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Old 08-28-2013, 06:46 PM   #1
Join Date: Apr 2013
Location: MA
Posts: 279
Ok, Brake question

This is a totally foolish question, but I need it answered as it would have got me into a small accident if I did not react quickly.

2013 JKUR, manual

I'm on a long down slope, waiting for a green. There is a long line in front of me. I do this stretch everyday.
Now I decide to do the most foolish thing. I turn off the jeep as traffic is crawling. Hmm.. to save gas . I know that I will loose power steering, which I considered as it is a straight line. But.... as I am coasting, my brakes have to power. Fortunately, I don't panic, but do go into an Oh Shit! mode. I immediately put my hand on the handbrake... but before I do that, push the clutch out and turn the jeep on.. instant brakes. Phew. Phew. Phew!

Lesson learned. But could you nice people tell me why the breaks need engine power. Are they hydraulic? My car(mazda3) did not do this.

I also think I scared the jeep. The moment I started it on the slope, I got a low fuel warning and it kept saying low fuel.. it normally does not.
A restart fixed it. Anyway, I skipped two heartbeats I think and my clutch leg hurts.

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Old 08-28-2013, 09:14 PM   #2
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Welcome to the world of electronics. Brakes need brake pressure - and when it comes to the lovely world of electronics, that means they need some of your electrically powered gizmos to generate that brake pressure.

To be less flippant, many vehicles now incorporate brake assist technology. Believe it or not, some folks have a real problem using their own leg to apply sufficient force to the brakes.

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Old 08-28-2013, 09:40 PM   #3
Join Date: Jul 2013
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The brakes are 'assisted' by a brake booster. Without the enginer running your reserve 'boost' goes away quickly. They should still work, just like steering will still work without the car on, but it will be unresponsive and hard to use.
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Old 08-28-2013, 10:14 PM   #4
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The brake booster utilizes engine vacuum pressure to assist in braking, thus no assist when engine isn't running.
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Old 08-28-2013, 10:34 PM   #5
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Location: Colorado
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Probably more information than what you want but here it is anyway.

Typical gas motor brake booster:

Pedal on the left, master cylinder on the right and in between is the booster.

The booster is roughly split in half between the pedal side and the master cylider (MC) side. On the MC side you can see a pink hose which is a vacuum line running to the engine. Under normal operation the engine supplies a constant vacuum to the MC and when you push the brake pedal the force you apply is boosted by the vacuum pulling from the other side. Without the engine running there is no vacuum assist and you are now applying force against the hydraulic pressure in the brake system as well as whatever mechanism is between the pedal and the MC which makes the pedal harder to push than the pedal in a manual brake system.

In a manual brake system the pedal is mechanically connected, as directly as possible (no booster between the pedal and MC), to the piston in the MC which allows the majority of the force you apply to the pedal to be transmitted to the MC. Over the last few years though, manual brakes have been relegated to either the cheapest or smallest of cars and sometimes pure sports cars will have manual brakes.

In vehicles with a diesel motor, the brake booster performs the same function but it is generally operated through hydraulic preassure or a separate vacuum pump because diesels produce little to no vacuum when operating.

As to your Mazda 3 not behaving in the same fashion as your Jeep, the only thing I can think of is that the Mazda (assuming power versus manual brakes) is a smaller vehicle with smaller brake components which made it somewhat easier to operate the brakes without vacuum assist.
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Old 08-28-2013, 10:41 PM   #6
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Power brakes need the engine running. The brakes will still work with the engine shut down but you need to push on the brake pedal pretty hard. Just like power steering. You'll be able to steer with the engine off, but it's harder to do. These functions are designed to work this way. If your engine dies while driving down the road you will still be able to steer and brake to a safe stop.

It's safer to just keep the engine running.
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Old 08-29-2013, 07:46 AM   #7
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Mebee need buy horse..............burn no gas!
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by its clean end!
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Old 08-29-2013, 08:14 AM   #8
Join Date: Mar 2011
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That's why real off-road vehicles have electrically boosted brakes with a huge pressure reservoir, so that you don't loose braking if the engine stalls. But Jeep is too much of a toy to have real off-road brakes.

Vehicles with electrically boosted brakes: Land Rover Defender, Merceges G-Class, Toyota Land Cruiser and FJ Cruiser. These guys know about what a real off-road vehicle should have.
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Old 08-29-2013, 09:59 AM   #9
Join Date: Apr 2013
Location: MA
Posts: 279
Ok.. thanks for all your replies.. so in case of an emergency, i need to apply more force.
Make me feel better when I know what is going on.
Thank you for the image rtone.

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