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Old 08-14-2013, 09:12 AM   #11
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Re: Brakes losing power under 4x4 Low Gear.

Diesel 2010 SMB 4x4.

I did find this post on the automotive forums.

http://www.automotiveforums.com/t521713 ... iesel.html

In a "normal" vacuum assisted situation, you have the maximum vacuum available during braking with the throttle plates closed on a gasoline engine. Well these (and most) diesels do not have any throttle and cannot make engine vacuum at all.

When you depress the brake pedal, a small valve is opened allowing stored vacuum to be vented to a diaphram. Doing this lowers the vacuum in the storage "tank" that is part of the booster assembly. In a gasoline engine, there is so much vacuum pressure and volume available during closed throttle deceleration, that you never even notice any vacuum was "used" as it is replentished immedaitely by the engine.

Diesel engines installed in vehicles with vacuum assisted brakes all require some form of vacuum pump to be fitted. The most common way is to drive them off the engine belts where the amount of vacuum is related to speed and time (it takes some time to produce or replentish used vacuum).

So, when you have a truck like yours with an engine driven vacuum pump, and you are aproaching a stop, you remove your Rt foot from the accelerator (GOOD thing), the engine speed drops (reducing the vacuum pump capacity), and you depress the brake pedal (which causes the vacuum in the booster to drop). As you are slowing down, the vacuum pump continues to run and replentish the used vacuum, THIS causes the pedal to "sink" because as vacuum is replentished there is ever-more "assist" available to the brake system.

The brake booster is basically a canister with a diaphram inside that is sealed on one side, and vented to the atmosphere on the other (brake pedal) side. The sealed side is placed under vecuum by the engine or vacuum pump in this case. The assist available (in PSI) is the area of the diaphram times the pressure difference between the two sides ofthe diaphram (A*Pd). Since the area does not change, but the Pressure does (and does so MUCH more in a system with a vacuum pump) you will have more assist as the engine driven pump replentishes the used vacuum.

Per the FSM, if the vehicle stops "normally" DURING braking, and the pedal does NOT SINK DURING braking, but ONLY while sitting stopped, there is not "problem".

Sounds like the issue I was having.

Will test again today in low, high, and 2WD.

Thanks everyone!
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Old 08-14-2013, 10:25 AM   #12
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Re: Brakes losing power under 4x4 Low Gear.

I think you are barking up the wrong tree - I doubt it is vacuum related.

The system has a vacuum reservoir that is good for multiple stops, even with the engine not running.

The "sinking pedal" they are referring to is a result of an additional vacuum spike but does not result in loss of braking force - as I understand it you were losing braking.

There should be no correlation to being in 4wd or Hi or Lo Range.
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Old 08-14-2013, 10:43 AM   #13
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Re: Brakes losing power under 4x4 Low Gear.

I want to point out what is perhaps obvious -- that your vehicle has 4 times the wheel-torque when in 4-low, so you need 4x the brake to stop your van from rolling when the engine idling in gear. You really need to step on that pedal. Combine that with a steep incline, and you're putting two feet on the brake pedal.

I have witnessed a (low-speed but expensive) van-to-van collision where the driver did not understand how much pedal pressure is needed to keep the van stopped with the transmission in Drive with 4-low.

What you're experiencing might very well be more than this, but something to keep in mind.

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Old 08-14-2013, 11:07 AM   #14
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Re: Brakes losing power under 4x4 Low Gear.

Quote:
I want to point out what is perhaps obvious -- that your vehicle has 4 times the wheel-torque when in 4-low, so you need 4x the brake to stop your van from rolling when the engine idling in gear. You really need to step on that pedal. Combine that with a steep incline, and you're putting two feet on the brake pedal.
Precisely my experience. I can have both feet on the brake pedal giving it all I have to give and the van can still be creeping forward in 4 Lo.
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Old 08-14-2013, 11:17 AM   #15
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Re: Brakes losing power under 4x4 Low Gear.

If your idle speed (engine RPM) is set correctly this should not be a problem as idle is well below torque converter lockup RPM. Heck, I use Lo Range to compression brake down steep hills.

Quick thought - are you sure your anti-lock brake system is not getting activated because the tires are losing traction. That will make your pedal real strange.
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Old 08-14-2013, 11:21 AM   #16
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Re: Brakes losing power under 4x4 Low Gear.

When I'm driving down steep trails I'm alway at the ready to pop into neutral to remove engine torque. And there's been a few times I've put it into gear the other way to get some braking help from the torque converter.
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Old 08-14-2013, 11:31 AM   #17
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Re: Brakes losing power under 4x4 Low Gear.

Quote:
Originally Posted by BajaSportsmobile
Quick thought - are you sure your anti-lock brake system is not getting activated because the tires are losing traction. That will make your pedal real strange.
Good point -- that can definitely feel like you have no brakes. But you should hear the pumping ABS sounds if that's occurring.

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Old 08-14-2013, 11:56 AM   #18
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Re: Brakes losing power under 4x4 Low Gear.

Yes, you should hear it if you know what to listen for and other noise is not too great. I disable my anti-lock brakes when in the dirt.

Quote:
Originally Posted by geoffff
Quote:
Originally Posted by BajaSportsmobile
Quick thought - are you sure your anti-lock brake system is not getting activated because the tires are losing traction. That will make your pedal real strange.
Good point -- that can definitely feel like you have no brakes. But you should hear the pumping ABS sounds if that's occurring.

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Old 08-14-2013, 12:36 PM   #19
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Re: Brakes losing power under 4x4 Low Gear.

Quote:
Originally Posted by BajaSportsmobile
I disable my anti-lock brakes when in the dirt.
Oooh do you have a trick for that? What van do you drive?

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Old 03-11-2016, 11:29 AM   #20
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I have (partly) the Same problem. So I am bringing this thread to new life.

2008 e350 V8 gas. Quadvan 4x4.

When i switch to low gear and push the pedal ... Nothing happens. Does not rev up. Van just starts driving at about 1 mph. Just when I push the "TCS" button it drives normally. Strange enough. (Any thoughts about that?)

No problems there in 4x4 high!

But often when i am driving in low gear (Tcs button pushed) and need to brake I have symptoms like in this thread. Pedal low. Just hesitatingly braking. Feeling somehow like ABS.

Any ideas??

Here in germany not to much knowledge about e350 is acumulated.

Philipp
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