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.

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

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.

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.

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.

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.

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;)

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.

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

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.

Here is the long answer.

The short answer is that a diesel will run as long as you supply fuel, no spark plug is needed, so in order to stop it you have to cut the fuel. You could just kill the injectors, but the newer TDs might now like that, so the ECU has its own stopping procedure. There are a few ways to invoke this safe procedure, for example when the ignition key is turned to OFF, the engine starts to stall, etc. If you cut the signal from the camshaft or crankshaft sensor, then the ECU might(*) not know the engine speed, so it'll go into shutdown mode.

*) Some ECUs calibrate the two sensors at startup, so if one is lost during operation the engine can still turn. In my case unplugging the camshaft position sensor wouldn't allow the engine to start, but wouldn't stop it once it got turning. The crankshaft sensor, on the other hand, is needed both for starting and operation.

Sounds like a good idea, and should be great in your Beetle.

But just a warning to other who might be keen to try this out, if your car isnt EFI then this is a very bad idea, keying off will cut the ignition but leaving it in gear the engine will still suck in fuel from a carby (or mechanical injectors?) so when you turn the ignition back on you have a lot of unburnt fuel through the car and get get a large explosion, this can destroy intakes or mufflers and makes a frighteningly loud bang.

I read your linked thread Piwoslaw.
I take it the check engine light turned off after you switched the crank sensor back on ?

First plan is to pull the crankshaft sensor wire with the car running to see if it will shut down.
If that works then a switch will be wired / placed so a unplanned deactivation will not be likely and it is still easy to reach.
My car is small, everything is in reach so placement should not be much of a challenge..

bobo i think this mod is a must if, like you say ,your car is fuel injected, scary scenario you described, that would certainly wake a person up in the morning!
But kidding aside i think this mod is going to catapult my mpg to new heights.

I take it you know that a turbo'ed car shouldn't be EOC'ed too often?

Everything goes back to normal, only the ScanGauge sees a P0335 code, which I clear every now and then.
A few members have their kill switch on the gear shift knob.
You can also wire a second switch in series, only this one hidden, to use as anti-theft protection.

We're drifting off topic...

Glad to hear about the code, its nice to retain the use of the check engine light.
The switch is a important part of engine off coasting, having the car there to coast with is important to.

I will now be looking for a spot that is somewhat hidden yet easy to reach and difficult to switch by accident..

The plot thickens:

As the oil heats and thins.

I did not think about my turbo's lubrication, but i better.
Often if not always i have to climb a hill to coast a hill, heating the oil/motor and spinning the turbo at near max boost in the process , making cooling a very real concern.
I think if i cycle it in and out of gear i can still coast and cool the turbo as well as provide vacuum for braking.
The complexity is growing.

I found my crankshaft speed sensor, after lunch i will disconnect it and double check the engine shuts down, then buy some shrink tube and get started.

After inspecting and thinking.. I deduced i am going to cut into the crank speed sensor at the diesel ECM as the wires already run that far so i'll cut and splice in that location,off to the side and away from the engine. I was hesitant to splice into the thin wires running from the crank sensor as the area is very exposed.
I needed to use my car yesterday and again today, so the switch will not be put in until tomorrow, too bad as i will be doing alot of engine On coasting on the :turtle:150 mile drive today.

The EOC (engine off coasting) begins.

The kill switch is in, it was a piece of cake once i took the plunge and cut the first 2 wires leading to the ECU, from the G28 engine speed sensor.
The G28 is the RPM sensor / engine speed sensor, not the vehicle speed sensor.
If the G28 sensor fails the ecu shuts down the motor. If i flip the switch the ecu see's this as a failure and kills the engine.
I would say this method is the absolutely best way to install a kill switch in a VW tdi that i could have hoped for. A BIG thanks to Piwoslaw for discovering this method and telling me about it !

1st- Mount the switch where you can reach it conveniently.
2nd- Near the ECU cut the tape and unravel about 6 inches.
Give the wires some space to ease the reattachment by exposing 5 inches of wires then cutting in the middle to share the length evenly, as instructed below.

Cut into black wire sheath that contains a yellow wire, white wire and black wire. There is two black sheaths the one with a blue wire inside is the wrong sheath.
Once you found the right black wire sheath to cut, cut it along the top to expose the wires inside, cut the black wire , cut the white wire.

3rd- Wirer from the cut to the switch then the switch to the cut in the wire harness.

Strip the wires a 1/2 inch or so and wire together then apply some solder paste to the wire and solder it fairly quickly so you do not melt the wire sheath.
I clean up the solder paste remnants with alcohol and a rag to insure a clean surface for the tape.

4th
Once all 4 wires (2 out 2 in) are soldered & cleaned you can tape the exposed wires, then tape it all to the wiring loom and finally re-wrap the loom with the original tape you cut to gain access and secure it with fresh tape.

what he said, and to elaborate, engine controllers will read the crank sensor for timing up to around 2500rpm then switch to the cam sensor above that, because it runs at half the engine speed.

Once the engine is killed, how do you restart?

And how is your steering? My Beetle's steering gets extremely heavy with the engine off, that's why I will not do EOC, but I am curious.

