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This thread is specific to '98-00, but some of it is likely applicable to earlier years. Consider it brainstorming at this point ... contributions welcomed!
My '98 SC400 is the only one of my three SC's that came with traction control (TRAC), and boy do I hate the feature. Any moisture on the ground, and I can barely touch the go pedal without spinning the inner rear wheel, triggering TRAC which cuts throttle to limp mode levels, and hoping I don't get t-boned while being half in traffic and waiting for the power to kick back in. It's the most dangerous safety feature imaginable. Even dry pavement will often result in the same thing, and that's with 275's out back.
That left me wanting to permanently disable it, but I ran across this post that indicates there's actually an upside to TRAC that I haven't encountered:
TRAC in the SC is a really good system (especially for the time), actively taps brakes when it loses traction even at speed (ie. tested at 80 km/h on a frozen lake with patches of snow). You can turn it off easily if you ever wanted to as well. Snow mode pretty much desensitizes your accelerator, and shifts sooner.
(I have Snow mode, which is all the throttle limitation I care for in normal driving conditions.) So, I've been sort of getting in the habit of turning TRAC off whenever I start the car. That's annoying, though, as '98 moved the switch to a spot mostly hidden by the steering wheel and driver's left knee. At night, you can't see it at all. And it's an inch to the right of the trunk release, making it risky to reach for by feel while driving, if you forgot to turn it off initially.
BTW, '98+ has TRAC integrated into the fly-by-wire throttle body, rather than a separate TB like previous years that can be easily eliminated.
A bunch of SC-related searching turned up next to nothing, because, well, there's very little info on '98-00 models. Most of what's out there, especially electrical, is for LS and GS VVTi's, which may or may not have the same wiring. But, I did find several useful ideas from other car forums, such as:
Basically, the TRAC on/off switch is a latching switch that transfers a ground signal, so it can be defaulted to off by starting the car with the switch's output grounded. I pulled my switch out and saw that it is 2-wire (brown and yellow/red), which was encouraging. Brown is the common ground color, so y/r is presumably the signal wire. I then confirmed that jumping the two pins turns TRAC on/off, and connecting the y/r pin to ground prior to start-up results in TRAC being off. All good there! A simple toggle switch wired to the output signal would do the trick. But, it would be nice to come up with something that didn't require both the toggle and stock switch to re-enable TRAC (for highway driving) like the above methods...
Of interest is that the switch (part # 639-0K8) is 5-pin and the connector is 6-pin (with most slots empty) despite having only the ground and signal wires. That makes me think it might have more functionality buried in it, but I'm not sure how best to test that theory. Probably some combination of hooking various pins to ground and checking continuity on others. It would be nice if it could receive an added ground wire as a default (off) condition, while still being able to bounce between on and off with each press of the button.
I got to thinking, what if we could eliminate the worst of TRAC while not affecting the best of it, disabling it at low speeds and enabling it once above 20 mph or whatever? That led me down the rabbit hole of VSS systems... The '98+ A650e transmission has two VSS output signals, but according to GS threads, they're only for the ECU's A/T shift point control. The actual vehicle speed signal comes from the ABS sensors.
I believe our wheel sensors are passive, not active (see: this or this), but they could still be AC or DC in theory, with maximum voltages of 1.0 or 5.0, so I don't have a great way to test that or a clear picture what signal range I should even be looking for, let alone how to translate that into a good cutoff point for triggering TRAC. (And I haven't yet researched how best to do that last piece.) I know the '98 SC300's VSS signal is the blue/yellow wire to the ECU:
I would guess that's the same for the SC400, but I haven't pulled the carpet and ECU cover yet. I'm also curious where that signal comes from, as the ABS/TRAC/VSC computer (89541-24080, I believe) would seem to be a preferable source to tie in. That's probably behind the stereo, but it's been so long since I had that opened up, I don't remember for sure. I searched to no avail for what Pioneer Avic-D3 HU's do with the VSS signal, as far as determining speed from whatever voltage it reports. It must be a consistent range for them to be able to process it, right? Like, does 5v correspond to the vehicle's top speed or a "universal" top speed of 200 mph or something?
Does the SC's head unit have speed sensitive volume control? I don't think so, but can't remember for sure. That would be a convenient spot to tie into, if so.
Here's an LS400 thread showing that the ECU's GL1 and GL2 pins swing from 0-5V when slowing to a stop (other way around, presumably), but it has four connectors unlike the SC's three, so I'm unsure how closely that relates. Does anyone have a ECU pinouts diagram for '98 SC400? I haven't been able to locate that anywhere.
Here's some fun reading on various circuits and definitions that could be applicable to a non-VSS approach:
Reading a bit more on how VSS signals in general are interpreted, rather than searching for specific HU input stuff, has my head spinning. It would appear that measuring voltage isn't reliable, rather oscilloscopes should be used. And the voltage scale could be relative to just about anything, so I have no idea how HU's know how to interpret it.
