Hand brake Light

[mordecaibrown]

1977 911S the parking brake light will not turn off is that a sensor or switch that is needed? Was turning off fine until now

Thanks

Sam

[EC900]

There’s plenty of threads on this in the Search.

The reservoir fluid level could be low. You can disconnect the switches on the master cylinder to check if these are defective, this is easiest to begin with, then move on to the handbrake switch adjustment.

Basically it’s probably the handbrake switch, you can try and adjust it but difficult to reach with the passenger seat in. Car has to be running to properly set the height, though there’s only about 3/16” or less of adjustable movement.

On my ‘77 after several attempts to adjust the copper contact band finally broke and light went is common and part still available, disassembly of handbrake is necessary to change. If working properly and adjusted, the handbrake warning light will turn on after 2-3 clicks up.

[mordecaibrown]

Thanks for the information. Do you have a part # on the contact stitch and wiring?

Sam

[spuggy]

My '77 did this a few years ago. Contact strip lost the original shape under the unrelenting pressure (flattened out to a ~40 degree bend instead of ~90 degree) - and the handbrake light just stayed on with the ignition one day...

There's other things it could also be, but that was the issue with mine. Fix was fairly painless; put the passenger seat all the way back, dismantle enough to get at the internals and peer at it - it was pretty obvious the strip just needed the bend restored - eg to be the correct shape. Put it back together. The switch adjustment had been at the very top, reset it back to the bottom. Worked perfectly. Carefully put the unused replacement contact, rivets and washers away, went for a drive.

I found this 2014 thread by speedking very useful https://forums.pelicanparts.com/porsche-911-technical-forum/844678-fixing-parking-brake-warning-light-switch.html

[HarryD]

You could have a short in the switch on the MC. The handbrake switch is in parallel with the MC switch.

See my comment in the topic.

Originally Posted by HarryD

Another possibility is a failed switch in the master cylinder.

I had my brake light start blinking.

I checked the handbrake and it was operating properly and the switch was functioning as designed.

I checked my brakes and all was normal. Pedal effort and stopping distances were as expected.

I reviewed my wiring diagram and found that the circuit for my brake light went like this:

Power --> flasher --> light on dash --> switch on master cylinder --> Switch on Hand brake. Both the switch on the master cylinder and handbrake would go to ground if activated (i.e. the switch is normally open).

I pulled the wire off the master cylinder (it also acts as a jumper to the handbrake switch so it remained functioning) and my light stopped flashing. If I pulled up the handbrake, the light came on, as expected.

Once I replaced the switch on my master cylinder, the flashing was stopped.

[mordecaibrown]

Thanks all for the advice!!!!!

Sam

[spuggy]

Originally Posted by mordecaibrown

Thanks for the information. Do you have a part # on the contact stitch and wiring?

Sam

As it could be several things, you should probably take a look and figure out what is actually in need of attention.

I didn't - but I felt it was very unlikely to be brake switches, given how the 930 M/C conversion had required new ones (different type) relatively recently. Of course, shortly after the original 911S ones began wanting 5 million PSI to activate the brake lights and also got replaced.

You'd probably find a copy of PET useful. illustration 7-105 shows:

901.613.523.20 - contact bow (#8)
900.055.001.20 - rivet (#9, 2 req'd)
900.031.012.10 - washer, 2,8 (#10, 2 req'd)

The "contact piece" that mates with the "contact bow" is literally a piece of plastic with a piece of copper held in place by it, with final position adjustable within a range via the screw, our host has good pictures of it:





from https://www.pelicanparts.com/More_Info/90161352120.htm?pn=901-613-521-20-OEM

Seems unlikely anything would go wrong with that - but:

901.613.521,20 - contact piece (#3)
900.015.025.02 - screw fill hd. AM 4 x 15 (#4)
N 012.116.1 - serrated lock washer A 4,3 (#5)

"wiring" seems to be covered by "911.612.001.28" for "harness tunnel" for 77 - #1 in the diagram. Although I seriously doubt you're going to replace that, as it runs from the fusebox/gauges to the 14-pin connector in the engine bay and includes bulkhead connector plugs for the ignition switch/steering column stalks. It's NLA from Porsche anyway - although Kroon do list an exact replacement (for a surprisingly reasonable price). Gee, I think that'd be extraordinarily painful to replace...

[EC900]

In my case, I found the broken contact bow (the one riveted to the handle) in the tunnel. Try looking thru the holes.




Here’s a link to the PET that I have bookmarked:
https://www.9xxteile.com/media/wysiwyg/pet/E_911_77_KATALOG.pdf

[HarryD]

Originally Posted by EC900

In my case, I found the broken contact bow (the one riveted to the handle) in the tunnel. Try looking thru the holes.




