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Brake Warning Light (not Brake Lights or Lining warning)

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Chris Wright
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Brake Warning Light (not Brake Lights or Lining warning)

Post by Chris Wright » Sat Feb 18, 2006 6:21 am

The brake warning light popped on today on my '87 635 and stays on.

Now, the brake fluid level is fine and when I unplug the leads to the brake fluid level switch the light stays on. So I unplug the two switches on the regulator and sure enough the light goes out when I unplug the switch on the side of the reg. (all with eng. running).


So, here is the question:

What does the switch on the side of the regulator measure/indicate?....
(The parts cat. lists this as a 20 bar or 292 psi switch)
Anyone have a flow diagram or system description of the regulator?

I've pumped the brake pedal and discharged the accumulator and the fluid level rises to the top and disappears when the engine is started. So the accumulator seems good.

The website realoem.com calls the side switch an "oil pressure switch" and the top switch the "brake pressure differential switch".

While the component locating section of the 1985 BMW 635Csi Electronic Troubleshooting Manual Online, calls the side switch a "brake accumulator pressure switch" and the top switch the "hydraulic pressure switch"

and the Bentley manual just calls them both "hydraulic pressure switchs"
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'87 635Csi
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MG 1100

demet
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Post by demet » Sat Feb 18, 2006 2:55 pm

I think a lot of us have been there. Before considering expensive repairs replace the pressure switches. They are known to fail.
demet
current: 85 735i, 86 528e, 89 535i turbo, 88 535is, 95 530i
past: 86 635csi sold :(

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Chris Wright
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Location: Columbus, IN (INDY 500 area)

Post by Chris Wright » Tue Feb 28, 2006 6:54 am

Well, first things first; do the cheap, simple things. Since there have been several posts about the filter in the hydraulic reservoirs crumbling, I decided to change it and the fluid first before trouble shooting the light problem.

Well the fluid was kind of dark and although the filter was not deteriorating, it, as well as, the inside of the reservoir was coated with a fine gray film. I cleaned up everything and installed the new filter and fresh ATF and ran the car to circulate the ATF. I noticed that the light went out for a while but came back on. I then removed the ATF a second time and refilled with fresh fluid. This time the light went out and has been out ever since. I would have replaced the fluid one or twice more, but I ran out of ATF. I will probability do this in a couple of days and then run the car for a couple of hundred miles and then check/change the filter. I'm guessing that this film may have been causing a valve in the regulator to malfunction causing a drop in pressure.

Now the big question is, where is this film coming from? It's not gritty to the touch and could be carbon from the pump if the pump is a vane type with carbon seals. Anyone know about the hydraulic pumps? Or it could be very finely powdered aluminum and who knows what is wearing!
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ZesCoupe
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Re: Brake Warning Light (not Brake Lights or Lining warning)

Post by ZesCoupe » Mon Jun 11, 2018 11:16 pm

Did you ever answer that film question Chris? I've been searching for a definitive solution for my problem and found the same grey film in my reservoir. Following a hint from the old Norm Grills FAQ back in the day I had installed a magnet in the reservoir and have noticed that most of the gunk was stuck to that.

Regards,
Martyn
"If a job's worth doing, it's worth doing well" My Dad (1921-1995)

1986 635CSi
www.automotivelife.nl

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Brucey
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Re: Brake Warning Light (not Brake Lights or Lining warning)

Post by Brucey » Thu Jun 28, 2018 9:26 pm

FWIW (it is a bit late now.....) but if pressure switches become intermittent such that they stay off and then come back on again after a while, this is symptomatic of a leak in the switch diaphragm. What happens is that the switch works normally (but leaks) until the leak rate through the diaphragm becomes higher than the leak rate through the back of the switch housing to the outside world. At this point there ceases to be as much net force on the diaphragm (because the fluid pressure is about the same on both sides) and the switch doesn't give a fair reading any more.

If the system is left without pressure in it, the fluid may leak away from the air side of the diaphragm so that (for a short length of time when the system is repressurised) the switch works normally, at least until enough fluid leaks past the diaphragm again.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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