1968 911 master brake cylinder warning switch

[helidoc]

I am replacing my 1968 911 master cylinder and the new one came with a single pole switch. My old one has a 2 pole switch and there a 2 wires going to it, one brown and the other brown with white stripe. The factory manual does not show this switch and the wiring diagram only shows a brown with white stripe wire going to the emergency brake handle switch. I have seen some articles on the internet indicating to use the brown/white wire only and not use the brown wire since the latter is a ground wire (those authors appeared to be upgrading from a single to two circuit master cylinder on earlier cars). My thought there is how would the switch get grounded in that situation an does that ground wire work with another circuit. I tried to use my old 2 pole switch in the new master cylinder but, with a volt meter the switch is always on (closed) which I suspect is from the the pin being too long. I thought about using a banjo type gasket but was concerned about leakage since the switch uses an o-ring and was not sure if I could find one thick enough. Anyone else had this issue and any suggestions?

[ed mayo]

Try using a piggy back spade and attach both wires to the single spade pole.

[helidoc]

Thanks for replying Ed. I guess though that I am having a hard time with that logic. What would be the difference of just hooking up both wires without even using the switch? Isn't the switch there to either have an open or closed circuit?

[timmy2]

New switch body is grounded to the master cylinder through the switch case and thus to the body.
Old switch body was isolated from the case ground and required the brown wire to go to the body ground stud nearby.
Just connect the brown/white to the switch and ignore the brown.

It is a US option for the warning light, and didn’t make it into some factory drawings.
It runs in parallel to the handbrake switch.
Connecting the two wires together at the terminal would cause the lamp to always be on.

[Arctic Rat]

Helidoc,

Just upgraded my 66 single circuit MC to a dual circuit. Original (66) 2 pole switch, the New MC came with a single pole switch. Just replace the single with a duel, about $50 bucks with shipping. The "Porsche" part is over a $100!!!!!

ATE Brake Light Switch


No heavy lifting, unscrew single pole, screw in 2 pole switch.

Joe D

[Arctic Rat]

Original single pole from MC in my hand, new 2 pole installed.

Joe D





From parts store I ordered MC from

"Includes a single pole pressure switch. For models with a two pole switch, also use (1) 901 613 115 01, or reuse switch from original unit."

[helidoc]

Arctic Rat, did you not find the plastic pin in the new 2 pole switch to be to long causing the circuit to be closed when in the master cylinder?

[Arctic Rat]

Helidoc,

Neither the single pole nor the 2-pole switch had a plunger on it. My original switch from the single circuit MC also does not have a plunger on it. Different style switch? My original MC setup (single circuit) the switch was in a fitting on the line, NOT IN THE MC. I will confirm the switch is not "CLOSED" as mounted with a meter tomorrow.

Joe D

12/6 0840

****CORRECTION: The single pole switch DOES have a plunger. The new 2 pole and my original 2 pole DO NOT have plungers.*****

[helidoc]

Well the plot thickens. I decided I would use the brown/white wire on the single pole switch and the brown wire I would put a connector to one of the MC studs to ground. When stripping back that wire I found out that it was brown and white too. I put a ohm meter between both wires and also checked each wire separately to ground and the meter went to 0 on all. Confused, I then realized I had the E brake on so I released it and checked again. Everything went to 0 again on the meter except the one brown and white wire I thought was to ground, which stayed at 1. This would probably be consistent with timmy2's post.

I next measured the 2 and 1 pole switch plastic plungers at rest and depressed to close switch circuit and made a spreadsheet to figure out the difference between the two with various measurements with and without an ohm meter. Basically the hole depth of the master cylinder is about 19.74 mm (for the new unit that came with the single pole switch). The single pole switch plunger is depressed about .74 mm in place and requires about another 1.02 mm depression to close the circuit. The 2 pole switch in this master cylinder is depressed 1.51 mm and the amount of depression to close the circuit is 1.48 mm or -.03 mm. That is why the circuit is closed all the time with the 2 pole switch. You need about a 1.05 mm spacer/washer to match the single pole switch function. Therefore one option is to use the single pole switch with the correct brown/white wire to the switch and maybe ground the other wire to the MC case through the mounting studs or another ground point. Alternatively you can use the 2 pole switch with both wires going to it with a 1 mm (1.05 mm) sealing washer so it does not leak. I am not a 100% sure on this as I don't know how it will effect the E brake circuit. Maybe you don't want that 2nd wire going to ground all the time.Very confusing especially without a wiring digram. Any thoughts?