UFO, Once the engine is killed i have to cycle the key then i can switch the switch back to the on position and bump start it, or use the key, i bump it.
The beauty of the rocker type kill switch( on or off) is that you can coast in gear with the engine off with full brakes and power steering.
In neutral I will only have the break reserve vacuum and no power steering, if i need brakes or desire some power steering I put it in gear.
The car does coast surprisingly well in 5th gear, engine off with power steering & brakes.
On the steeper hills i will be in gear engine off maintaining my brakes and steering.
I myself find the powerless power steering manageable but it is indeed heavy, i look at as exercise..:p

That is a good piece of information. For my TDI, highway rpm is less than 2200 rpm

I doubt there is much benefit on steeper hills (engine braking), as fuel is cut anyway, but obviously coasting out of gear saves 0.8-0.9 gallons/hour, idle consumption.

Looking forward to results.

The kill switch is more effective at saving gas then engine on braking on the steep hills, as follows.
It will be best to use neutral when ever the full power brakes and steering is not needed. The steep grade hills will be approached engine off and coasted for the first few 100 meters until the speed increases then put in gear, at the bottom of the hill it will be put back in neutral,still engine off and coasted until i need to restart the car.
Simply put there is nothing more fuel efficient then a non running motor.

The idle consumption is more then i anticipated, .8 or .9 of a gallon a hour is large. I should be seeing a hefty mpg gain. With a modest 10% mpg improvement it would mean 6 mpg with my car.

This shouldn't be a problem for us hypermilers since we hardly ever rev that high, plus the engine will get killed after returning to idle at the soonest, to allow the turbo to cool off somewhat.

Ecomodded, someone here once remarked that kill switches should be of the momentary type, since it is possible to push and forget, especially if something suddenly grabs your attention. I believe there was something about a situation when he needed to bump start fast, but forgot to flip the switch and the engine wouldn't start. Lots of panic but no serious consequences, thankfully.
Just something to keep in mind.

That is high. My 1.6 turbodiesel consumes ~0.52 liters per hour (~0.14 GPH) when warmed up. Even with A/C on it's between 0.8-1.1 LPH (0.21-0.29 GPH).

BTW, is your PS pump electric, or is it run off of the engine?

I have a decimal point off. I think the idle consumption is more like 0.08 - 0.09 gallons per hour.

I'm sort of anxious to ruin my automatic, I want to convert mine to see what I could do with a manual.

I was originally one of the people thinking the momentary switch would be the best way to go, because of safety.
Then (see 1st post) i realized that i could have brakes and power steering if i used the on/off rocker type switch.

I would not be able to engine off coast in gear if it was the momentary type of switch, the car would just start up.
I put the rocker switch in because i wanted to EOC with brakes, my safety concerns were met plus some, as i also have power steering. Beats a momentary switch, for my purposes.

a) if you find it, please re post, you may of been right the first time.

b)If you replaced it,you could sell the auto !

Quote Piwoslaw:

That is high. My 1.6 turbodiesel consumes ~0.52 liters per hour (~0.14 GPH) when warmed up. Even with A/C on it's between 0.8-1.1 LPH (0.21-0.29 GPH).

Aha ! this also clarifies UFO's first numbers as incorrect, they may of been .8 or .9 Lph ? rather then gph a posted.

My Scangauge is set to 1.9 liters, dieselA and -21% fuel. This gives me accurate mileage estimations for my daily commute, but I suspect it is significantly off on idle fuel consumption. It shows 0.09 gph when warm at idle.

However, I don't know what it really is because I went on a 360 mile road trip through the mountains last weekend, and with all the coasting and engine braking, as well as all the hill climbing, my scangauge mileage was 65 mpg at the end of the trip. Actual tank mileage: 50mpg.

My car uses about 1 gal. per hour while driving, so the less time the motor is running the better. I do a lot of engine on coasting now so my gains will not be as much as someone else, who doesn't coast around constantly to begin with.

The switch is going to take some time before it becomes routine,in two weeks i should be adept at it..
I have to flip the switch off then reset the ignition to have it ready for the next start , move the key down one position then back up.
I find it bothersome all the same but i will get used to it shortly as with all new things.

I have to comment on the quietness and the great feeling you get coasting engine off/ in gear with power steering and brakes, the motor becomes powered by the wheels the wheels by the hills or momentum, its reversed, its perfect.

So far i have messed up three times trying to start with the switch in the off position, eventually it will will become routine, which will be nice :D.

My power steering runs off the belt so i get it when in gear, it coast's quite well in 5th gear, i was surprised by that.

Install an electric 12v vacuum pump and switch it on as needed.
Like when ever the key is off and the brake is applied. Or when the engine is off, but the key on and when brake booster vacuum falls below 5 to 10''Hg.

I would like a electric brake pump, I may end up picking one up from the wreckers, at some point. What i am looking into now is a way to fine tune the kill switch. The bug is not so much in the switch system but the cars ignition.
With my Beetle if it stalls i have to cycle the key a notch down and then back to crank or to on for a bump start.
Also if i crank the motor and it does not start, i have to cycle the key back a notch or it will not engage the starter.

a Stupid quirk that i want to rectify.
Hopefully i can bypass a wire or some such thing, Vag Com might help. I am not 100% sure but suspect the problem is with the security system,possible key recognition. My car came out a year or two before the chip keys were used but the ignition still may want to be appeased. At this point i am going to have to trouble shoot it.