New thoughts on "Ice Mode" & possible solution?

SonicVQ

Over the years, there has been a lot of talk and worry about "ice mode".
I don't know why, but I find this topic very interesting...

After reading this thread Lock up and ice mode I started to dig into what could be going on...

From what I have read, the common item is sudden hard braking after a straight and if you get off and then back on the brakes, it is back to normal.
That got me thinking about brake booster vacuum. Low vacuum = low brake boost = very little brakes.

When the VVEL is controlling the air intake to the engine (typically during light-mid engine load) the intake manifold has very little vacuum of about 4 in/Hg (data logged via OBD)

Also, when the VVEL system is controlling the air intake to the engine, if you lift off the accelerator pedal, the throttle bodies quickly close have vacuum for the brakes. It is like a quick "recharge"

Because of this, our brake booster has its own pressure sensor. This is used to make sure there is a specific amount of vacuum in the booster. If it gets low, the throttle plates close for a moment to "recharge" the booster.


However, during wide open throttle, there is almost no intake vacuum.

What if the brake booster or check valve in the brake booster hose has a slight leak, allowing the booster to loose the stored vacuum while going WOT down the straight?

Then add a very quick transition from WOT to brake, where booster vacuum may be low and if your transition from WOT to brake is quicker than the time it takes for the ECU to close the throttle and create vacuum, you won't have much brake boost or vacuum and little brake function.

People have said that when it happens to them, if they get off the brakes and back on, full braking is restored... I wonder if that slight bit of extra time, allowes brake booster vacuum to build up?


So, how to test this theory?
I have two thoughts:

• Since the brake booster pressure sensor's voltage is available through enhanced ECU diagnostics (OBDLink or OBDFusion with $14 enhanced add on) the voltage can be easily data logged and monitored.

• Add in an electric vacuum pump to the brake booster before the check valve.


So, that's my thought/theory.
What do you think?

Rusty

A while back in one thread. Someone was adding a extra vacuum reservoir to the brake booster. Never did find out how it worked out.

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Evildky

I had this thought shortly after I first experienced ice mode. Hooked up an external vacuum canister, did not help.

SonicVQ

Interesting...
Where did you connect the vacuum canister to and did you happen to measure/view the brake booster pressure or sensor voltage?
(I assumed you didn't, but wanted to verify)

I'm going to go for a short test drive and data log the throttle position, MAP and brake booster voltage while driving around under different engine loads.

AND with the engine off/ignition on to see how quickly the pressure in the brake booster changes.

The rate of vacuum drop (pressure increase) may be helpful to troubleshoot this issue. -OR- it may not be helpful at all... You don't know until you look

Evildky

I used an off the shelf vac canister, I plumbed it inline with the brake booster line from the manifold as one typically does for cammed cars that don't generate enough static vacuum. Verified everything was working property and that it was holding vacuum. Drove the car for a bit so it would have a chance to build vacuum. Got the same result.

SonicVQ

Thanks for your quick reply.

If you connected the can on the manifold side of the one way check valve, the can would be vented to the atmosphere as soon as the throttle opened.

If you connected the can to the booster side of the check valve, then that would provide a little extra vacuum.

The one way check valve is built into the hose that connects to booster to the intake manifold. It isn't like other designs where it is part of the brake booster elbow/connector.

Do you left foot brake?

Evildky

I do not left foot brake. The canister I bought has it's own check valve. It's been a while but as i recall I ran a new hose from the OE port on the manifold to the vac canister (pretty side that the side that had the check valve on the canister, then form the canister back to the OE vac tube that crosses back over to the booster (check valve is located on the booster iirc). I verified that the booster and check valve worked correctly and held vacuum and that the canister was holding vacuum and that the check valves was working properly. The only problem with this method is it still relies on the engine to build vacuum, while I did not measure the vacuum I did verify that it held vacuum, so it's possible wiht a vac pump you could pull more vacuum, the reason I left it there was that under moderate or ramped up braking it had plenty of boost with good pedal feel, no reason to suspect lack of vacuum at that point. Feel free to pursue this, I expect you'll find the same result I did.

SonicVQ

Understood.... and just to be very clear, I am VERY grateful for you sharing what you have done, and I am not trying to pic apart/belittle what you have done. Your insight to this is GREATLY appreciated!!

I have set up OBDFusion to record the MAP/Brake booster/throttle position sensor voltages and went on a short test drive.

I'm going to look at the data now...

SonicVQ

For the 370, here is the location of the check valve / one way valve:
(This valve only allows air to flow out of the brake booster)

Attached Images

booster_check_valve_sm.jpg (55.1 KB, 84 views)

SonicVQ

After doing a lot more research on ICE MODE, it isn't a brake booster issue.
It seems to happen on multiple OEM ABS modules when used in track days / racing.

A common tigger seems to be multiple small bumps going into a corner.

Here is a post from Travisjb way back in 2012:
I just spoke with Bosch and with the head of their major west coast distributor

Not going to quote anyone directly but basically the theories discussed in this thread are correct. The DOT approved versions of Bosch ABS induce ice mode when the wheel rate sensors detect variation across the four sensors. I am not clear if it is front/rear overall or any one wheel or both, nor did I find out what % difference the computer looks for nor timing involved in the algorithms... but it is a safety mode designed to help in the majority of street situations... hard to blame them for that.

Link to full text:
(Ice mode solution?)

Too bad... For a moment I thought "just maybe"!!