whatever reason.

Clearly as a brake it is not very good, but will eventually stop the car, assumng the throttle is closed. Testing the efficiency of this brake is a key part of the UK Government roadworthiness test, (the so-called MOT Test). A minimum level of efficiency has to be acheived or no certificate is granted. No certificate = illegal road use.

 

I saw the part in the manual about the warning light illuminating and the "Parkbrake On" message, but it didn't actually say if the parkbrake applied or not. That's why I didn't want to try it.

Frasier's right - technically it's an "emergency brake" so you would want to be able to apply it while the car is in motion. 3 or 4 pulls to fully actuate the brake is logical - that's enough to get the brake fully applied without a single inadvertent pull locking the wheels.

Next question - if you do fully apply the brake, how do you release it? Would you have to let the car come to a stop and put it in park?

 

I assume, either just let go of the lever, or push it down past its normal resting place.

 

Interesting... I've always wondered what would happen if the computer decided to apply the emergency brake on it's own accord for whatever reason, while the car was moving.... I wonder what failsafe loops it has built in...

 

Interesting footnote: The parking brake/emergency brake on my wife's Pontiac Solstice stopped working. It took four trips to the dealer to get it to hold, however the car still didn't feel right to me. Way too much nose dive under braking. I checked and found that while the brake would hold the car, yanking it up while the car was moving had no effect. Worse, the rear service brakes didn't work either. I could lock the front wheels and the rear ones kept on turning.

According to the service manager at Sir Walter Chevrolet, the rear brakes on modern cars are not supposed to work. He said they are only there because the law requires it, and they are not supposed to provide any stopping force. I took the car to a different dealer and he admitted the rear brakes should work, but said he couldn't find anything wrong with them (other than the fact that they didn't work) and suggested I wait until all the brakes failed and then bring it back.

Perhaps the fact that the car was less than 1,000 miles from the warranty expiration influenced their opinions? Anyway, I rebuilt the rear calipers and all is well.

 

If that service manager said that, then avoid that dealer ! What nonsense !

Of course a car needs all its brakes. However, it is the fronts that do the "heavy lifting" of stopping the car, the rear brakes provide a steadying function, and, in a Jaguar are used by the stability control to stop wheel spin whilst accelerating. Modern powerful cars tend not to have limited slip diffs these days, wheels are stopped from spinning by the system braking the wheel that has started spinning. That's why the rear brake pads tend to wear out quite quickly nowadays.