Heading for my recall flush this morning. Will check for DTC's before I go.

 

Why would the "normally open" valve stick closed without an abs activation? The valve has to move to block the passage and its not moving under normal braking. Crap buildup blocking the passage is the only other possibility because its such a simple device. Theres no fail safe for a failed master cylinder or calliper so why aren't you complaining about that too? They don't last forever.

 

When you start your bike, doesn't the ABS test itself? If it does test itself, would it activate the ABS control module, thereby causing the "normally open" valve to stick closed in the Hydraulic Control Unit (HCU)? I don't know. Just asking...

 

Most (all?) abs systems will do a self-check actuation upon start-up of the vehicle. A great many of the failure reports are of the initial brake application after a start-up. I can't say with certainty, but it appears the failure is frequently happening upon that initial self-check.

Harley abs failures occur more catastrophically than other manufacturer abs failures. Others leave you with fully functional brakes, just no abs function. Harley leaves you without any brakes. That's significant, at least in my opinion.

Not arguing that a master cylinder or caliper can't fail. Rarely to those go instantly and completely, leaving you without brakes. If a manufacturer were to be having frequently reported failures of master cylinders or calipers I would expect a recall to correct the problem.

 

The initial ABS self test is the ECM only checking the system as to whether or not it can "see" the ABS electrical components. It does not cycle the pump or valves.

 

OK. Dang.

 

A little boring background... (I feel like Cliff on "Cheers")... "It's a little known fact that"....

When the silicone based fluid first came out (as a result of testing at the request of the military in the 60's & 70's), it was superior to DOT 4 (glycol based). The silicone fluid had the most stable temperature related viscosity of the prior DOT fluids. It also had higher wet/dry performance. Thus it was given the "DOT 5" specification... It was also later noticed that it didn't affect paint nor would it absorb water like Dot 3/4... Many accepted its superiority and went to DOT 5 (prior to ABS)....

However, DOT 5 (silicone) has a molecular structure that is less dense than DOT 3/4. For this reason, it is possible that O2 can dissolve into DOT 5, without increasing volume. I've heard this called "micro bubbles" by some. The DOT 5 liquid, with dissolved O2, can actually, slightly compress... Michael Grant, a writer from Moss Motors in the US, reported the dissolved air can cause a "spongy" brake feel. That is why it is considered no good for racing applications. Racers totally avoided DOT 5 fluid. These "micro bubbles" are also the reason manufacturers claim DOT 5 doesn't work well with ABS, and won't use it with ABS....

So we're back to DOT 4 as the standard, which absorbs water, and requires flushing on a periodic schedule...

FWIW.... I have personally noticed a difference between DOT 4 & DOT 5 fluids.

Two of my older, non-ABS Harleys use DOT 5. I find it easy to work with, but a bear to bleed the front reservoir. Even after flushing with a vacuum bleeder, I often get a spongy brake lever on the front reservoir. It has never happened on the rear reservoir/pedal.

Don't know why, or how, it works.... but I just zip-tie the front lever to the grip overnight, and it's firm in the morning..

I've never had that issue when flushing my cars, trucks, or other two bikes with ABS and DOT 4....

 

Why would the ABS self-test not activate the ABS control module and, therefore, the HCU? It would be easy for the engineers to program that feature into the bike. After all, the ABS control module is a major part of the ABS system.

 

Could be one of any number of scenarios but I suspect not enough memory in the ECM for the additional code. Remember there's bean counters involved in the creation process and after all it is a Harley.

 

The military wanted a brake fluid that was more environmentally stable, among other standards. A fluid that would not absorb water in a humid environment, a fluid that would not freeze in cold environments, a fluid that could be left in vehicles for long term storage. Silicone satisfied that requirement. Current milspec is MIL-PRF-46176B.

You can download a copy of the current milspec here:
http://everyspec.com/MIL-PRF/MIL-PRF...F-46176B_6492/

It is worth note that the milspec is different than the NHTSA DOT5 spec. The various NHTSA DOT brake fluid specifications can be downloaded here:
https://www.gpo.gov/fdsys/pkg/CFR-20...sec571-116.pdf

Silicone fluids have no special attractive properties related to oxygen. However silicone fluids can suspend or otherwise retain more air suspended in the fluid than glycol. Excerpted from SAE J1705, Appendix A, A.2.2.8,
"It has been reported that dimethyl polysiloxane fluid, which is a major part of silicone-based, low water-tolerant type brake fluids can typically contain dissolved air at a level of 16% ±3% by volume at standard temperature and pressure. This compares with a typical level of 5% ±2% by volume of dissolved air for glycol ether based type fluids."

This suspension of air can create a more spongy brake pedal feel than glycol fluids. Coming from that same SAE J1705 standard; "An increase in brake pedal travel may be experienced under severe operating conditions, especially at higher altitudes and high temperature conditions."

I know of no free download of SAE J1705. However one can download it here:
https://www.sae.org/standards/content/j1705_199505/

This air retention and foaming or sponginess has been used as the basis of the claim that silicone brake fluids are unsuitable for use in ABS systems. The claim being that the rapid cycling of the hydraulic components in the ABS module would froth the fluid. Where the air would come from is left unexplained.

However, the Army commissioned a study on this. The results of this study showed silicone fluid to be incompatible with ABS systems. Not for frothing reasons, but for wear problems owing to the lower lubricity of silicone fluids.

This military report can be downloaded here:
https://events.esd.org/wp-content/up...quirements.pdf

And as a semi-sidebar, the military also investigated claims of silicone brake fluid suddenly becoming highly compressible in non-abs vehicles, resulting in catastrophic brake failure. The results of the study contra-indicated these claims, showing the cause to be conventional brake fade.

This military report can be downloaded here:
http://www.dtic.mil/dtic/tr/fulltext/u2/a251835.pdf