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Troubleshooting Master Cylinder switches
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malcolm2
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PostPosted: Sun Jan 29, 2023 5:47 pm    Post subject: Troubleshooting Master Cylinder switches Reply with quote

To remind folks..... bus has been being worked on for 5 years. Everything has been replaced.... New and waiting to be on the move.

Engine is in, battery is in and wired. So today was the first day that the key activated stuff being properly wired.

Starter turned the engine over.... GOOD.
All the light systems worked with switches pulled... GOOD

BRAKE LIGHTS did not lite up. Brake light on the dash stays on... NOT GOOD

Here is what I have. 2 pressure switches on the MC. 3 male spades per switch. Like a clock, let's say I have a spade at 3, 6 and 9 o'clock.

Diagram says 1 goes to the brake lights, 1 goes to the warning lite, 1 comes from the fuse panel.

The connectors are waterproof molded, so I can't really tell what wire goes where. I made up some wires so I could get to the connections with a meter.

KEY ON. Brake UP: Dash brake light is on. I get 12 volts at both 6 o'clock terminals. 0 volts on 3 and 9 o'clock

KEY ON. BRAKE DOWN: Same.

KEY OFF. NO VOLTS at the MC... As expected.

How should i TS from here?

Confused since the dash light is on, shouldn't I get 12 volts on 2 terminals at the MC?
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PatJr
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PostPosted: Sun Jan 29, 2023 6:01 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

you got this already?

https://www.thesamba.com/vw/archives/info/wiring/bus_1972_BrakeLampWiring.jpg

I think the B light is telling you something funky is up with the MC
peddle feel OK?
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dodger tom
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PostPosted: Sun Jan 29, 2023 6:04 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

this thread came up last month.

hope it helps.

https://www.thesamba.com/vw/forum/viewtopic.php?t=776214&highlight=master
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Jeff Geisen
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PostPosted: Sun Jan 29, 2023 6:26 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Jump the two wires together at the MC brake light switch. Turn on the key, brake lights?
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malcolm2
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PostPosted: Sun Jan 29, 2023 6:32 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

PatJr wrote:

I think the B light is telling you something funky is up with the MC
peddle feel OK?


tuff to say. have never driven it. It will have power brakes once the engine runs. Just sitting there, the pedal has some pressure. Not like nothing is pushing back.

Clark
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kreemoweet
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PostPosted: Sun Jan 29, 2023 7:27 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Brake warning lite will stay on until engine starts, and alternator puts voltage on blue #61 wire on warning lite. Dash lite gets its own voltage supply from fuse box; it gets no voltage from m/c switches unless one of the switches goes on, and the other stays all the way off. Both switches staying off, or one getting into a "half-way" position results in no warning lite coming on.

But warning lite aside, if the pedal makes pressure in either or both brake circuits, there should be voltage on brake lite wires. Seems like you have 2 bad switches.
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malcolm2
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PostPosted: Sun Jan 29, 2023 7:36 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Make senses... I guess the 12 volts I get is from the fuse panel.

To further test, can I just jump the 12 volts to the wire going to the brake lites?
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Last edited by malcolm2 on Sun Jan 29, 2023 7:50 pm; edited 1 time in total
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malcolm2
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PostPosted: Sun Jan 29, 2023 7:48 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Jeff Geisen wrote:
Jump the two wires together at the MC brake light switch. Turn on the key, brake lights?


Not done that yet, but will be simple... thanks
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sjbartnik
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PostPosted: Mon Jan 30, 2023 4:25 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Ignore the dash light for now, it's behaving as expected (e.g. it comes on with the ignition on as a bulb check and will go out when the engine starts - it's tied in with the alternator light so they both go out at the same time).

The brake light switches are simple pressure switches. With no hydraulic pressure, they are open (brake lights off). With sufficient hydraulic pressure, they close (brake lights on).

There is one switch for each circuit so even if you lose hydraulic pressure completely in one circuit, the brake lights will still work.

However, the downside of this arrangement is that if one switch fails, you'll never know until the other switch finally dies and then you have no brake lights.

If you have a situation where there is pressure in one hydraulic circuit and none in the other, the difference is what makes the warning light on the dash come on when the pedal is pressed.

You can test the switches directly (with a helper) - pull the wires off, put your meter across the 2 main terminals (the ones that the black and black/red wires go to) set to resistance. With pedal up, resistance should be infinite. With pedal depressed firmly, resistance should go to near zero. If not, the switch is bad OR you have insufficient hydraulic pressure in that circuit (e.g. air in the lines).

Test both switches, if they both fail the above test and you're sure all the air is bled out of the brake lines, the switches are bad.

Jeff's suggestion above is excellent for testing the actual lights - if you jump the wires with key on, you are bypassing the switch and the lights should come on. Both lights, it's not one switch per light. Each switch independently switches both lights.
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malcolm2
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PostPosted: Mon Jan 30, 2023 4:44 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

Thought I would do both of those tonight. It is tuff to tell which wires go to which switch spade. The connections for the switches are molded and the wires twist up as they enter the molded cover.

I will figure it out. There are only 3 connections per switch, so just check continuity across all combos....

Thanks for your input here.

Clark
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malcolm2
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PostPosted: Mon Jan 30, 2023 6:19 pm    Post subject: Re: Troubleshooting Master Cylinder switches Reply with quote

That did it. No continuity on either switch with the pedal down. And the brake lights lite up with the switch jumped out of the circuit.

I did bleed the whole system not too long ago and the pedal does feel nice and firm.

Found these at FCPeuro. ATE brand and $16. I forgot that I had bought from them before. Cool thing is they have a lifetime warranty. check them out.

https://www.fcpeuro.com/products/porsche-brake-light-switch-ate-113945515g
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