ABS VI no or intermittent rear brakes fix

[GnatGoSplat]

Here's one for the archives.

 

I've had ABS disabled on the 94 for awhile, because it would shut off the rear brakes and light up the ANTI-LOCK and BRAKE lights. For some reason the ABS power-on self-test was disconnecting the rear brake circuit and not re-connecting it. I noticed sometimes stepping hard on the brake while starting the car would cause the rear brakes to work. As a fudge-fix, I managed to make the rear brakes work all the time by yanking the ABS relay one of those times. That resulted in the ANTI-LOCK light staying lit, and some weeks later the BRAKE light decided it wanted to stay lit as well. The lights were very annoying and technically should fail state safety inspection.

 

I'd found this interesting post on Google. http://ls1tech.com/forums/suspension-brakes/1076739-no-rear-brakes.html#post14259357 I had the airbox out for a different annoying job, so figured while I had it out, I'd pull the bottom cover off the ABS VI unit to see if I might have the same problem. The bottom cover is just held on by 2 Torx T-25 screws, no gasket or anything, it comes off with no resistance. Front one is easy to remove with a 1/4" ratchet and T-25 bit. Rear is a bit harder to reach, managed to loosen it with the ratchet and bit, but had to spin it out by hand. Once I had the cover off, I could feel the nut on the center gear (rear circuit) was completely loose. The gear had dropped to where it wasn't touching the motor drive gear. I tightened up the nut best I could, re-installed the ABS relay, and both ANTI-LOCK and BRAKE lights went off! Somehow the rear screw on the cover is 10x more annoying to reinstall. Took it for a drive and everything seems fine.

[RobertISaar]

http://www.rockauto.com/catalog/moreinfo.php?pk=301677&cc=1056417

 

second image shows the locations of the screws.... i looked/felt for a little while and couldn't find them.

 

http://www.rockauto.com/catalog/moreinfo.php?pk=1416080&cc=1056417

 

first image shows gears....

 

http://www.rockauto.com/catalog/moreinfo.php?pk=301781&cc=1056417

 

and what they will mesh with.

[GnatGoSplat]

Yep, those are the screws in that 2nd pic. I was able to see the front one, it should be visible on the front edge. The back one I just had to feel around for. The back one I could feel 2 little pokey protrusions on the cover itself, you can kind of see them on either side of the screw in the pic. Once I felt those, I knew the screw was somewhere in-between them.

[Nicutlass_Supremacy]

Since I bought my cutlass I have had the ABS light on. I don´t have the brake light on but could this loose gears be the problem? I´ve also made and system check with and X-Ray diagnostic tool and it says everything its ok.

[GnatGoSplat]

ABS light could be on for many reasons. If your brakes still work properly, I'm not sure this would be the reason as the loose gears will cause certain circuits of the hydraulic brake system itself to be shut off. Now if your rear brakes sometimes don't work at all, then I would say it could definitely be a loose gear.

[Brenni40]

I'm a new member and I need help trying to figure out what is going on with my brakes. When both my ABS and Red Brake Light is lite on my dash, my brakes work. Now it acts like the power booster is not working because the brakes are super hard and you press fairly hard but the brakes work. Then I will shut my car off and then restart it and both lights are off on my dash and then the pedal goes to the floor and I have basically very little brakes. Is this the ABS pump causing this problem, working randomly? It just does not make sense, because wouldn't this work in reverse. I have thought about changing the Master and Power Booster but I really don't know if that is the problem because it happens randomly. Any help would be great! Thank you so MUCH!

[RobertISaar]

since I had everything off already for line replacement, I pulled my master cylinder off to get the bottom cover off and look at stuff.

 

none of my gears were loose, but I gave them a little bit extra torque to make sure they don't do so in the future. I do notice that if I turn the master over, I hear something jingle like a couple of washers going up and down a shaft. it's kind of odd and have no idea if it's related to why my ABS hasn't functioned since ~1997.

[RobertISaar]

after looking at it, what is jingling is the little bar that is used by the center cylinder to apply/release the rear channels equally, so I assume it is normal.

 

also played with it quite a bit, found out that if you want to manually move the pistons for whatever reason, here is the magic behind it: there are 6 pins on the modulator, they are paired with the pin next to it, making 3 pairs. applying positive voltage to the left pin(from the battery or some external power supply) and a ground to the right pin will move the piston up. positive on the right and ground on the left will move it down. I didn't time it, but the front wheel pistons seem to have a lot more travel to them and take a little over a second if you feed them 12 volts to hit their limits in either direction. the center pins control the rear wheel's piston and it probably took half of that time.

 

messing around with a power supply, if I powered them using 5V, they tended to travel at around 3.5 amps and when they hit their limits, current jumped up to 10.x amps. at 12 volts, I seem to remember seeing around 10 amps traveling and when they hit their limits, it trips my power supply's current limiter(around 20 amps), so significant current can be drawn. if a motor were held on continuously, it probably wouldn't take long to start overheating it and causing issues.

 

from what I'm reading of the system, if the rear wheel's piston isn't at the home position for whatever reason, fluid quits flowing to the rear wheels(and possibly pressure is kept in their lines?), so to bleed the rear wheels, the piston needs to be homed first. this is the least messy way of going about it unless you have a tech1/similarly capable scanner. from what I can see, the other way is using the angled bleeder valve that is on the modulator itself. knowing me, I would probably do both(rehoming first and then using the modulator's rear wheel piston bleeder) to be 100% assured of nothing being in the lines other than brake fluid.

 

if you plan on taking the motors off, you need to run the pistons down to prevent injury/damage.

 

http://www.motor.com/magazine/pdfs/062001_04.pdf

 

that was helpful in determining a few things about how all of this works.

 

http://www.j-body.org/forums/read.php?f=3&i=137251&t=137206

 

had to read that to get a good explanation of what the solenoids actually do.