Brain Trust Help needed on Parking Brake Light, Parking Brake Light Always on...... Options

[Superhawk996]

I’d say ‘don’t be surprised if there is no reset button to push’ … I have 3 old 914 master cylinders and none of them have a reset button. Am I missing something? They all have some type of a square plastic “tower” where the spade(s) are … is that somehow the reset “button”? I lightly tried prying the plastic block off on one and that didn’t seem to be the case (IMG:style_emoticons/default/confused24.gif)

Not all switches have the latching function and the reset button. The later model two pole switches are not latching - when there is a problem in the master cylinder the pressure differential between the hydraulic circuits will cause the light to turn on.

If you’ve converted from an early single pole to double pole switch (or vice versa) it needs to be wired accordingly.

[Superhawk996]

No need for warning light.

(IMG:style_emoticons/default/blink.gif) (IMG:style_emoticons/default/headbang.gif) (IMG:style_emoticons/default/screwy.gif)

[Gatornapper]

SH -

Have electrical schematics for '87 944?

GN

What in the world does this wire to the MC do? Make sure there is fluid in MC? Why is there a reset on it?

The master cylinder switch senses a loss of pressure in either of the two braking circuits.

If there is a pressure differential across the sensing piston inside the master cylinder, it trips the latching switch.

This will occur during brake bleeding - thus the reset switch.

If you haven’t bled the brakes and this circuit is being tripped, it is a serious issue and shouldn’t be ignored.

[Superhawk996]

SH -

Have electrical schematics for '87 944?

GN

Appear to be available online in a couple places. I haven’t looked for 87’ but you should find some general info that you’re looking for to at least get you going.

https://www.pelicanparts.com/944/electrical...KXI-hEDyEc0bAkW

[Literati914]

I’d say ‘don’t be surprised if there is no reset button to push’ … I have 3 old 914 master cylinders and none of them have a reset button. Am I missing something? They all have some type of a square plastic “tower” where the spade(s) are … is that somehow the reset “button”? I lightly tried prying the plastic block off on one and that didn’t seem to be the case (IMG:style_emoticons/default/confused24.gif)

Not all switches have the latching function and the reset button. The later model two pole switches are not latching - when there is a problem in the master cylinder the pressure differential between the hydraulic circuits will cause the light to turn on.

If you’ve converted from an early single pole to double pole switch (or vice versa) it needs to be wired accordingly.

My car came with a separate ground wire at the MC input - this is the only reason I switched from the single pole unit (which has an obvious button to push) during installation, besides already having one on hand.

So, as for the single pole switches like I posted above and you posted in the other thread (showing MC internal workings) - how do they get reset?

[Superhawk996]

I’d say ‘don’t be surprised if there is no reset button to push’ … I have 3 old 914 master cylinders and none of them have a reset button. Am I missing something? They all have some type of a square plastic “tower” where the spade(s) are … is that somehow the reset “button”? I lightly tried prying the plastic block off on one and that didn’t seem to be the case (IMG:style_emoticons/default/confused24.gif)

Not all switches have the latching function and the reset button. The later model two pole switches are not latching - when there is a problem in the master cylinder the pressure differential between the hydraulic circuits will cause the light to turn on.

If you’ve converted from an early single pole to double pole switch (or vice versa) it needs to be wired accordingly.

My car came with a separate ground wire at the MC input - this is the only reason I switched from the single pole unit (which has an obvious button to push) during installation, besides already having one on hand.

So, as for the single pole switches like I posted above and you posted in the other thread (showing MC internal workings) - how do they get reset?

@literati914

Bear with me I’m not trying to be an (IMG:style_emoticons/default/bootyshake.gif)

Push the button!

Having said that, not all single pole switches have the button. This means they are non-latching and will “reset” themselves (like the 2 pole switches) when the underlying pressure imbalance within the master cylinder is corrected.

Here’s the thing - these are 50 year old cars that have been (IMG:style_emoticons/default/sheeplove.gif) for decades. It’s not possible to tell exactly what you have. There were so many variations on these switches across OEMs (and now the Chinese aftermarket (IMG:style_emoticons/default/headbang.gif) ) it’s hard to say exactly what you have. At some point, you’re going to have to use a DMM to understand what switch you have and to troubleshoot your particular car - including the wiring.

