Disable ABS for rallyx.

Is there a clean way to disable ABS in newer (like 08+) cars?

Want to go to rallyx/snowx with this car some time over the next year, but given the over-sensitive ABS - want to be able to disable it. Preferably 'on the fly', so that I can use the switch when I'm driving in the deeper snow and want to stop faster, but I'll settle for a way that only requires a re-start to completely re-set everything.

So far I've tried replacing the 'abs/vdc' fuse under dash with a switch and its inconvenient.

The most annoying thing (apart from all the blinking lights it causes) - SI-drive gets stuck in 'S' mode, which is not what I'd like for rallyx.

Are there any options other than doing the same to the abs/vdc fuse under the hood (which supposedly does not trigger CEL, which is probably responsible for SI-drive issue)?

Are 07 models any different? Someone on nasioc with 07 N/A impreza claims he can switch ABS control unit on/off on the fly (but did not return my PM as of yet)

P.S. Creating this in the 'motorsports' forum, in hopes of staying on topic, and not turning into another worthless abs thread, like the numerous I came across while searching for this issue on a bunch of subaru forums.

UPDATE: temporary solution to have ABS=off and EBD=on is in post #23: pull the fuse that's on the relay block under dash.

Anyone?

I tried taking out Yaw/laternal G-sensor out (since FSM shows it as a possible cause of wheel locking during braking, which is what I want), but that only disabled VDC. Next on the list - taking the power wire off the steering angle sensor, which should also be capable of disabling ABS. And there's a little note in the FSM that gives me hope. It says "ABS/VDC operates normally when the power to the sensor comes back", so it might be possible to do an 'on the fly' abs switch.

Disabling the ABS also disables the EBD. Be ready for a a lot of push under braking.

Not according to the FSM. It has a list of things that could go wrong (and which I'm trying to induce) and a list of symptoms they cause. For some it lists "VDC/ABS/EBD won't work", for others just "VDC/ABS", and for some just "VDC".

In my 2005 Outback I just pulled the ABS fuse for the Rally X races. It also disabled the cruise control, but that was it.

Like I said - it forces SI-Drive in "S" and causes a CEL that does not go away after re-inserting the fuse. So I want smth a bit cleaner.

You won't stop faster without it.

Don't need comments from someone who has no idea what they are talking about, thank you. ABS increases stopping distance in deep snow/gravel/dirt. Plowing that stuff with locked-up wheels stops the car faster. Its a fact.

Even worse then

Look, I'm with you as far as people offering opinions that you didn't directly solicit, but in this case ... if this was as prevalent as you seem to suggest, I'd venture a guess someone would have already offered you a solution.

What's worse? A lot of people with older subies routinely disable ABS for rallyx/snowx, otherwise one would risk ending up in the part of the course not meant for driving or a hard snow bank. Its easy with older cars (with fixed proportioning valve), and hard with newer cars with lots of electronix in'em (EBD acting instead of proportioning valve).

Primary issue is whether its possible to make VDC computer to give-up doing the ABS thing, while still doing the EBD function. If its not possible - then yes, disabling the ABS+EBD is probably a bad idea even for rallyx, since (while decreasing braking distance in straight line) it will make the car extremely tough to control. If it is possible (and what gives me hope is the FSM language) - than its worth the effort for rallyx applications.

Like I said - I know what I want to try, I know why I want it and I know the downside. And the reason I created this in motorsports forum - is exactly to avoid comments like yours.

Anywho, back on topic.

I attach few pics from FSM. Two show that some issues disable everything including EBD (which makes sense since the error shows no signal from all wheel sensors whatsoever), and some don't (and might even work without reset is power comes back). And the third one shows the harness plug the steering angle sensor connects into. Somehow it seems to be in the middle of the firewall, and not somewhere under the steering column, where I'd expect it to be.

I guess I'll take out the stereo/nav and look if I don't see the connector in the steering column.

On a somewhat related topic, what type of tires are you using for the rally/snow crosses?

I have no experience with rally cross but I do know that when I went form all seasons in the snow to Ice/Snow General Altimax Arctic tires my car now stops w/o even using the ABS no matter how slick it is. FYI, they are studless.

What about unplugging the harness from the ABS valve unit?

Of course - you may have to connect a dummy load to the harness in order to "trick" the ABS unit into thinking that it has a working ABS valve unit.

I'm of course not sure that it will work.

On a somewhat related topic, what type of tires are you using for the rally/snow crosses?

 

I have no experience with rally cross but I do know that when I went form all seasons in the snow to Ice/Snow General Altimax Arctic tires my car now stops w/o even using the ABS no matter how slick it is. FYI, they are studless.

Sigh, the tires comment....

And FYI the Altimax Arctic is a Chinese/Korean (don't remember which) knock off of the older Gislaved NF4 tire design (if I remember right), except for the tire compound. So they are decent in deep snow/slush, but are only marginal relative to the real thing on the slick surface, where the compound actually matters. So I find it hard to believe in "my car now stops w/o even using the ABS no matter how slick it is".