I think I'll drop the hopeful VSS tie-in (unless people chime in) and focus on figuring out whether the TRAC switch's extra pins can handle additional functionality.
There are variations of the TRAC/Snow/Power switches across years and SC/non-SC models, and a 6-pin connector with one unused seems to be common. The other terminals are for illumination 12v/ground (which would be nice for the '98+ position down by the knee!) and Snow outputs. So, sadly, that likely rules out my hope that it could accept dual ground inputs, with one being the OFF default, and switch between modes with no external toggle.
After thinking about it some more, it looks like there ought to be a pretty easy solution. I re-checked the behavior to make sure I was visualizing things correctly...
Start with Y/R wire grounded: TRAC off.
Remove ground from signal wire: TRAC stays off.
Re-ground signal wire: TRAC on.
Remove ground from signal wire: TRAC stays on.
etc.
So, the stock setup acts like a momentary switch, changing state whenever the button is pressed, but not when it is released. As such, replacing the stock switch with a normally closed (NC) momentary switch should do the trick. It would be closed (off) at start-up, and would be cycled on/off with each button press. Unless I'm overlooking something, there's no need for a jumper bypass of the stock switch and inline toggle. It helps that the switch doesn't need to be held down for 3 seconds like on some of the systems I read about.
I don't have any momentary switches on hand to test the theory, so I'll order the nicest looking one I can find, and see if I can't come up with some sort of TRAC switch blanking plate to mount it in.
A few misc. items while awaiting some momentary push buttons I ordered:
I opened up the TRAC switch to see if it could be modified. Nope, not that I can see... It is spring loaded to press down on one contact; the housing is directional so can't flip it around to default to the other contact. (When the button is resting in the down position, its peg on the backside is in the middle of the housing -- neutral position for the brass rocker between the contacts -- pushed up rotates peg toward the bottom.)
I checked all 5 pins for continuity and only the 2 for BR and Y/R are connected to each other. The others are probably for illumination and Snow controls not used by that particular switch.
A nice thing I discovered, you can test the switch's operation with key ON. You don't need the car running to test start-up conditions.
I went shopping the other day and found a SC blanking plate and a couple Toyota/Lexus ones in grey that are identical, as well as a TRAC OFF button (0.40" hole; 10mm) that could be glued to one of the blanking plates for a decent looking label. The momentary switches I ordered are Eatons with a 0.407" threaded body, so fingers crossed that that works well without covering up the lettering which is quite close to the hole. I didn't want to bother with all the no-name switches all over Amazon. Sure, I can get 10 for $10 and swap them out regularly, but who wants to deal with routine failures?
I also verified that a regular toggle switch will work ... just have to flip it twice (off-on or on-off) to change state. That's what I have in place at the moment.
The new switches arrived this weekend and it all went together fairly nicely, albeit in possibly unfinished fashion.
I dug around in my stash of spare connectors and wiring, and found a weatherpack 2-pin that takes the same shape pins, so I could hook the stock B and Y/R wires into it, have a quick disconnect for the new switch, and be able to revert back to stock if desired. Of course, one of the wires turned out to have no continuity -- not discovered until it was all hooked up -- but I had a second one of that type that had a functional donor wire despite the weatherproofing holding the weather inside...
The Eaton 8411K10 switches are nice in that they take a full spade connector, none of that tiny-lugged stuff you see everywhere online that melts the internals if you try soldering wire straight to it.
A rocker switch would be nice, more like the stock one, but momentary rockers are not a common thing. I looked at some hinge lever micro limit switches, but they're basically just a push button switch with a lever over the top, so not really an improvement. I considered building a recessed panel for the momentary switch, so it wouldn't protrude much and be angled toward the line of sight above, but bonding the plastic sides and not recessing it too much to be fully pressable made me back burner that idea:
Here it is mounted:
It looks better with the front hex nut turned slightly, but that takes fiddling with the rear one which is perfectly locked into the backside grooves of the blanking plate and wasn't enough to bother me at the time:
I used the grey blanking plate instead of the color-matched tan one, in case I decide to revisit building a recessed mount. That would look best with the proper tan one. I also considered chopping the "TRAC OFF" portion off the top of that smaller black switch mount, and gluing it to the top of the blanking plate for more lettering clearance, but decided just tightening it down in place works well enough and allows me to reuse it in the future for a recessed mount. (I couldn't find any basic "TRAC OFF" decals.)
Here are the various pieces:
I'm very happy with the modification, being able to drive in wet weather now without worrying about gentle starts totally ruining the fun. In fact, it's raining again today (and all week), so I think I'll go take it for a "spin"... And that concludes this session of talking to myself!
09-26-23, 04:35 PM
'98+ TRAC override and improvements
