General question about ABS operation and the 'pump' (89 300SE)

Can someone explain what happens/goes on with the ABS. When I switch on the ignition, I hear the sound of a pump start up (not the fuel pump) in the engine bay. This runs till I hear a click
(5 secs or so), then the process repeats again at infinitum. I don't understand what exactly is going on. I am sure it is something to do with the ABS. I can guess that it needs pressure in some part of the system to pulse the brakes (if required), but I don't see a need to keep charging this pressure. The light on the dash seems to work OK. Any suggestions?. The CD manual doesn't tell what is happening when I switch on the ignition. Only how the system operates in the event of the ABS needing to modulate the braking pressure. Have I got a fault?. Should this thing keep running/recharging continuously. If I look on the circuit diagrams (electrical), I don't see a pressure switch that could cause this.
Any ideas or enlightenment would be appreciated.
Cheers... Rob.
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I had a similar issue on an old Honda. I seem to recall that the ABS needs to maintain pressure but a leaky valve inside of the system was allowing the pressure to escape (or something along those lines). Sometimes these systems can be "exercised" via computer to un-stick the valves. I had that done and it solved the problem for about six months.
Josh
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You haven't cheered me up Josh.
Can't I excercise the valves myself by finding some loose gravel and anchoring on?.
Still doesn't sound too good a prognosis. I recall I had a Ford with ABS. That used a citroen suspension sphere as a volume to charge with pressure. When you turned on the ignition, an electric pump could be heard running for several seconds, then a click and it stopped. I guess that was it charging the pressure. In this caes, the thing just repeats and repeats. Funny thing is that they don't label the motor as a compressor, more some sort of return pump. The Electrical schematic also doesn't seem to show a pressure switch. The description of what happens in the cd manual skips over what happens as you energise the system (ignition on). It goes pretty much straight into what happens when you lock a wheel. Very interesting, but not completely explanatory.
Cheers... Rob.

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Hi Rob might sound silly but are you sure that you are hearing the abs pump and nothing else ie: the heater control valve opening and closing to maintain temp in car (try turning heater control dials to max hot and see if noise stops cycling) also is the noise present with ignition on or engine running? there is also an air pump behind the glove box which starts to run when you put the ignition on.
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Hello
Sorry for the slow reply. Been away a couple o days.
I'll investigate this phenomina with the heater. I believe this happens both with the engine stopped (ign on) and running.
Thanks for this info.
More when I have it. Cheers... Rob. snipped-for-privacy@aol.com wrote:

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