Here’s a link to the PET that I have bookmarked:
https://www.9xxteile.com/media/wysiwyg/pet/E_911_77_KATALOG.pdf

Before you go through those heroics, check your MC Switch. It can be accessed from under the car.

[mordecaibrown]

This is great info Thank you!!! I went ahead and ordered the switch as if I am going to take it all apart the switch is only $24 and I need a new rubber cover anyways. But I check that contact rivet as well.

Sam

[mordecaibrown]

I looked at it today and could not locate the switch is it under the plate with the 3 bolts?
Looks as though I will have to take the passenger seat out to get in under the plate if thats where the switch is

Thanks,

Sam

[HarryD]

Originally Posted by mordecaibrown

I looked at it today and could not locate the switch is it under the plate with the 3 bolts?
Looks as though I will have to take the passenger seat out to get in under the plate if thats where the switch is

Thanks,

Sam

Sam,

Did you check the switch on your master cylinder? A lot easier to access.

[mordecaibrown]

No not yet, where exactly is that switch on the MC? A couple of weeks ago the lights did turn off and I used the hand brake and it then no longer turned off so I was looking at that area. I assume that switch is under that plate by the hand brake?

Thanks,

Sam

[Showdown]

Here are the flow charts for 1977 if you want/need them

https://www.dropbox.com/sh/4e5tj2d6t0fp020/AABMXkMKLmaXWHYZhWvOQGU7a?dl=0

https://www.dropbox.com/sh/4e5tj2d6t0fp020/AABMXkMKLmaXWHYZhWvOQGU7a?dl=0

[HarryD]

Originally Posted by mordecaibrown

No not yet, where exactly is that switch on the MC? A couple of weeks ago the lights did turn off and I used the hand brake and it then no longer turned off so I was looking at that area. I assume that switch is under that plate by the hand brake?

Thanks,

Sam

The switch is not near the handbrake. It is on the brake master cylinder.

On my non-assisted brakes, the MC can be seen under the car.

If vacuum assisted, I think you see it in the Frunk.

[mordecaibrown]

This is the Switch I ordered based on recommendation in this thread. where is it located?



Is it located here under the hand brake?

[spuggy]

Originally Posted by mordecaibrown

This is the Switch I ordered based on recommendation in this thread. where is it located?

That is extremely unlikely to be your issue; mine still looked almost perfect after 5 seconds attention from a fiberglass pen - despite 275,000 miles on the tub.

The copper strip riveted to the back of the handbrake that completes the circuit with that switch is prone to losing its shape after nearly 50 years of constant pressure. And the warning light comes on.

If that's even the issue - the pressure switches (2, one for each circuit front/rear) on the master cylinder in the trunk - can also cause the same symptoms. Although they often get replaced either because they require too much pressure to activate or because the internal contacts arc out from switching too much current first.

[mordecaibrown]

So if it is the rivet on the hand brake how is that fixed? Regarding the switches on the MC I will look into those Also As long as I ordered this switch where is located?



Thanks

Sam

[EC900]

Sam,
The two copper rivers that secure the copper bow are seen on the top of the handle. I don’t think the bow being broken is your problem. The bow breaks either from old age or maybe maladjustment.. As stated previously if this copper bow breaks - the dash light will not turn on as the circuit can’t be completed. The curve in bow touches the copper band on switch when you pull up.
There is no easy way to reset the rivet should you buy a new bow, and the entire handbrake assembly needs to be removed. Set the rivets either with a tool or punch as any other riveting which can fit in the crevice of the handle.

Secondly, your arrow is where the switch is s installed, under the handle. Not sure but I believe the screw backs out that holds the switch, slide new one in and again, the handle needs to be removed.
Lot of tedious work to replace the switch/bow especially if you haven’t explored the rest of the areas mentioned as to what else might be the issue.

Before my dash light was always On, (while engine on and driving, lever down) my attempts to adjust the height of the switch, it would flicker on bumps. eventually the bow broke off and with that the light doesn’t come on except for part of the start up regiment (key in, ignition On) that wired to the master cylinder. The warning light was always an annoyance while driving in the dark, the fact mine is off now I don’t need to have it activated by pulling up the handle….pretty much know now when my handbrake is set unlike when I was a teen driving a slow VW !

So…the brown/yellow stripe wire is for the handbrake switch, in your pic it’s the covered wire leading under the handle. If you have access, disconnect it from switch and that will let you know the bow exists and making contact and switch needs adjustment. Don’t attempt or bother to disconnect the dash light switch as it’s connected to the master cylinder circuit.

[mordecaibrown]

It don't sound like the connection bow broke then as the dash light is on all the time?