ATTN: People who have 5-speed conversions. ABS trouble?

[vrg3]

I wasn't sure whether to put this in the Wheels/Tires/Brakes forum or here, but I figure my target audience is here.

People who have swapped 5MTs in for their 4EATs (DLC, aludeman, anyone else), do you ever have problems with the ABS system? Like, it activating when it shouldn't, or the ABS light coming on in the instrument cluster?

5-speeds came from the factory with a simple G-sensor under the hood while automatics didn't. The only bit of documentation I've been able to find about why is a single sentence in an End Wrench article that says:

The G-sensor is a two stage mercury switch which detects the rate of deceleration of 4WD MT equipped Legacy vehicles. It is required due to the small wheel speed differential caused by the FT4WD system.

Doesn't seem to make sense to me, since I'd expect all the wheels to go the same speed anyway, but... if it's true, then it would imply that putting a 5MT in a car originally meant for a 4EAT (without modifying the ABS system) would interfere with ABS operation.

[THAWA]

does the abs system from the 4eat use input from the transmission or tcu? If so, that could be one of the reasons only 5mt have that sensor. Of course I'm assuming 5mt's don't have tcu's, is this correct?

[vrg3]

does the abs system from the 4eat use input from the transmission or tcu? If so, that could be one of the reasons only 5mt have that sensor.

See, that's what I thought too at first, but...

I have spent more time looking over the ABS wiring diagrams than the people who designed the system probably did, and from what I can tell, the only connection between the ABS control unit and the TCU is a signal from the ABS unit to the TCU; it's connected to the switched side of the ABS motor relay coil. Basically, it tells the TCU when ABS braking happens. Apparently the TCU responds by locking into 3rd gear and disengaging the MPT cluch.

Aside from the one wire going to the G-sensor on 5MT vehicles and the tap off the motor relay coil going to the TCU on 4EAT vehicles, the wiring appears to be the same for both types.

Of course I'm assuming 5mt's don't have tcu's, is this correct?

Yes.

[THAWA]

does locking into 3rd gear and disengaging the mpt do anything to the physics of the car?

[vrg3]

My guess is it's just to prevent the drivetrain from having any unpredictable effects on the car's stability. 3rd gear means the engine's torque isn't multiplied too much. Locking it in gear means it won't shift while the tires are struggling to grip, and disengaging the MPT clutch lets the front and rear axles turn completely independently.

[DLC]

I seem to remember my ABS light coming on once...but not since. For some reason my BRAKE light is on, i think i cracked a wheel speed sensor a bit ago and it might be causing problems, but it's more likely the very cold weather and a sticking e-brake sensor.

ABS works, can't say it's any better or worse than it was with the 4EAT, but it is definitely working.

[scottzg]

could it have to do with the mt having the clutch type differential and the auto the viscous? i would think that if one wheel locked in a skid, and the others didnt, you would have some torque bind, and that would rip the other wheels loose. I have an incomplete thought here, vrg, finish it.

[DLC]

I completely see what you're saying, you can't stop one wheel when the others are pulling it along.

I usually try to brake on the slick stuff in neutral, and i can remember a feature Volvo added to it's AT-equipped car that would disconnect the engine from the transmission on braking, nifty.

[vrg3]

I seem to remember my ABS light coming on once...but not since.

Did you by any chance pull the code?

For some reason my BRAKE light is on, i think i cracked a wheel speed sensor a bit ago and it might be causing problems, but it's more likely the very cold weather and a sticking e-brake sensor.

Our cars' brake fluid level sensors seem to act up in the cold. Before I replaced my master cylinder, I got the BRAKE light whenever it was below like 20 degrees. With my new cylinder (which includes the sensor), it comes on often when it's below 0 degrees. This is with the fluid filled to the upper line.

The cracked sensor could cause ABS trouble, but wouldn't turn the BRAKE light on; the BRAKE light comes on only if the fluid level sensor says fluid's low or if the handbrake is on.

could it have to do with the mt having the clutch type differential and the auto the viscous?

Well, yeah, that's what Subaru implies in that End Wrench article, so I guess so...

i would think that if one wheel locked in a skid, and the others didnt, you would have some torque bind, and that would rip the other wheels loose. I have an incomplete thought here, vrg, finish it.

Yeah, I see what you're saying. Whether accelerating or decelerating, the viscous coupling will always try to limit speed differences between the front and rear axles. This is avoided on the automatic by disengaging the MPT clutch.

But I don't see why that would affect how the ABS computer would operate. And why a sensor that tells it that the car is decelerating strongly is the solution.

