69 VW Bug master cylinder question
Gary Steed
I just replaced the master cylinder in my '69 bug. The old one had 3
switches. (one for the front brakes, one for the back brakes and one for
the pressure differential warning lamp on the dash) I was informed when
I bought the new master cylinder that they don't make the 3 switch version
anymore (haven't for 20 years) but the 2 switch (3 prong) version would
work just fine.
Well, the brakes work just fine, as do the brake lights, but the
dash light doesn't work. (not as a pressure differential indicator anyway)
The third prong on the switches normally has +12 volts with no pressure
on the brake. When pressure is applied, the +12V goes away, unless there
is a loss in pressure in the front or rear brakes. The diagram that I saw
for wiring a master cylinder with two, three prong switches had them
wired in parallel. The third wire went to the lamp.
I have mine wired with the third wire on the third prong. Currently,
the the dash light stays on all of the time, but it does turn off once
in a while to let me know that I have my foot on the brake, not that
I really need a visual aid. :')
I think that something else is needed, possibly an additional circuit
to sense a difference in pressure. I have an idea that might work using
a dual relay, but I'd like to find out how it's suppossed to be done.
The answer I usually get is "leave it disconnected, and don't worry
about it". I'm hoping to get the right answer here.
------------------------------------------------------------------------------
Gary Steed Hewlett-Packard Co.
CC&S Network Engineering
Unix: st...@hpcc26.corp.hp.com Palo Alto, CA.
Jeffrey Mulligan
> I just replaced the master cylinder in my '69 bug.
...
> I have mine wired with the third wire on the third prong. Currently,
> the the dash light stays on all of the time, but it does turn off once
> in a while to let me know that I have my foot on the brake, not that
> I really need a visual aid. :')
Disclaimer: I don't have any bug-specific knowledge, but...
Maybe you have the thing wired up correctly, and the light is on
because you didn't properly "centralize" the pressure differential
switch when bleeding the brakes. On my car the pressure differential
switch is a separate unit external to the master cylinder, with
4 hydraulic connections one 1 electrical (not counting the chassis ground).
The switch body consists of two hydraulic chambers separated by a piston,
which moves to one side or the other when a large pressure differential
exists, actuating the electrical switch. When bleeding the front
or rear brakes a genuine pressure differential is created which can move
the piston; one method to keep the light off involves inserting
a screwdriver to prevent the piston from moving, while another
involves bleeding fluid from the opposite side of the system
to "centralize" the piston.
--
Jeff Mulligan (j...@eos.arc.nasa.gov)
NASA/Ames Research Ctr., Mail Stop 262-2, Moffet Field CA, 94035
(415) 604-3745
Gary Steed
>Maybe you have the thing wired up correctly, and the light is on
>because you didn't properly "centralize" the pressure differential
>switch when bleeding the brakes.
The pressure differential switch is what disappeared on the "newer" version.
My old master cylinder had another bore that the third switch used to sense
a difference in pressure.
The repair manual that I have shows the "two switch" version for a different
year, and it doesn't show an external pressure differential switch. It does
show the third prong on each switch going to the brake warning light assembly.
Now maybe the warning light assembly is different for this year. If there
was another circuit that could tell if the brake pedal was pushed (brake lights
on) and still +12V on the third prong (no pressure sensed) that could be
used to turn on the light. That's what I may end up doing with a dual relay.
What it needs is an Exclusive OR gate between the switches and the warning
light. Now if only they made one for +12V systems.
Thanks for the response.
Gary