Howdy,
Thought you all might be interested: found stuck pistons in both front and rear modulators on my new-to-me 1993 ST.
I dove into the modulator using the excellent tutuorial here: http://www.st-riders.net/index.php?topic=7784.0
This is what I found on both modulators:
The piston is stuck in the spool on the right, and the spring that brings the piston back to battery is completely collapsed.
I held the spool in a padded vise, then pried the piston out and made sure that it now moves smoothly. I advise wearing eye protection, as brake fluid ~will~ eject from the ports on the side of the spool.
Per the tutorial, there is an elaborate procedure to bleed the modulator. I did not split the top and bottom halves, but rather pulled the motor with it's eccentric bearing, then removed the cap over the pump piston.
I pushed the spool and piston out, freed it, then reinstalled it after reinstalling the motor assembly.
In the process nearly all of the DOT 4 fluid is lost from the modulator. I removed the flat plate secured with 2 4mm screws and refilled the modulator with fresh DOT 4, making sure that all of the bubbles come out.
I cycled the motor and solenoids to prime and purge the pump.
On the front modulator I didn't get all of the air out. This caused the bellows that surround the limit switch plunger to distend and depress the limit switch even when the solenoids were operated to purge the accumulator.
I re-removed the modulator, then removed the limit switch and manually pumped the limit switch plunger to remove the air. Upon refilling and reinstalling the modulator, it appears all is well. Because the front modulator is tipped forward, any air will tend to rise into the accumulator.
Once the body work is reinstalled, I'll give it a test ride and see how effective the repair is.
For anyone who's curious, here's the sequence as I tried to get the ABS self test to complete:
1. Code 1. Found ABS motor fuse blown. Discovered that it would only blow after bike is started and the power on self test is running. Does not blow the fuse during the ABS code dump.
2. Removed motor housing and manually turned the motor armature. Very stiff to rotate initially, then less so. Suspect that the piston was stuck in battery, the motor was stalled and blowing the fuse.
3. Code 1. Checked limit switch, found it not closing upon power on self test.
4. Pulled modulator off of the bike completely, discovered piston stuck in the collapsed state. Freed piston, refilled and bled modulator (or so I thought).
5. Code 2. Rear modulator fault. Rechecked motor fuse (still good). Suspect stuck piston on rear modulator.
6. Removed rear modulator with footpeg plate and rear master cylinder as an assembly, with the only open portion of the brake system the rear caliper hose.
7. Dumped rear reservoir (old, brown fluid) and pulled the motor and piston out of modulator. Found piston stuck, but not as collapsed as front modulator piston.
8. Freed piston, reinstalled motor assy, refilled and bled rear modulator.
9. Reinstalled modulator on RH footpeg plate, then reinstalled that assembly onto bike. Refilled reservoir, bled rear brake system.
10. Code 3. Consider abandoning project, swearing off motorcycles for good.
11. Chase troubleshooting in Factory manual for a bit, get nowhere.
12. Check front modulator, find the bellows for the limit switch distended and "squooshy".
13. Unbolt modulator, limit switch, and reservoir cover. Burp a surprising quantity of air from front modulator, then refill with DOT 4. The modulator must be tipped "vertical" with the reservoir plate at the top. I found that I had to remove the bolts clamping the brake hoses to gain enough slack. The air trapped in the accumulator was pushing the bellows out, holding the limit switch closed even when the solenoids cycled to dump the pressure in the accumulator. This causes the ECU to cycle the modulator twice, then set a code 3. Not taking the brake lines off of the modulator saved having to bleed the front brakes a second time.
14. Finally. Passes the Power On Self Test. Both the front and rear modulator thrum then chatter during the test. (as I suspect they should have when new)
Because the spool and piston are steel and not chrome plated or stainless, I suspect that simply sitting for any length of time may cause the piston to seize such that the battery spring cannot return the piston.
I am staggered to think about how many thousands of dollars have been spent changing out modulators that could have been revived. The brake fluid that lives in the modulator really doesn't have any exchange with the fluid from the master cylinder, and as such should probably be changed more often than once in 23 years.