What about unplugging the harness from the ABS valve unit?

 

Of course - you may have to connect a dummy load to the harness in order to "trick" the ABS unit into thinking that it has a working ABS valve unit.

 

I'm of course not sure that it will work.

Not sure its doable. VDC contol module and hydraulic part with valves etc is a single unit, or so it seems. And I think all the error codes for valve malfunctions seem to disable EBD as well.

Sigh, the tires comment....

 

And FYI the Altimax Arctic is a Chinese/Korean (don't remember which) knock off of the older Gislaved NF4 tire design (if I remember right), except for the tire compound. So they are decent in deep snow/slush, but are only marginal relative to the real thing on the slick surface, where the compound actually matters. So I find it hard to believe in "my car now stops w/o even using the ABS no matter how slick it is".

Wow! You ARE an absolute prick. I try to throw some helpful info out there and get blasted.

I'm glad that you have no experience with my tires and can judge how good they are by what you think that you recall. Me, having actually driven on them, do have experience with them in ice, snow, and dirt/mud conditions for 2 seasons.

^ its because you're turning my thread into typical ABS thread, where 99% percent of people write about either:

a) disabling ABS being a bad idea

b) suggesting changing tires or slowing down.

Although your comment is probably my fault, since I wanted to write smth about my tires in the first post but got lazy.

Not sure its doable. VDC contol module and hydraulic part with valves etc is a single unit, or so it seems. And I think all the error codes for valve malfunctions seem to disable EBD as well.

That's why I had the idea of replacing it with a "dummy" to trick the ABS control unit that it does it's work.

The other way around would be to fool the sensors into thinking that there never is a lockup. But that means that you will have to inject a signal into the system instead. Of course - a simple test would be to just replace the wheel sensors with a dummy load, but I suspect that the ABS unit also get a central speed indication from the gearbox.

That's why I had the idea of replacing it with a "dummy" to trick the ABS control unit that it does it's work.

I'm not sure I understand what you're suggesting. So I attach a schematic of the abs system (vdc is similar). The only thing I can do - is to cut power to valves, the rest is inside the abs unit.

The other way around would be to fool the sensors into thinking that there never is a lockup. But that means that you will have to inject a signal into the system instead. Of course - a simple test would be to just replace the wheel sensors with a dummy load, but I suspect that the ABS unit also get a central speed indication from the gearbox.

Yeah, I thought about it too.

I also thought that there's a central speed signal somewhere, but it seems (from digging through FSM) that ABS wheel sensors are the only source of vehicle speed for a MT legacy. ABS/VDC unit calculates it and transmits it to ECM/BIU via CAN/LAN interface (which scraps my next though to cut the communication between VDC and ECM, nothing good will come off it). When ABS works - it calculates some 'dummy' speed and engages/disengages valves in reference to this dummy speed. Here's a quote from FSM:

Depressing the brake pedal increases the brake fluid pressure in each wheel cylinder, which in turn

decreases the wheel speed (or increases the wheel deceleration rate). When the brake fluid pressure

is increased to a level of point “A” of the brake fluid pressure curve in the diagram (at which the

wheel deceleration rate exceeds threshold “−b0”), the ABSCM makes a pressure “hold” control.At

the same time, the ABSCM calculates a “dummy” vehicle speed which is a reference speed it uses

in the next stage of control.

When the wheel speed then drops below the slip ratio setting, i.e., a speed lower than the “dummy”

vehicle speed by the predetermined value (at point “B” of the pressure curve), the ABSCM makes

a control to prevent the wheel from locking, or a pressure “decrease” control.

So, with ABS sensors being the the only speed source its possible to mess with their inputs. But its fairly involved. ABS signals are alternating current, with rectangular waves, so to mess with those I'd have remember a lot of stuff I once knew, but completely forgot since.

2008FSM 3749.pdf

I'm not sure I understand what you're suggesting. So I attach a schematic of the abs system (vdc is similar). The only thing I can do - is to cut power to valves, the rest is inside the abs unit.

My idea was to create a fake ABS valve unit that you reconnect the harness to when you don't want ABS. This unit will look like the real deal to the ABS control unit but don't do anything at all with the brake pressure.

Yet another idea would be to have a bypass setup to circumvent the ABS unit using a 3-way valve for the hydraulic pressure.

Anyway - regardless of which setup you use it depends on how smart the ABS electronics is.

But have you tried to disconnect the ABS box completely instead of pulling the fuse? If it isn't present on the CAN bus at all it may not cause an error. There may also be test modes for the ABS unit that changes how it behaves.

My idea was to create a fake ABS valve unit that you reconnect the harness to when you don't want ABS. This unit will look like the real deal to the ABS control unit but don't do anything at all with the brake pressure.

