Tuning brake pedal feel/bias with brake light switch adjustment - EBD sorcery

I recently installed some Carbotech AX6 pads on the front of my 2011 Mazda2.

My initial impression of them... They have some RIDICULOUS initial bite, especially after 2-3 moderate stops and they are warm. I'm talking it was like 1/2in to 3/4 pedal travel being equivalent to standing on the brakes with the stock pads.

I could get pretty nutty ABS engagement just trying to come to a stop in my neighborhood.

I figured it was just something I'd get used to, and I started to. I put about 400mi on them this week.

Well last night I noticed that my brake lights were coming on just a bit further down in pedal travel than I'd like... as in the car was just starting to begin to slow down before the lights came on if pressed slowly. The brake light switch is a self-adjusting type, when brand new you push the pedal down, insert the switch, release the pedal upwards, it compresses the switch and internally it ratchets, so now when you press it, the switch extends. A certain distance down, the lights come on. Technically not manually adjustable.

So because of new pads and stainless lines and now a very nice firm pedal(and very little effort needed to stop the car with the new pads), I put a small screwdriver(figure 1.0mm thickness or so) in between the switch and pedal, to make the internal adjuster move one more click when I let the pedal up. So now the brake lights come on about 2mm of travel sooner.

So I take it on a test drive... The brakes feel COMPLETELY different. They can now be modulated easily and controllably, much like stock. Push harder(but not as hard as stock) and it slows down in a hurry like you'd expect. Much more balanced, assuming more rear bias. Less ABS engagement unless you really want to. Basically freaking amazing.

I was in disbelief so I taped a penny to the brake pedal bracket(to simulate going back to similar how it was before brake light engagement timing-wise)... and sure as hell it goes back to being touchy as hell, front end doing most of the work biting hard. Unable to easily modulate initial bite.

Mazda doesn't publish anything particularly useful on description and operation of their systems, so I looked up the EBD info for a Ford Fiesta(which should be similar or at least comparable).

Here's the excerpt from the Fiesta FSM:

[QUOTE]The ABS module incorporates a strategy called Electronic Brake Distribution(EBD). The EBD strategy uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain threshold, the ABS module closes the appropriate solenoid valves int he HCU to hold rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of a rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

If there is a DTC in the ABS module specifically for the HCU or there are 2 or more wheel speed sensor DTCs set, EBD is disabled. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and the stability/traction control indicator(sliding-car icon) illuminate.

[/QUOTE]

My BEST GUESS as to what is going on... Maybe the pads were biting RIGHT before the brake switch was pressed, so when I started pressing the pedal it WAS doing it's rear-bias thing like it's supposed to, with just a bit of front bite, then right at/near the brake light switch point the bite of the front pads are slowing the car down just fast enough that it pushes the bias forward, and due to the nature of the pads they bite harder, more bias, bite harder yet, etc etc.

I know someone on here works for Bosch(IIRC) programming ABS modules... hoping they chime in on what kind of logic would be getting used to cause such an incredible change from a simple brake switch adjustment.

I dunno, I'm pretty stoked I won't look like a complete idiot trying to heel-toe a car with an on-off brake pedal like I thought I was going to have to relearn I was just hoping to change the brake light switch point to hopefully lessen my chances of being rear ended on my commute

I don't think the switch necessarily "adjusts" the bias per-say, but if the module doesn't think the pedal is being pressed, it will not attempt any bias adjustment(I'm ASSuming).

To add to the information: I pulled both DSC fuses last night(for science!). This completely disables EBD due to no communication with the HCU.

When I drove it, the pedal was moreso like after I adjusted it, not super touchy on initial application. So by default they are not touchy.

My revised guess is that with EBD disabled, it defaults to a predetermined fixed bias rate.

With fuses in and EBD and functioning, it starts off with primarily rear bias supposedly... But feels more front biased since it bites so hard...

Im thinking if it was just barely starting to grabgrab(fronts) before light illumination(but pressure is building in the rear), maybe it sees pressure up in the rear, but doesn't perform any bias adjustment until the lights come on(it just lets whatever happen as long as the pedal isn't pressed)... So when they do come on it abruptly sends pressure forward for the desired brake bias? My best theory for now

What pads are in the rear? I ran AX6 on the front and 1521 on the rear on my Miata w/o ABS and it was extremely front biased. I am now running 1521's on front and rear and get a more balanced feel. Thinking about putting a set of AX6 on the rear leaving 1521 on the front to see what happens.

Not much help about the light. Seems the AX6 pads are working before the light engages.

Using the switch (an adjustable switch to boot) as a feedback to the system sounds like it could have drawbacks much like you have detailed. Since the default appears to be usable and the failure mode for a broken switch would also give the same result, it sounds like at least the programming is doing the right things, FOR THE FACTORY BRAKES DESIGNED FOR THE APPLICATION. Glad you seem to have found a way to get the reaction you want with the existing hardware and software. It does seem that the further down the electronic control/feedback path we go, the more we are going to need to think about the software side as much as the mechanical side when we start modifying our cars.

wlkelley3 wrote: What pads are in the rear? I ran AX6 on the front and 1521 on the rear on my Miata w/o ABS and it was extremely front biased. I am now running 1521's on front and rear and get a more balanced feel. Thinking about putting a set of AX6 on the rear leaving 1521 on the front to see what happens. Not much help about the light. Seems the AX6 pads are working before the light engages.

Completely different systems. You have a prop valve. It gives the same bias no matter what.

Read again how putting a penny in to space the switch contact point I can go between good feeling and super overly grabby brakes. It's some kind of wizardry going on inside the HCU.

With the HCU fuses pulled the bias is normally feeling. But when its active... Depending on switch engagement point it can be touchy or perfect feeling basically. It's very bizarre.

stafford1500 wrote: Using the switch (an adjustable switch to boot) as a feedback to the system sounds like it could have drawbacks much like you have detailed. Since the default appears to be usable and the failure mode for a broken switch would also give the same result, it sounds like at least the programming is doing the right things, FOR THE FACTORY BRAKES DESIGNED FOR THE APPLICATION. Glad you seem to have found a way to get the reaction you want with the existing hardware and software. It does seem that the further down the electronic control/feedback path we go, the more we are going to need to think about the software side as much as the mechanical side when we start modifying our cars.

Yeah, I had no issues with stock brakes. I'm currently partnering with another Mazda2 owner on stock brakes to see what/if anything occurs if the same adjustment is done.

Also it's technically self adjusting. I just fooled it into clicking once more to make the lights come a bit sooner.

I could see a brake pressure switch affecting things, but this is just a basic on/off type switch.