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Disabling ABS via various methods and effects on braking and brake bias

It's been a while since I last updated the thread but I've continued to do some testing and making observations with the electronics disabled, continuing to look at handling and chassis dynamics but focusing on torque steer and anti-lock braking. In this segment I’ll be covering braking and the methods of disabling the torque vectoring type functions which might interest those planning on taking their car to the track to see how it behaves without the torque vectoring type functions enabled.

To recap, without access to the programming in the ABS HCU (hydraulic control unit) controller I would be surprised if there is any way to fully disable the torque vectoring stability and traction control functions via current software tuning solution. The AdvanceTrac stability control does offer three modes, the “Normal” full on, “Sport” mode where the computer allows for more wheel spin and high yaw and slip angles before stability control steps in, as well as the “Off” mode. With it off you’ll find it easier to light up an inside tire under heavy acceleration mid-corner and the stability control system won’t selectively brake individual wheels to rotate the car to assist in keeping the vehicle going on the directed path of travel the system is still applying braking on the front axle for the torque vectoring, curve control type features to help put the power down and aid in rotating the vehicle. You can get the car to slip and slide all over the place and the conventional stability control functionality won’t interfere but the other features still have some impact on the chassis dynamics and stability (but aren’t acting on their own to bring the car under control). From the ST Owner’s Manual Supplement Ford describes it as follows:

ENHANCED TORQUE VECTORING CONTROL
Enhanced torque vectoring control (eTVC) is comprised of two elements:
• Torque vectoring control which applies brake torque on the inner
wheel in a curve for better traction and less understeer
• Cornering understeer control which controls the yaw response of the
vehicle under braking and acceleration on high and low friction
surfaces.
Unlike ESC, eTVC control does not slow the vehicle but does help
control excessive wheel slip and gives the vehicle cornering agility. The
system only increases performance. Because of this, eTVC is not disabled
when the AdvanceTrac system is off.

Since we can’t disable it in software (the tuning tools like the currently available SCT and Cobb solutions are only for the Bosch powertrain controller and not the ABS controller), that leaves us with three somewhat easy methods to full disable all brake-based electronic aids: 1.) disconnect a front wheel speed sensor, 2.) disconnect a rear wheel speed sensor, and 3.) pull the ABS module fuses. I differentiate between the front and rear sensors because each method produces slightly different results discussed below.

Probably the easiest method without having to crawl under the car to disconnect a wheel speed sensor is to remove the ABS fuse from the underhood power distribution box. According the manual there are three fuses that affect the ABS, fuse F19 for the controller (5A, “Anti-lock brake system/Electronic stability program 15 feed), fuse F8 likely for the solenoid pack in the HCU (30A, “Electronic stability program valve”) and fuse F7 for the pump motor used to build hydraulic brake pressure without the operator having to press the brake pedal (40A, “Anti-lock brake system/Electronic stability program pump”). All three of these fuses are located in a module and can be removed together or individually by opening the cover on the fuse carrier.












This method will completely disable the ABS HCU and all anti-lock braking and stability related functions but is generally not the preferred method for disabling stability or traction control on cars that lack a true on-off switch like the old SVT Focus (that car did have a switch to disable traction control but only killed the powertrain controller traction functions such as reducing ignition timing to decrease power to bring a spinning wheel under control but it couldn’t disable the ABS-based braking features which were still used and active below 25 mph/40 km/h even with traction control set to “off”). The reasons why pulling the ABS fuse is often frowned upon is due to most newer cars using the ABS for electronic brake force distribution (EBD) as discussed earlier in the thread. Without a mechanical proportioning valve to control brake pressure gain between the front and rear wheels and no EBD active many cars will prematurely lock the rear wheels.

