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ecomodded 08-15-2012 04:51 PM

How To maintain brakes while engine off coasting
i have been reading up on brakes today and came to a realization that there is a technique to maintain your brake booster pressure while coasting, engine off.

My tdi uses a mechanical vacuum pump that runs off the camshaft, when in neutral with the engine off, the camshaft is not turning, put it in gear and cam turns the vacuum pump, keeping the brake booster loaded.
My theory is that coasting with the key in off ACC on position can provide a unlimited amount of brake use.

In short by putting it in gear before braking i will maintain my brake boosters vacuum without starting the motor.

I am going to the lake later, a 20 mile country highway with lots of hills for me to try this ignition off brake in gear theory of mine.

UFO 08-15-2012 05:02 PM

That should work for any manual transmission vehicle, as brake vacuum for gassers can be generated with engine braking.

PaleMelanesian 08-15-2012 05:02 PM

I'd go ahead and key back to the "on" position. There's a bunch of electronic stuff you might like to have functioning. Things like ABS and airbags.

I preserve brakes by not using them. You get 3-4 pumps before the vacuum runs out. If I use 2+ pumps, I'll bump-start for a second to restore vacuum, then key off again. That way I have full boost if an emergency appears.

ecomodded 08-15-2012 05:07 PM

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I am thinking stopping with full brakes may prevent the use of the air bags.

I am actually excited about this new technique to try out, it will save ME a LOT of fuel and increase my coasting safety.
I was originally looking into a electric vacuum pump such as the ones used on some VW passats & jettas or this Volvo brake vacuum pump below. I may not need to spend a dime to get the same result.

Piwoslaw 08-15-2012 05:26 PM

A diesel has to have a mechanical pump, which is almost always actuated when the engine is turning. So, if you coast in gear, you can restore your vacuum, but you lose some momentum (energy) in the process. If the 2-4 brake pumps' worth of vacuum in the system isn't enough for you, then I can see three options:
  1. Make a reservoir out of a PVC pipe, increasing your vacuum system's volume by 1-3 liters.
  2. Add a small electrical pump to replenish vacuum while EOC.
  3. Replace your mechanical pump with an electrical one, like from an EV. This will reduce the load on your engine since the electrical pump will turn on only when needed.

BTW: Make an engine kill switch, it should be much better and safer than keying off. In my turbodiesel's case the engine kill switch keeps the power steering on and doesn't reset the computer. It may also keep the airbags and other important stuff on.

ecomodded 08-15-2012 05:39 PM

I am thinking technique alone will give me the desired results, which is full brakes while approaching stop lights or vehicles or coasting down hills.
A electric booster was my first thought until i realized i could just put the car in gear and turn the vacuum pump.

Piwoslaw do you have a thread on your engine kill switch mod.
I should be using a killl switch, the key switch is a distraction and a pain to do.

Piwoslaw 08-15-2012 05:57 PM

I don't think I started a thread on the details. I just found the crankshaft position sensor and put it on a relay which is actuated by pushing a button in the dash. I get a beep and warning about low oil pressure, and the ventilation fan stops, but other than that everything seems to be fine.
I just can't EOC for longer than 30 minutes at a time or the computer will go into economy mode;)

ecomodded 08-17-2012 02:22 AM

The brakes maintained there effectiveness without losing vacuum, while engine off coasting / in gear for braking.

I feel like the kill switch really is necessary, It is risky to be fooling around with the keys while driving plus there is a possibility of turning the ignition key to far, unknowingly locking the steering wheel.
I do not engine off coast every chance i get now because of brake issues and the key issue.
With a kill switch & in gear braking i could engine off coast in a lot more circumstances, such as shorter coasts with the convenience of the kill switch and steeper hills with the in gear braking ability.

Piwoslaw i am going to look for the crankshaft position sensor. What does the motor think happened when the switch is opened ? it sounds unconventional but effective. I like it.

ksa8907 08-17-2012 12:31 PM

Why can't you just go to the junk yard and get a vacuum canister? More than one if you need more vacuum.

ecomodded 08-17-2012 01:34 PM

I feel if any parts will be bought it would be the electric vacuum pump as the vacuum tank is not unlimited, i want a unlimited amount of braking so the 1 to 10 km steep hills i frequent can be safely engine off coasted.

I live in a mountainous region with many opportunities to coast, One is a 10 km coast from the top of the Pass leading to my city.

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