And I don't understand what this "small wheel speed differential caused by the FT4WD system" the article speaks of is.

I usually try to brake on the slick stuff in neutral, and i can remember a feature Volvo added to it's AT-equipped car that would disconnect the engine from the transmission on braking, nifty.

Yeah, I usually declutch when braking hard, which is similar. But the viscous coupling is always acting, no matter what, since it's part of the center differential.

It seems Subaru could have very easily done what Volvo did, since the TCU can just as easily switch to Neutral as it can to 3rd... I wonder what their reasoning was, since Neutral seems to make more sense to me (and, clearly, to Volvo).

[evolutionmovement]

The only speed differential I can think of would be a slight variance in wheel diameter as the more heavily loaded tires would be more compressed and have a smaller effective diameter. I can't imagine something this insignificant as being an issue wrth even accounting for, but what do I know?

As for neutral vs. 3rd gear under breaking, keeping power to the wheels would probably be a better solution for stability purposes, although even unpowered differentials linking the wheels would have a stabilizing effect. I have an intersting article here from Racecar Engineering that discusses how the former Benetton team and Williams were experimenting on having front unpowered differentials after observing Audi's fwd A4 touring cars after they had been forced by regs to drop the Quattro system. They ended up re-fitting the rear differential and axles for its stabilizing effects on the rear wheels (helped prevent wheel lock up). The FIA later banned this idea on the touring cars, and later in F1. I hope this makes sense, but its 2:30 in the morning and I'm taking a break from writing (my book anyway ).

I personally always like to have power to the wheels for the same reason, though when I convert to AWD I'll have to relearn handbrake turning as I presently don't need to clutch in.

Steve

[scottzg]

dont handbrake turn with an awd car- torque bind.

i would think that with the viscous setup, the abs would have to react faster and be more sensitive so it could start abs-ing before the torque bind had ripped all four wheels from the pavement. with that sensor, it could predict better. even when you have the clutch in, all the wheels are connected, right?

[vrg3]

Hmm, yes, I can imagine that load-compressing-tire thing, but that ought to happen regardless of the drivetrain...

About the other stuff -- You're talking about limited-slip differentials, right?

The way I see it, it would help prevent just one wheel from locking up because that's what it does... But that would mean the 5MT setup would aid ABS.

The racecars you're talking about weren't allowed to have ABS, right? So they were trying to use LSDs as a very simple mechanical ABS.

Keep in mind that declutching won't make e-brake turns safe on a 5MT Subaru; the center limited-slip is still acting. It'd be kind of cool for 4EAT people to set up a relay to force Duty Solenoid C on (which disengages the MPT clutch) when the handbrake is pulled. That could be as easy as letting the "BRAKE" light short the contacts in the FWD fuse holder.

[vrg3]

The above reply was to Steve. Scott posted while I was writing. :)

Scott - Hmm, I guess you're right. The limited-slip effect works both ways. It could be that if one axle locks up, the viscous coupling tries to make the other side lock up, so the ABS computer has to anticipate it.

Huh, I think you might be right! If the ABS control unit sees all four wheels decelerate very rapidly, it could mean one of two things: the car is finally stopping, or one axle locked up and the center LSD is making the other side lock up. The G-sensor would allow it to distinguish between these two, since the former will read as a deceleration but the latter won't.

The mystery is potentially solved...

Though apparently it's not that big a deal, since Dave's ABS is still functioning.

[evolutionmovement]

I can't speak for touring cars, but yes, ABS at the time was outlawed in F1 and the reason for banning such a system was that it violated the no-ABS rule. As for what the F1 teams used - they wouldn't divulge although I'm sure it was an LSD-type diff.

I remember reading that the USDM WRX had an oversensitive ABS system so I think that points more to Scott being right.

So would handbrake turns damage the drivetrain in a 5MT? Keep in mind this is a technique I only employ in low friction situations like dirt or snow. I usually only use it for fun and employ left-foot braking for serious driving, but I don't want to damage anything just being stupid.

I still think Subaru's idea of keeping power to the wheels in the automatic under braking would be more beneficial than neutral as engine braking could also add a greater element of deceleration and control than just drivetrain friction. Plus I trust Subaru's ideas for AWD systems over Volvo's.

Steve

[scottzg]

I can't speak for touring cars, but yes, ABS at the time was outlawed in F1 and the reason for banning such a system was that it violated the no-ABS rule.

ya dont say!

i knew what i meant and why and whatnot almost immediately, but i couldnt express it.