I wanted to find out if this was the case so I headed out to a dirt county road where I could practice varying levels or brake application force, application time and braking from different speeds. Upon pulling the ABS fuses the first thing I noted was that the speedometer and odometer stopped working and the expected ABS and warning lights came on with one exception, the red “Brake” light was also on. This generally comes on only when there’s a problem with the mechanical braking system such as low fluid level in the reservoir due to the pressure switch reporting a fault on the master cylinder.



After driving a short distance another unexpected light also came on, the supplemental restraint or “airbag” light. This is pretty logical as the airbag controller needs to know the vehicle speed to use in combination with the deceleration sensors to calculate if you’re going fast enough and have an impact with enough force to trigger an airbag deployment (normally around 12-18 mph in most cars to prevent a deployment in low speed collisions such as in a parking lot, for example).



With the fuses out and still on asphalt pavement I performed a braking application at about 35 mph with moderate force and engagement speed and was rewarded with the left rear wheel locking up and the rear of the car stepping out hard to the side. Without immediately modulating braking pressure this likely would have required an application of counter-steer has this been on a surface with less traction. After proceeding to my dirt testing area I confirmed that with the ABS fuse pulled there is a dramatic change in rear wheel bias once the EBD function is disabled. On dirt and regular pavement it is still possible to brake with a good amount of stopping force but if the physical master cylinder ramps up pressure too quickly or if the roads are poor the rear wheels always lock up first and in a manner that is unforgiving at higher speeds and also affects lower speed braking if you lay into the pedal too much.

A couple other items of note, with the ABS fuse disabled the car has no way to obtain vehicle speed from any of the active-style, powered ABS sensors (which work like a Hall effect sensor and are more accurate than older, passive sensors). As such the powertrain controller can’t tell how fast you’re moving and it appears to slightly reduce power. Without digging through the calibration and programming/algorithms in the powertrain controller it is probably registering a default and falling back to a fail-safe or certain tables and cells aren’t active and being used with no vehicle speed input. The last minor item to note is that in the driver information screens on the instrument panel display, even with close to a half dozen warning lights lit up the System Check screen doesn’t provide any additional information, instead showing all systems as normal.

Next up was a comparison to normal, fully enabled operation. The first start after replacing the ABS fuse turned off all lights with no lingering check engine or braking system indicators. Both on pavement and the dirt road the ABS works well with smooth modulation and excellent vehicle control.

The next test was to disconnect an ABS sensor and I started with the rear wheel. As noted earlier in the thread if you disconnect a front wheel speed sensor the speedometer stops working and the electrically assisted power steering defaults to a very low, fixed level of assist. I did note this time that the odometer kept working with either a front or rear sensor disconnected. With the rear sensor unplugged the speedometer works and you have normal power steering assist. With just the ABS wheel speed sensors unplugged, either at the front or at the rear (I didn’t test with multiple sensors unplugged) the amber colored ABS malfunction indicator lamp comes on but the red “Brake” light does not compared to what happened when pulling the ABS fuse. Why this happens becomes pretty apparent after the first moderate stop on poor road conditions.

Unlike what happened when pulling the ABS fuses where there was extreme rear wheel brake bias and both tires would lock up harshly, with just the ABS wheel speed sensors disconnected it does appear the EBD function is still active. The rear brakes still have a tendency under quick and hard braking application to lock up first but it is nowhere near as bad as with the ABS disabled via the fuse. The system may not be able to modulate brake bias front to rear to account for vehicle load, weight transfer, speed and force when the brake pedal is applied among other factors but it still gives a somewhat normal brake bias.

To summarize:

1.) Remove ABS fuse block
  • Disables speedometer and odometer.
  • Disables anti-lock braking and all functionality relying on it.
  • Disables electronic brake force distribution (EBD) biasing for the rear wheels.
  • Rear wheels of car extremely likely to lock up under moderate braking.
  • Disables supplemental restraint system (airbags).

2.) Disconnect front wheel ABS speed sensor
  • Disables speedometer (odometer still works).
  • Power steering defaults to fix, low level of assist.
  • Anti-lock braking and functionality relying on it disabled but EBD still active.
  • Slight tendency towards rear wheel bias under braking (likely but not always first to lock up).