I still don't follow. The only relevant ABS valve unit wire in the ABS harness is the valve relay power wire. The connection between ABS control unit and ABS valve unit is inside the ABS case, and not in the form of a harness, but I suppose some soldered circuitry. I did try to cut valve relay power (fuse under hood does that) - did not give me a CEL, but cuts the EBD off.

But have you tried to disconnect the ABS box completely instead of pulling the fuse? If it isn't present on the CAN bus at all it may not cause an error. There may also be test modes for the ABS unit that changes how it behaves.

Pulling the fuse (under dash) cuts off power to ABS control unit (which disables ABS+VDC+EBD). Generates CEL exactly because ABS CU is on the CAN bus.

I still don't follow. The only relevant ABS valve unit wire in the ABS harness is the valve relay power wire. The connection between ABS control unit and ABS valve unit is inside the ABS case, and not in the form of a harness, but I suppose some soldered circuitry. I did try to cut valve relay power (fuse under hood does that) - did not give me a CEL, but cuts the EBD off.

OK, so there is no connector on the valve unit at all then? My idea was to disconnect it and connect it to an electrical "dummy" that from the view of the ABS control unit looks like a valve unit, but isn't. That way the ABS control unit thinks it's really doing the right thing but it isn't. Of course - I suspect that it also kills VDC in that case.

Pulling the fuse (under dash) cuts off power to ABS control unit (which disables ABS+VDC+EBD). Generates CEL exactly because ABS CU is on the CAN bus.

OK, but does the ABS control unit lose all power or will it still be partially powered? It's still possible that it's partially powered and answers on the CAN bus through a secondary power feed so a complete disconnect of the box isn't the same as pulling the fuse.

I'm just suggesting various routes to take to circumvent the ABS. From what I understood the main problem was that you did lose S# when you tried to disable the ABS.

OK, so there is no connector on the valve unit at all then?

None at all. In the sense that there's no harness to disconnect.

OK, but does the ABS control unit lose all power or will it still be partially powered? It's still possible that it's partially powered and answers on the CAN bus through a secondary power feed so a complete disconnect of the box isn't the same as pulling the fuse.

Some parts are probably on (since the speedo works with the fuse off, and ABS sensors being the only source of that info). But that already gives CEL (related to communication error from ECM) and disables EBD, and I want less parts disabled, not more, so pulling the whole harness off ABS unit does not make much sense.

I'm just suggesting various routes to take to circumvent the ABS. From what I understood the main problem was that you did lose S# when you tried to disable the ABS.

The problem is two-fold: the loss of S# and the loss of EBD (represented by emergency brake light staying on after releasing the handbrake).

Anyhow, I just discovered that the lazy have to re-discover a lot of stuff that has already been done for them. The well known write-up on ABS on/off switch for earlier models has what I need, I just didn't pay close enough attention to what was done there and why.

There are a couple of fuses on the ABS/VDC system:

50A (SBF-1) motor fuse in engine compartment

30A (#1) valve relay fuse in engine compartment

7.5A (#33) VDC/ABS control unit power fuse under the dash

and another one:

7.5A fuse that's on the relay block also under the dash.

The first two fuses disable ABS motor or valves, so no way for EBD to work as well. The third disables the VDC/ABS brain, throws a CEL and kills everything including EBD. The last fuse controls only one thing: signal of the brake pedal sensor to the VDC/ABS brain and it seems that pulling it does what I need:

-ABS+VDC shut-off,

-'Brake' light goes off with parking brake release, so the chances are that EBD still works (need to find an empty lot and see which wheels lock first).

-No CEL

It's not 'on-the-fly' (requires re-start to re-enable VDC/ABS), and it disables cruise control, but I'll take it if it turns out that EBD indeed works.

I will still play with the steering angle sensor when the weather improves and maybe disconnect one of the rear speed sensors to see if I can get the 'on the fly' capability.

I suspect that the EBD also will get disabled. Assuming that the EBD you mean is the Electronic Brakeforce Distribution.

In that case what you need may be a way to manually tune it so you can make a setting that is suitable for the conditions you are driving in. But many cars have through time been working OK even without EBD.

Just pick your poison.

If you need to keep the EBD and disable the ABS I suspect that you will need to reflash the ABS module with different software or construct a completely new one, but that is probably a really tricky thing to do.

A good question is also how the brake balance is with the EBD disabled. Is the default setting useful or does it cause trouble?

I suspect that the EBD also will get disabled.

That should turn the 'brake' light on solid, which it did not (and the other two fuses did).

In that case what you need may be a way to manually tune it so you can make a setting that is suitable for the conditions you are driving in. But many cars have through time been working OK even without EBD.

Not sure how does one manually tune EBD in LGT. Its all electronic, using the Hydraulics of the ABS system to adjust the pressure front/rear.

A good question is also how the brake balance is with the EBD disabled. Is the default setting useful or does it cause trouble?

It is very rear-biased in Subies with electronic EBD. That's why I want EBD on. If its off the rears will lock-up first and I'll spin out too easily.