Problem with 2-pole brake warning switch in 914R MC, Solved - I think... Options

[bbrock]

Long story short, I had the phantom relay clicking from the day I put my car together. I traced it to the brake warning switch on my 914R master cylinder. I did make sure the reset button was pushed in but the relay still constantly clicked without lighting the warning light unless I disconnected the switch.

I bought a genuine Porsche 2-pole switch and finally got around to installing it, but now the warning light flashes all the time. The switch works - closing contacts when the plunger is depressed about 1mm. The problem is that screwing the switch fully into the MC depresses the plunger enough to close the contacts and activate the warning light. Backing the switch out about a turn is enough to open the contacts again.

I'd like to understand how the switch plunger is actuated under low pressure condition to make sure I don't defeat the warning system with the wrong "fix." I can think of two possible cause/fix scenarios:

Best Case - Need a thicker O-ring. The 914R MC/switch did not include an O-ring so I grabbed what I had on hand that fit the diameter of the switch base, but is rather thin. I don't know the correct size for the OE O-ring, but the PET has a note for the part that reads "12,42 - 1,78" If that means 12.42mm I.D. x 1.78mm thickness, then that would likely space the switch back far enough that the plunger isn't depressed. Easy peazy!

Worst Case - looking inside the switch port I can see a small bore in the center about the diameter of the switch plunger. I have a sinking feeling that the plunger is suppose to fit inside this bore. If so, it means the bore in the MC is too small and the warning system is not functional.

Either scenario might also explain the phantom relay issue because it is possible pressing the reset pushed the plunger in far enough to shut off the warning light, but not far enough to completely open the switch contact and allow enough current to leak to ground to run the relay constantly. The 2-pole switch has the self resetting spring loaded plunger when might not go in as far as pressing the single-pole reset.

I'm really hoping it is the "best case" scenario. Otherwise, it raises the question if the warning system is working properly on any 914R MCs. Has anyone actually tested the low pressure warning on these MCs by having an assistant crack open a bleeder while the brake pedal is pushed and ignition on per the FM procedure? If "worst case," I'll have to bite the bullet and do what I should have in the first place - buy an ATE MC. (IMG:style_emoticons/default/dry.gif)

Any thoughts?

[peteinjp]

I'm interested in this too. Maybe you could use a thicker crush washer instead of the o-ring?

I have developed an issue where my brake warning light stays on. 3/4 of the time the car is on. Upon inspection I do not see any leaks ( a very slight leak at the rear caliper adjuster but...) so its hard to imagine any other cause than the switch itself. Not to derail your thread but- when you replaced the switch did you need to bleed the whole system again? Or can one just pull it out and throw in a new one and be good to go? Mine should be good as it is low miles on a 21mm ATE master.

Pete

[bbrock]

I'm interested in this too. Maybe you could use a thicker crush washer instead of the o-ring?

I have developed an issue where my brake warning light stays on. 3/4 of the time the car is on. Upon inspection I do not see any leaks ( a very slight leak at the rear caliper adjuster but...) so its hard to imagine any other cause than the switch itself. Not to derail your thread but- when you replaced the switch did you need to bleed the whole system again? Or can one just pull it out and throw in a new one and be good to go? Mine should be good as it is low miles on a 21mm ATE master.

Pete

A crush washer was my first thought but without knowing how the switch is actuated, it would be a gamble. It would stop the warning light from flashing, but may be the equivalent of disconnecting the switch completely. Not something I'm willing to accept..

You do not have to bleed the system when replacing the switch (I had the same question some time ago). There should not be brake fluid in the switch port.

@Mikey914 As this is a potential safety issue, it would be good to know if the low pressure warning system has been tested on 914R MCs configured as shipped (single pole switch with no O-ring) using the FSM procedure. If so, that would quickly rule out the MC itself. Perhaps something as simple as installing the correct O-ring is all that's needed to position the switch for correct function. I'm pretty sure 12.42mm x 1.78mm is the correct O-ring size as that seems to be a common size. Bottom line is that I (we) need to be confident the warning system is working properly.

BTW, I've confirmed that the switched that came with my MC was defective. It reads ~5Mohms regardless of plunger position so never operated correctly and leaked enough current to ground for the phantom flasher clicking.