[vrg3]

So would handbrake turns damage the drivetrain in a 5MT? Keep in mind this is a technique I only employ in low friction situations like dirt or snow. I usually only use it for fun and employ left-foot braking for serious driving, but I don't want to damage anything just being stupid.

Well, think about it -- any time you try to brake the rear wheels but not the fronts, the viscous coupling in the center differential will try to fight you. If the braking is stronger than the viscous goo, you shear the goo. Even if it's not, you slowly weaken the goo anyway.

I still think Subaru's idea of keeping power to the wheels in the automatic under braking would be more beneficial than neutral as engine braking could also add a greater element of deceleration and control than just drivetrain friction.

But they're disengaging the MPT clutch, so that engine braking would happen only to the front wheels. Or maybe that's good.

[evolutionmovement]

Now that I'm more awake, I see that the front wheels are the only loaded ones... I guess it would still serve a little purpose, but I would think having all four would be more beneficial. I guess the system was set up in the interest of the center diff rather than performance.

What would heavy left foot braking do? Would the clutch still disengage or would it try to distribute the power? Would the ABS activate at all?

Steve

[vrg3]

I guess the system was set up in the interest of the center diff rather than performance.

I dunno... The MPT clutch is pretty strong; strong enough to overcome the traction of the wheels usually...

What would heavy left foot braking do? Would the clutch still disengage or would it try to distribute the power? Would the ABS activate at all?

Wait, we're still talking about 5MTs right? There is no clutch in the 5MT system (except for the one you control with your foot of course). I haven't tried it myself, but I'd think that left-foot braking wouldn't have too much of an effect, or maybe even the opposite of the effect you want.

On a FWD car, left-foot braking has the effect of slowing the rear wheels, right? Since the front brakes are overpowered by the engine's torque? Well, in a 5MT AWD Subaru, all the wheels get the engine's torque, so... the front wheels get half the torque fighting big brakes, while the rears get half the torque fighting small brakes. So the motor may be able to overpower the rear brakes but not the fronts...

At the same time, the center LSD will be trying to make both the front and rear go the same speed, so if you succeed in causing a speed differential, it'll be unhappy.

[scottzg]

would it be possible to mount those mini drum brakes on the front, and then Y the brake cable into your handbrake? I know its more trouble than its worth, but would it be possible? Also, didnt the GLs and them have the Ebrake on the front wheels?

[evolutionmovement]

vrg3 - I was actually talking about the auto in this case (for once trying to stick to the original post ).

I use left foot braking more for the purpose of balance and weight transfer than power distribution - its like dancing the car. I can't dance for shit, but my ex was a dancer and she coined the term when I was trying to describe it to her. It's great when controlling the car high speeds - almost like how a rudder contorls yaw on an aircraft, while steering input would be like ailerons...maybe. Power distribution would be more the hand brake technique, which I understand as a problem with AWD.

Scott - yes the EA81 equipped GLs (all models until '84, hatchbacks and BRATs until '87?) had the hand brake to the front calipers.

[vrg3]

Scott - Small brakes in the front aren't good. :)

evolutionmovement - Hmm... What is the functional difference between left-foot-braking and hand-braking? All I can think of is that the foot brake is easier to modulate than the hand brake...

[THAWA]

foot controls all 4 brakes also

[vrg3]

But in a FWD car, the engine torque cancels out much/most/all of the front brake torque.

[THAWA]

kinda helps if I read and then absorb the information previously posted huh? I'm sorry.

[evolutionmovement]

You're still thinking in terms of engine torque when the real benefit seems to be weight change. When the car's floating at over 120 it's more accurate steering with gas and brake modulation than steering input. I had some idiot road rager try to run me off the road a couple times last week (what a blast!) and used this very effectively against him - even though he had the power advantage, I was able to maneuver like rabbit and even stay ahead of him. Dumb ass was lucky I'm in much better shape mentally now than a few years back, I could've spun him into a tree and killed him so easily with only a cracked headlight on my part, but I digress...

Tight roads at lower speed also benefit and in slippery conditions, you can rotate the back easier. The other thing is smooth transitioning of weight at high speeds as just throwing on the brakes at 120+ on undulating roads could send you off the road with the sudden weight shift. So you ease the brakes on with the gas until the front gets the weight transfer and you can back off the gas. Also takes any drivetrain sloppiness out of the equation.

I would guess this technique would be unnecessary in an all out race car on asphalt as there's little to no slack to be accounted for.

I'm not exactly sure on the physics myself (I'm not an engineer), but I know how much faster safer I can drive than when not using it.

Steve