3.) Disconnect rear wheel ABS speed sensor
  • Anti-lock braking and functionality relying on it disabled but EBD still active.
  • Slight tendency towards rear wheel bias under braking (likely but not always first to lock up).
  • Speedometer, odometer and power steering operate as normal.


With the ABS fuses pulled I’d say it isn’t safe to drive the car on the street. With no brake force distribution between the front and rear brakes the rear end of the car will lock up and step out with ease. This likely explains why Ford triggers the red, mechanical service brake “Brake” light when the ABS system isn’t powered. You still have normal braking power but it is undirected with too much force going to the rear. For someone on the track, if you’re running giant, sticky, non-staggered size tires or running a much more aggressive brake pad compound up front it might be a way to get more braking force to the rear or lock up and slide the rear if that suits your style but for the majority of people they’re probably best served not to pull the fuse.

That brings us to the wheel speed sensors. If you want to explore what the ST is like without the torque vectoring and corner control features behaving the car disconnecting a front sensor will increase steering effort and give some extra weight to the steering but you lose the speedometer while disconnecting the rear sensor leaves the speedometer and steering unaffected. Without the torque vectoring the car isn’t as cohesive and tossing it around corners leads to more understeer. The tail will still want to step out like on a stock, normally operating car but you won’t have the electronics there to help control this in a progressive manner. As such, with the increased steering effort afforded by disconnecting a front sensor it seems a little easier to dial in the right amount of counter-steer.

With the rear wheel sensor disconnected and normal steering behavior the once playful variable-ratio steering rack and reduced driver effort from the power assist (with assist still quite noticeable at speed compared to with the front sensor disconnected) the steering is twitchy and doesn’t harmonize with what the chassis is doing. On public roads that appeal to enthusiast you aren’t rewarded with a sense of confidence that a stock behaving car gives and the balance front-to-rear just doesn’t seem there.

On a short auto-x or safe road course it might be something to play around with if you’re trying to figure out how the car behaves without the electronics and to help determine what suspension changes may do to the chassis. I mentioned earlier in the thread but the ST really was developed as a complete package of suspension and chassis tuning and the electronics which when working together is pretty impressive. Without them the ST feels more like a regular, nose heavy front drive sporting car, more prone to push in corners but with less balance in just the chassis than the best of the breed with a fast but numb steering that doesn’t provide a ton of feedback and filters out pavement imperfections.

One last thing that I might get around to testing is to see if the ABS pump fuse is pulled (fuse F7) if the EBD is still active. Pulling the whole carrier module of the three fuses completely kills the system including broke force distribution but I suspect pulling just the pump fuse will disable the brake-based functions but the controller and HCU solenoid pack should still receive power and be able to default to a fixed brake bias vice being unpowered and a majority of brake force going to the rear. This would make it a little easier to explore the car’s behaviors without the need to disconnect wheel speed sensors.

Speaking of which, there is a small slip near the cable/wiring side of the sensor that needs depressed in order to disconnect the sensor. If you go off feel alone the main body of the sensor will flex and feels like it has a locking tab on the side of the body but you need to press this smaller clip near the end/top (also there’s no locking tab on the sensors that needs disengaged prior to pressing the removal clip). Even disconnected they stay in place fairly well and don’t look like they will get caught up in or bind up on suspension components as the vehicle is driving. If someone wanted to leave it disconnected long-term it might be good to relocate it to reduce vibration and stress on the internal wiring and cover the open end of the connector with a weatherproof tape.
Hi, I find removing F7 is perfect. Then you can turn off ESC manually as normal with the button. I did this on my Focus SE and ST. Just pulling F7 removes ABS and Traction Control but leaves the speedo and steering as normal.
Maybe you're already aware but thought I'd share. I love the results. I live where it snows and ABS is such a hindrance stopping on snow. Take care!
 
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