In reply to a message from JagMan2 sent Fri 13 May 2005:
It sounds as if you have a failing brake accumulator or perhaps a
dodgy combined pressure switch on the Teves pump motor unit.
Here’s a note I did for another list member which may give you some
idea of what you should be seeing:-
Firstly, the Teves pump motor combined pressure and warning switch
has two separate functions. One circuit switches the electric pump
on and off, the other circuit illuminates or switches off
the ‘Brake’ and ‘ABS’ warning lights on the dash.
Here’s the sequence of events (using mean nominal pressures)
When you first switch on the ignition (pump running without
starting the engine) :-
-
As the pressure in the system reaches 104 Bar the
warning switch is actuated and first the ‘ABS’ warning
light extinguishes followed shortly after by the ‘Brake’
warning light
-
As the pressure continues to rise reaching 183 Bar (the
upper switching point), the pressure switch opens and the
pump stops running.
-
If, after one or more brake applications (or natural
‘leak-down’), the system pressure falls below 139 Bar (the
lower switching point), the pressure switch closes and the
pump re-starts until the pressure again reaches the upper
switching point and the pump stops.
-
In this way, the ‘working pressure’ is maintained between
a nominal 139 Bar and 183 Bar (~ 2000 and 2700 psi)
-
If due to hydraulic or electrical; failure, this working
pressure is not maintained and falls below the level of 104
Bar (~ 1500 psi), the warning switch contacts close and
the ‘Brake’ warning and ‘ABS’ warning lights illuminate.
That’s how my car works and to give you some idea of the
timing of these activities, here’s a check I did on my car a
little while ago:-
When my car has been sitting unused for five days or so, any
residual pressure in the accumulator has long since dissipated.
On first turning on the ignition (without starting the engine),
the pump motor runs, the ABS light extinguishes after 32 seconds,
the ‘BRAKE’ light extinguishes after 35 seconds and the pump stops
after 42 seconds.
With the ignition still on, the pump will re-start on the
second application of the brake and run for 3 seconds to recharge
the accumulator.
Then, with the ignition switched OFF, it takes about 30
applications of the brake before the pedal goes ‘hard’ when
servo assistance is lost.
These timings are not absolute and YMMV, but how does your car
compare?
If the ABS warning light illuminates without the ‘Brake’ warning
light, that suggests that there is an electrical problem with the
ABS system, not related to system pressure. On your car there is an
on-board ABS diagnostic facility - details are in the archives, but
if you can’t find them, post back here.–
The original message included these comments:
I have a early 93 XJ6 and increasingly the brake system check upon
startup seems to take longer and the ABS Light and Brake light
flicker through its/the cycle and sometimes fails to rerach an OK
status and the ABS Warning light stays on. If after the car is
started and then the car is stopped/Ign. Turned off and the
electrical compleately shuts down and the Tach and Speedo fall to
an absolute rest and then the car is Re-started, the light almost
always goes out. There have been times where this proceedure
needed to be repeated.
–
Bryan N, '91 Sovereign 4.0 L, (RHD)
Cambridge, United Kingdom
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