Thought you all might be interested: found stuck pistons in both front and rear modulators on my new-to-me 1993 ST.
I dove into the modulator using the excellent tutuorial here: http://www.st-riders.net/index.php?topic=7784.0
This is what I found on both modulators:
The piston is stuck in the spool on the right, and the spring that brings the piston back to battery is completely collapsed.
I held the spool in a padded vise, then pried the piston out and made sure that it now moves smoothly. I advise wearing eye protection, as brake fluid ~will~ eject from the ports on the side of the spool.
Per the tutorial, there is an elaborate procedure to bleed the modulator. I did not split the top and bottom halves, but rather pulled the motor with it's eccentric bearing, then removed the cap over the pump piston.
I pushed the spool and piston out, freed it, then reinstalled it after reinstalling the motor assembly.
In the process nearly all of the DOT 4 fluid is lost from the modulator. I removed the flat plate secured with 2 4mm screws and refilled the modulator with fresh DOT 4, making sure that all of the bubbles come out.
I cycled the motor and solenoids to prime and purge the pump.
On the front modulator I didn't get all of the air out. This caused the bellows that surround the limit switch plunger to distend and depress the limit switch even when the solenoids were operated to purge the accumulator.
I re-removed the modulator, then removed the limit switch and manually pumped the limit switch plunger to remove the air. Upon refilling and reinstalling the modulator, it appears all is well. Because the front modulator is tipped forward, any air will tend to rise into the accumulator.
Once the body work is reinstalled, I'll give it a test ride and see how effective the repair is.
For anyone who's curious, here's the sequence as I tried to get the ABS self test to complete:
1. Code 1. Found ABS motor fuse blown. Discovered that it would only blow after bike is started and the power on self test is running. Does not blow the fuse during the ABS code dump.
2. Removed motor housing and manually turned the motor armature. Very stiff to rotate initially, then less so. Suspect that the piston was stuck in battery, the motor was stalled and blowing the fuse.
3. Code 1. Checked limit switch, found it not closing upon power on self test.
4. Pulled modulator off of the bike completely, discovered piston stuck in the collapsed state. Freed piston, refilled and bled modulator (or so I thought).
5. Code 2. Rear modulator fault. Rechecked motor fuse (still good). Suspect stuck piston on rear modulator.
6. Removed rear modulator with footpeg plate and rear master cylinder as an assembly, with the only open portion of the brake system the rear caliper hose.
7. Dumped rear reservoir (old, brown fluid) and pulled the motor and piston out of modulator. Found piston stuck, but not as collapsed as front modulator piston.
8. Freed piston, reinstalled motor assy, refilled and bled rear modulator.
9. Reinstalled modulator on RH footpeg plate, then reinstalled that assembly onto bike. Refilled reservoir, bled rear brake system.
10. Code 3. Consider abandoning project, swearing off motorcycles for good.
11. Chase troubleshooting in Factory manual for a bit, get nowhere.
12. Check front modulator, find the bellows for the limit switch distended and "squooshy".
13. Unbolt modulator, limit switch, and reservoir cover. Burp a surprising quantity of air from front modulator, then refill with DOT 4. The modulator must be tipped "vertical" with the reservoir plate at the top. I found that I had to remove the bolts clamping the brake hoses to gain enough slack. The air trapped in the accumulator was pushing the bellows out, holding the limit switch closed even when the solenoids cycled to dump the pressure in the accumulator. This causes the ECU to cycle the modulator twice, then set a code 3. Not taking the brake lines off of the modulator saved having to bleed the front brakes a second time.
14. Finally. Passes the Power On Self Test. Both the front and rear modulator thrum then chatter during the test. (as I suspect they should have when new)
Because the spool and piston are steel and not chrome plated or stainless, I suspect that simply sitting for any length of time may cause the piston to seize such that the battery spring cannot return the piston.
I am staggered to think about how many thousands of dollars have been spent changing out modulators that could have been revived. The brake fluid that lives in the modulator really doesn't have any exchange with the fluid from the master cylinder, and as such should probably be changed more often than once in 23 years.
