On 24/08/2015 3:04 PM, fmiser via Mercedes wrote:
Randy wrote:

One issue that remains, and I had hoped would be solved in
all of this is the brake warning light on the dash still
does not wish to go out. I believe it to be the sender that
warns of an uneven pressure between front and back.
Anyone know anything much about this?
I have no experience with a 115, so I probably don't know
much. *smiles*

But on my 123s, the _red_ "brake" dash light warns of pressure
imbalance _and_ low fluid.  At least I presume it warns of
pressure imbalance, I know it warns for low fluid.

Because unplugging the sensor prevents the warning light from
coming on, we know you're on the right trail.  If the light
is off when unplugged, presumably the sensor is normal-open
and closes to send a warning.  That would suggest that if
there are multiple sensors they would be wired in parallel
meaning unplugging one of them would not effect the others.

Did the new master cylinder include a new sensor?

Does the dash light still come on for the self test when
starting the engine?  (I'm presuming the 115 dash lights test
even though the glow and start are not done with the key)

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I assume (always dangerous to do that) that the 76 115 300D may be more like the 123 in any event as the earlier 115 models did not have the pressure imbalance sensor.

You are correct that there are two sensors - 1 for pressure imbalance and the other for low fluid. They are wired together in the sense that the low level sensor has 2 wires for it and the pressure sensor has one lead that appears to come on from the first. Like an oil pressure gauge, I assume (there it is again) that it grounds out when it puts the light on. The bebuilt unit came with neither sensor. The level sensor is built into the resevoir and is not removable. I have not checked to see if it works. I could do so while bleeding the brakes as the level will no doubt go down. I am not pressure bleeding but just opening the wheel cylinder bleed screws while someone else steps on the pedal for me. I believe the issue to be with the pressure sensor as unplugging the lead from it puts the light out. It has a small white plastic pointer on it and I believe that the pointer will be moved one way or the other by the plunger in the body of the master cylinder when it is moved one way or the other by the pressure from the main plunger. One thought that comes to mind right now is that it might be in order to loosen it off enough to bleed it too just in case there is some air in there.
Will try that tonight.
If that does not work and it is still wonky after I bleed the brakes, I will remove it temporarily and test with a meter to see if it is grounded out all of the time.

RB


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