ABS 1 - Modulator Assembly Tutorial ( ST1100 ) - Part #5
Comments by
Norm Keller, STOC #8030:
In an effort to improve general ABS knowledge I will attempt to add individual observations as they occur to me and as energy allows.
Here is a side view of the ABS module with the white plastic limit switch cover and limit switch removed:

Note the ABS pump motor housing on the left. The spade lug electrical connector leading from the motor can be seen to the extreme left.
The motor (field) housing can be removed without removing the armature and pump drive assembly. This can be useful for hand turning the pump to check operation.
A problem in removing the motor field housing is that the flanges on the housing interlock with the reservoir housing. In order to allow the housing flanges to clear, remove the pump Torx screws (T27 drive bit) and pry the drive outward a short distance which will allow the pump motor to be angled to the side sufficiently to allow the field housing to clear the reservoir housing.
This technique will allow the motor internals, brushes, etc. to be inspected without opening the hydraulic portion of the modulator.
Note: I recommend adding a couple of drops of motor oil to the Oilite pump motor bushings to replace the lubricant lost to evaporation over the years.
The electrical connector and harness on the right side are the three leads which connect to the solenoid valve assembly. The solenoid valve assembly is the steel cadmium plated (slightly gold colored) cylinder immediately behind the black rubber connector boot.
The solenoid valves are not separate as shown in the diagrams but rather are included, back to back, in the solenoid valve assembly.
The mounting bracket which extends upward from the top of the photo is to the left of the reservoir/back-up spring housing. The reservoir cavity can be seen in front, immediately behind the hex bolt securing the mounting bracket.
Note the two threaded holes on the ends of the flange surface which secure the reservoir bellows and reservoir cap.
Behind the reservoir and higher, is the top of the back-up spring cavity where the limit switch bellows can be seen as a black donut with aluminium centre.
The limit switch stud (plate or what-ever) in the centre of the bellows is pushed outwards by the end of the back-up piston in order to close the limit switch to signal the ECU to shut down or start the pump motor.
The movement or not of the bellows stud serves as an indicator of the back-up piston positon when diagnosing the modulator operation. The bellows can also be "pumped" to move air bubbles from the spring housing into the reservoir during refill & bleeding.
The mounting platform for the limit switch is out of sight behind the mounting bracket but the limit switch cap's retaining groove can be seen around the end of the limit switch area.
During assembly and bleeding, the modulator is mounted in the orientation shown and fluid poured into the reservoir which will flow into the spring cavity with some help by pumping the limit switch diaphragm.
Fluid will also drain downward and drain out of the pump spool valve cavity unless the spool valve cap is in place.
It took some experimentation to derive the correct order for bleeding.
When operating the pump without cycling the inlet or outlet valves, the pump will fill the back-up piston cavity, compressing the spring and extending the limit switch stud.
Cycling the outlet valve will vent the back-up piston volume into the reservoir and mainly around into the spring cavity as the spring forces the back-up piston to battery.
When viewing the open, three-wire plug from the solenoid valve harness, the common connection for the two solenoids is the "corner" one which connects to the white/orange wire. Or at least the colours so appear to my aged eyes.
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Of course it wasn't until the modulator was packed for shipment that I remembered the other thing....
Mike's modulator is a front one so I wanted to compare it to a rear one to see how difficult it might be to substitute one for the other.
I'm not prepared to pull STella's modulator.....hmmm, that might be mis-understood in some company. So will have to settle for comparing with the modulator in place.
The top mounting brackets are different in shape but the bracket could easily be transfered from one to another. Fabricating a top bracket for either configuration would be easily done by anyone with access to a bench vise and basic tools. Top bracket = not an issue.
Lower brackets, ditto, move or make. Lower bracket = no problem.
Upper hose banjo mounting (to caliper) appears to be in an identical location on both modulators. Upper hose connection = no problem.
Lower hose mounting (from master cylinder) is part of the bolt-on lower section which means that these two parts can be exchanged between front & rear modulators.
Solution #2 for master cylinder hose would be to substitute another hose such as the front master cylinder hose, front caliper hose or match a hose from another source. Not a big problem.
Solution #3 to correct the orientation of the master cylinder hose connector ......no option but to pull the one from Mike's and....YES!!!!!!
It fits in the opposite direction!!!! The lower housing is simply assembled at 180 degree orientation, same part.
Lower hose = no problem
Pump electrical connector = same.
Pump ground connection (earth) goes to the same bolt = no problem.
Solenoid valve wire connector: Here there's a problem. The harness plug which connects between the modulator and the bike's wiring harness is different from front to rear units.
Solutions:
1) Cut wires and splice to harness. Do-able.
2) Disconnect pins from the old unit's plug and swap plug bodies= Do-able.
3) Simply unplug the harness from the solenoid pack and plug in the harness from the old unit = doable.
4) If both harnesses are not available (unlikely) then I could fabricate a plug from some terminals, heat shrink and epoxy/silicone. Do-able.
Conclusion: The front & rear modulators can be substituted with less effort than the remove & re-install.
On with the project. I have some photos of the units side-by-side but they really aren't of much value because they are pictures of twins. If anyone wants to see them. post & I'll stick them up here.
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Parts which can be removed without opening the fluid cavities to air are:
1) The white plastic limit switch cap.
2) The limit switch itself.
3) The pump motor's field housing. The two Phillips screws secure the housing which is a permanent magnet unit. The two T27 bolts which mount the pump drive assembly into the modulator housing must be backed off and the drive assembly pried back sufficiently to cock the motor and drive to the side, away from the modulator housing. The motor field housing (the steel cadmium plated/gold colored) has a pair of flanges which catch onto the modulator housing unless the pump drive is backed out to allow the pump motor to clear.
See the motor at left in this photo:

The motor drive can be seen as the aluminum saddle with one bolt partially engaged. It is necessary to pry the pump drive upward enough to allow it to cock to the side but not enough to pass the "O" ring seal or air may enter the modulator. This is not a difficult task because there is a fair amount of engagement. The solenoid pack's harness plug to which I will refer later is on the right side. The solenoid valve pack can be partially seen as gold colored.

