Hello to everybody & greetings from Odessa!

We have a discussion here in Ukraine on if there is a difference in brake fluid change procedure depending on whether a car is equipped with ABS or not. Some people tell that in case brake fluid is to be completely replaced on a car which is ABS-fitted, then it is impossible to do that correctly without activating of the ABS hydraulic unit and its pump (e.g. by means of special service tools etc.). However other people insist that there is not any difference and if it is necessary to fully replace the fluid on ABS cars then it is possible to do that in common way as on any other non-ABS vehicles.

What do you think? We will be very grateful every advice / information you may have. Many thanks in advance.

 

Conventional brake bleeding will not change the fluid out in the ABS resevoir. It does retain a small amount that is pressurized and some that is not to allow for automatic application or release of hydraulic pressure at individual wheels as needed. You would need to energize both an apply solenoid and a release solenoid for at least one of the wheels to get all of the fluid changed. Older cars had a separate resevoir that could be drained and refilled but newer is all self contained in the ABS/HU.

 

Thank you, babyhuey!

Can you please clarify, if possible, whether it is correct to do a full brake system flush and not to energize any solenoid of the ABS? In other words, is it mandatory that the fluid which is in the ABS HU changed simultaneously with conventional brake system fluid? And, is it possible that air get into the ABS HU when the brake system is being bled but the ABS solenoids are not energized?

Just for your info. As far I understand your post there should be two solenoids per wheel – one apply & one release solenoid. However there are only four solenoids in my 626 GF’s ABS unit. So, I have only one solenoid per wheel.

 

Would not consider it necessary to energize any of the solenoids or run the pump motor to perform an acceptable brake system flush if the flush is just part of a routine maintenance. You are only going to leave perhaps 50 milliliters or so in the hydraulic unit chambers and passages. It would be possible to get air in the hydraulic unit it you were to allow the master cylinder resevoir to run dry and get air in the system but should flush thru just as it would out of convention system lines. If air had somehow become trapped inside the hydraulic unit say between the solenoid and pump because the system was energized while air was present in the system, then you would then need to energize the solenoids and pump to purge that air out, other wise no.
Yes you are right that your 97 626 only has 4 solenoids, one for each wheel. The solenoids are used to decrease application pressure only. When the ABS pump runs and is applying pressure it is doing so to all 4 wheels and it is up to each respective solenoid to relieve pressure as needed to prevent wheel lock-up on your type of system. Later years use apply and release solenoids as there is more precise control to each wheel with this type of system. Using separate apply and release solenoids allows the ABS computer to specificly target individual wheels without affecting the others. Very important to be able to do this especially when vehicle has Traction Control or Dynamic Stability Control systems.

 

Thank you very much, Babyhuey, for your irrefragable answer!

Sorry to inundate you with questions but may I again ask you one. As you told, two valves / solenoids per wheel allow to individually control each wheel and do not affect other wheels. If so, what happens in my car with only one solenoid per wheel? Does it mean that my ABS is unable to control each particular wheel when necessary and it may influence (decrease pressure) on one or more of other wheels even if that is not needed for these wheels?