Mk3 ABS Manual Trigger (for bleeding brakes)

traingeek2000

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I've had horrible brakes and pedal feel for all of the 2 years I've owned my '91 Turbo. The first time I drove it, I had the calipers drag and eventually lock up on me. A master cylinder and 4 refurb calipers later, pedal feel still sucks and it's never felt like the brakes could be bled fully. Seemed as if there was still air in the brake circuit, and the only place left was the ABS block. ABS has never worked on my car due to lack of a driveshaft speed sensor, and even with one it wouldn't work since my rear gear has been swapped.

I came across THIS thread on Supraforums about bleeding woes on an ABS Mk3 Supra. User adxx was able to trigger his ABS manually on his Z20 Soarer by jumping the motor and solenoids. I followed his wiring, cross-referenced with the TEWD, and rigged up a breadboard circuit to give it a shot. ABS motor definitely fired and my brakes already feel better even before a bleed. I'll have more definitive results after I have the brakes properly bled. Until then, here's how I set things up.

ABS bleed img 1.png ABS jumper circuit.png

This problem has been plaguing me for over 2 years now; I had to do a write-up on it. I used a breadboard, jumper wires, and a pushbutton to set up my circuit; you can see it HERE. Not very pretty. I was able to pump up some brake pressure from the driver's seat, then reach over and use the pushbutton to trigger the ABS pump and solenoids. The pedal sank about 1/2" for ~6 short bursts of firing the ABS, after which the pedal remained relatively stationary even while firing ABS. Already the pedal felt much firmer despite having not done a proper bleed yet. I was excited to get on the road and test the brakes.

This is where I ran into a problem. THIS CIRCUIT DRAWS A TON OF CURRENT. My battery was drained even after only a dozen or so bursts of firing the ABS. It may be beneficial to use a separate power source to run this circuit, or to use it while the engine is running. I did jump the car to test the brakes on the road, as well as see if there was any further air to be drawn out of the ABS block, and firing the circuit with the car on caused my dash lights and headlights to dim. Lotta draw.

I'm very impressed and very excited that even activating ABS improved my brake feel by this much, despite having not bled my brakes yet. Goes to show how horrible my brakes are right now. Eegh. I will update once I have properly bled the brakes, and hopefully I will have the best possibly operating stock brake system before immediately swapping my calipers for the Merc/Evo Brembo calipers sitting on my shelf. Hopefully this guide proves useful to anyone with ABS that doesn't work and can't bleed their brakes properly, or anyone who's feeling that their brakes aren't up to par.
 
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debrucer

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I've had horrible brakes and pedal feel for all of the 2 years I've owned my '91 Turbo. The first time I drove it, I had the calipers drag and eventually lock up on me. A master cylinder and 4 refurb calipers later, pedal feel still sucks and it's never felt like the brakes could be bled fully. Seemed as if there was still air in the brake circuit, and the only place left was the ABS block. ABS has never worked on my car due to lack of a driveshaft speed sensor, and even with one it wouldn't work since my rear gear has been swapped.

I came across THIS thread on Supraforums about bleeding woes on an ABS Mk3 Supra. User adxx was able to trigger his ABS manually on his Z20 Soarer by jumping the motor and solenoids. I followed his wiring, cross-referenced with the TEWD, and rigged up a breadboard circuit to give it a shot. ABS motor definitely fired and my brakes already feel better even before a bleed. I'll have more definitive results after I have the brakes properly bled. Until then, here's how I set things up.

View attachment 85159 View attachment 85160

This problem has been plaguing me for over 2 years now; I had to do a write-up on it. I used a breadboard, jumper wires, and a pushbutton to set up my circuit; you can see it HERE. Not very pretty. I was able to pump up some brake pressure from the driver's seat, then reach over and use the pushbutton to trigger the ABS pump and solenoids. The pedal sank about 1/2" for ~6 short bursts of firing the ABS, after which the pedal remained relatively stationary even while firing ABS. Already the pedal felt much firmer despite having not done a proper bleed yet. I was excited to get on the road and test the brakes.