4) The harness which plugs directly into the solenoid valve pack and leads to the bike's harness plug. This is the harness which differs from front to rear modulators.
The flat connector plug (white) can be seen plugged into the black insulator of the solenoid. It can be seen that the solenoid valves face in opposite directions.

5) The mounting brackets can be swapped between the two modulators without opening the fluid cavities.
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I forgot to respond to the question regarding a seized pump. In discussing this issue with Mike Fisher, it seems most likely that a failure due to contamination of the brake fluid within the modulator will result in a seizure of the pump piston within the pump spool valve.
The diagrams in the manuals do not represent the pump components in a meaningful way as they do not resemble in size, shape, or location, the real components. I mention this to avoid someone being confused by trying to use the diagrams as a guide for understanding the operation of the modulator.
The pump is a mushroom shaped steel piston which strokes in the steel body of the spool valve. The piston is returned to battery (the outward end of the stroke) by the small coil spring which is located around the piston and tensions between the head of the "mushroom" piston and the body of the spool valve. The piston is forced inward in its stroke by the action of the eccentric single row ball bearing mounted to the pump drive shaft.
Any significant increase in friction affecting movement of the piston will likely exceed the ability of the pump piston's coil spring to return the piston to battery. In this event the pump will cease operation. The pump has a positive means of moving the piston inward which is the pumping action which forces the fluid under high pressure.
A seizure which prevented the piston from moving inward in the stroke would either stall the pump motor or break something. This eventuality is prevented by two effects:
1) Long before the friction reached a critical level for the pumping stroke, the effort of returning the piston would be beyond the ability of the spring to return the piston for another stroke.
2) The pump's spool valve is located toward the pump drive by the pair of concentric coil springs between the spool valve and the pump cover cap. In the event that pressure or seizure should become too high for these two springs to resist, the spool valve shifts away from the pump motor's drive shaft to reduce the pumping effect.
There is no external means of applying force to the pump piston.
The diagrams suggest that the pump is a rotary type by virtue of the circular pump symbol but this is simply mis-understanding based on a simple diagram.
While working on the modulator, I became convinced that a fluid change every few years should see these units lasting forever. I cannot see that these operate for sufficient time for motor or mechanical wear of the internals to be a factor.
If the modulator could be filled with a hydraulic fluid, automatic transmission fluid or some other, non-hygroscoptic medium, it should endure indefinitely. Seal compatibility would be the concern as even should the modulator seals be compatible, any tiny exchange of oil into the brake system, proper, would swell and destroy master cylinder cups and likely caliper seals. Not an option.
The modulator must come apart in order to change the fluid. As I explained in an earlier post, almost all of the fluid can be changed by removal of the housing end which contains the back-up piston spring and reservoir.
From this point, the reservoir and back-up piston cavity can be refilled. A more complete refill will be achieved by prying the pump drive upward which will allow that cavity to be dumped out and refilled as mentioned in Bleeding Step #1. I do note that the open pump drive cavity was not shown so here is a better view of the open cavities.

Note the pump drive cavity on the left. The pump drive mounting holes can be seen across the cavity. The locating dowel which seizes into the aluminum of the other housing is seen in the lower center of the photo. Mike is correct about the effort required to move this puppy as it was still tough when I removed the housing for the second time. Care is required so as to avoid damage.
The solenoid valve cavity is located to the lower right side and is readily opened by gently prying the solenoid valve pack upward. Having removed the pump drive and solenoid valve, the fluid can be poured from these two cavities. They can then be refilled as per the bleeding instructions.
Removal of the pump spool valve assembly, coil springs, spring plate and cap is a much more involved process which might best be avoided if the pump is functioning normally. There is little to be accomplished because the pump piston and pump bore require specialized tooling to re-surface. The check valve can be extracted but these valves are very easily, make that extremely! easily damaged. Given that no parts are available, risk must be avoided.
The "O" rings can be sourced from local supply but other parts will be more difficult. If someone can donate a "dead" modulator, I will be able to do more research without the concern of keeping a bike off the road.
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I think the only part not referenced is the ABS pump motor. As can be seen from the photo:

The field housing can be removed as outlined earlier in this thread but it may be worth mention that the brushes can be replaced.
In order to remove the brushes the armature must be removed from the pump drive. The snap ring must be removed from the end of the drive and the bearings, eccentric drive and motor drive housing must be separated from the armature.
The brushes can be removed from the brush holder, by removing the two brush holder screws and sliding the brush holder upward. Brushes can be matched from alternator or electric motor brushes available from auto supply or electric motor repair shops respectively. Brushes which are too large in cross section or length can be reduced by draw filing or grinding.
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Thank You for your contribution,
Norm Keller, STOC #8030.