In an effort to be helpful - again not trying to be an ass but I know I come off that way, it isn’t my intent. (IMG:style_emoticons/default/beerchug.gif)

Here is the pic you posted:
Single pole - no reset switch installed in master (none)

Single pole, new looking — appears to have reset but you’ll need to test with DMM (circled). You likely need to remove the black nipple or cover depending on how solid that cover is. That should expose the reset button assuming that is what’s there.



Here are other examples:


I’ve seen that button also be red, and a faded red that looks brown.

Here’s another that looks modern and like what you posted. picture came from one of the Euro parts sites.



The one I posted is a single pole non latching switch - the same as the switch shown in one of your master cylinders marked as “none”.



A DMM is your friend when working with an unknown. Play with the switch in hand to understand what you have and to see how it’s working.

[Literati914]

.. They all have some type of a square plastic “tower” where the spade(s) are … is that somehow the reset “button”? I lightly tried prying the plastic block off on one and that didn’t seem to be the case (IMG:style_emoticons/default/confused24.gif)

In the pic below, the separate warning switch does actually have a button (under a little cover that's meant to be temporarily removed). I switched it out..

-I tried to be clear here ^^ that I knew the separate new single pole (that I removed) does indeed have a reset under that tiny black cover - it's obvious on that one.

Also, you're last post answered the first part of my question/statement above (plastic tower) ^^ , thanks as always.

@Superhawk996 wrote: "..Not all switches have the latching function and the reset button. The later model two pole switches are not latching .."

-OK, that's good to know, thanks. Were there EARLY two-pole switches with manual buttons? Anyone got a picture of one? So probably lots of 914s with two wires to the MC would not have a button to push, by default. Which jives with my statement to the original poster, not to be surprised if there is NO button to push.

Superhawk996 wrote: "..If you’ve converted from an early single pole to double pole switch (or vice versa) it needs to be wired accordingly."

-Well Duh! ( (IMG:style_emoticons/default/biggrin.gif) ), I mean this is THE REASON I switched "back" to a two pole switch (because it matched the original wiring).

Superhawk996' wrote: "..Bear with me I’m not trying to be an (IMG:style_emoticons/default/bootyshake.gif)

Push the button!"

-But see what I mean? - I'm hearing it again while there is actually NO button to push! (see three random examples above). I assume you mean manually of course. Since my original wiring included two wires (+/-) I would have had a two pole switch... which probably didn't have a manual button to push, if I'm understanding correctly.

Superhawk996 wrote: "..In an effort to be helpful - again not trying to be an ass but I know I come off that way, it isn’t my intent. (IMG:style_emoticons/default/beerchug.gif) [quote]
-Of course I understand..me too, certainly not intended.. this is all very informative.

[Superhawk996]

Were there EARLY two-pole switches with manual buttons?

@literati914

I have never seen a two pole switch with a reset. Not an authoritative answer but a data point.

Why so many switches and ambiguity? The FMVSS regulation regarding the “Brake” warning has always been vague and can be met in different ways. There was never a requirement to have the latching & resettable switch. This is something the Germans choose to do.

As noted by this post (and others), the latching switches cause a lot of confusion.

I would speculate that VW moved away from the reset button switches both as a cost savings as well as a means to simplify the system.

[Lucky9146]

Finding this thread helpful in diagnosing my own situation. Unfortunately I have nothing to contribute with the more knowledgeable weighing in. Thanks for all the info!
(IMG:style_emoticons/default/driving.gif) (IMG:style_emoticons/default/white914.jpg)

[Gatornapper]

Finally got car on lift to find that there was no reset and sensor had strange resistance of about 300k ohms.

Disconnected it and yes light finally went out.

Someday I'll replace the sensor, don't see it as urgent need.

Thanks to all for help on this.

GN

[Spoke]

Finally got car on lift to find that there was no reset and sensor had strange resistance of about 300k ohms.

Disconnected it and yes light finally went out.

Someday I'll replace the sensor, don't see it as urgent need.

Thanks to all for help on this.

GN

Good to hear. Sounds like the switch is electrically leaky.