This is where I ran into a problem. THIS CIRCUIT DRAWS A TON OF CURRENT. My battery was drained even after only a dozen or so bursts of firing the ABS. It may be beneficial to use a separate power source to run this circuit, or to use it while the engine is running. I did jump the car to test the brakes on the road, as well as see if there was any further air to be drawn out of the ABS block, and firing the circuit with the car on caused my dash lights and headlights to dim. Lotta draw.

I'm very impressed and very excited that even activating ABS improved my brake feel by this much, despite having not bled my brakes yet. Goes to show how horrible my brakes are right now. Eegh. I will update once I have properly bled the brakes, and hopefully I will have the best possibly operating stock brake system before immediately swapping my calipers for the Merc/Evo Brembo calipers sitting on my shelf. Hopefully this guide proves useful to anyone with ABS that doesn't work and can't bleed their brakes properly, or anyone who's feeling that their brakes aren't up to par.
Good job. Nice write up. Thanks!
 
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mobes

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Great job. I picked up some pigtails from a junkyard Supra a couple years ago to build a patch harness to do just this - but I haven't gotten to it yet.
 
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traingeek2000

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UPDATE!

Today I went to the mechanic and had them vacuum flush my system. HUGE air bubble came out of the left rear caliper, using the LR-RR-LF-RF bleed method for ABS lines. I can't definitively say whether or not my ABS trigger freed said air bubble, but given that the car was flushed multiple times and still had poor pedal feel in the past, I am choosing to believe that made the difference.

I can, however, definitively say that my dinky little breadboard pushbutton is not rated for the amount of current drawn by the ABS system. Little wisps of smoke coming out of the poor guy when I fired the circuit today. But it's a cheap button and it still did what I needed it to, so... eh. Still, it may be worth wiring the jumper straight from the positive battery terminal and to a body ground, rather than using the pins in the ABS harness.

My brakes still aren't perfect. It still feels like stopping force only begins about an inch and a half down the pedal travel, even though it's much firmer now. The pushrod coming from the master cylinder is within the specified clearance of the brake pedal arm, so perhaps my master cylinder is what I should focus on now. Still, brakes are much better and much more confidence inspiring. Once I get the master sorted I'll go ahead with my Brembo swap.
 
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Z06gette

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At the shop, sometimes on older cars the proportioning valve will stick. Usually when this happens we either do an abs brake bleed if the application allows via the abs module. OR in some instances tapping on the actually valve when bleeding free's it up.
Nice work
 

Piratetip

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Its a good idea.

There is a factory service tool that is designed to do exactly what you have done.
It manually activates the ABS pump.

I almost bought one a couple years ago when one popped up for sale, but figured I would never use it more than a couple times in my lifetime... :D
PN: 09990-00150
Here is a copy of the brake bleeding procedure for reference.
GE and GTE, Brake ABS & Trac Systems.
TSB for the Application Sheets
s-l500.jpg
 

traingeek2000

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Its a good idea.

There is a factory service tool that is designed to do exactly what you have done.
It manually activates the ABS pump.

I almost bought one a couple years ago when one popped up for sale, but figured I would never use it more than a couple times in my lifetime... :D
PN: 09990-00150
Here is a copy of the brake bleeding procedure for reference.
GE and GTE, Brake ABS & Trac Systems.
TSB for the Application Sheets
View attachment 85171
I read about this SST on, I think it was a Tacoma forum? At the very least it was a similar tool for Tacoma ABS. But knowing that there was actually a tool designed to do this and it's not complete backyard hackery that could severely damage ABS components pushed me to follow through with my solution. Thanks for the info, always appreciated! Cool to see what the tool actually looks like.
 

hvyman

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Get a pressure bleeder and you won’t have that problem. It’s less messy if you use it dry as well.