>Does the HU cycle on its own at any regular (or irregular) intervals.. like
>startup..or quick deceleration (I think someone said so), or only when
wheel
>lockup occurs & during the test?
>
The hydraulic unit cycles occasionally when you turn on the ignition switch.
It is not a full operation of the system, however. Only the large electric
motor runs--this is what returns "dumped" brake fluid to the master
cylinder. The solenoids are what actually "dumps" pressure and diverts the
extra fluid to a separate storage compartment inside the HU.
When I first got my HU back from England, I discovered that it had been
reassembled missing one of the check valves in one of the return pumps. One
thing it did expose is that the electric motor activates on a sharp
deceleration--this is controlled by the ABS Relay. The system runs
something like this: 1) ABS sensor tells the computer that there was a
sharp deceleration (like sharply closing the throttle), 2) Computer
interprets as a situation where wheel lockup is likely, 3) Close contacts in
ABS Relay (starts electric motor to be ready to return any "dumped" fluid
AND applies 12V to one side of each solenoid), 4) Computer will ground the
other side of the appropriate solenoid to actually "dump" fluid pressure IF
lockup occurs, 5) Open contacts in ABS relay when deceleration stops (Shut
off motor, disable solenoids, return to normal braking.)
In short, any time that there is a strong potential for lockup, the computer
senses it, starts the return pump motor, and enables the dump solenoids to
be activated if needed. Obviously the whole shebang is operated during the
ABS diagnostic test with the jumper wire in the test coupler.
I hope this hasn't been too confusing, but I had to say something--I'm going
through GTS withdrawals!! ;-) Scrape a peg for me Jim!!
Terry Baker
