74 R90/6 Master Cylinder Switch

[mschmitz57]

My 74 R90 project has no wiring running to the hydraulic switch on the master cylinder. I assumed the P.O. removed it at some point. The front brake lever has a small mechanical switch built into the perch (the 74's have old style /5 controls).
The front brake light works just fine. So what is the purpose of the hydraulic switch mounted on the M/C ?

-Mark

[melville]

My 1975 /6 has wires in the harness for both the MC switch and for a brake lever switch. I think they wanted to make only one harness work on drum and disc bikes.

It's interesting that the wires have disappeared from yours. If you're looking for them, I'd expect to find green wires, probably one with a red tracer and one with a black tracer.

[RPGR90s]

The Master Cylinder switch is attached to a float under the cap that triggers a light on the instrument cluster (Brake Failure) if the brake fluid drops to a certain level.
Assuming you have the stock instrument cluster, then the removal of the switch, or disconnecting the two wires at the switch would not light the lamp since the circuit is permanently open.

[mschmitz57]

No, I'm referring to the hydraulic pressure switch that threads onto the front of the master cylinder.
Yes, My bike has the wiring for the brake fluid float level sensor.

-Mark

[jagarra]

Parts fiche calls it a brake light switch, must be part of circuitry operating brake lights. Interesting that yours has no wires attached and brake light operates when front brake applied.

[Rob Frankham]

The hydraulic switch on the master cylinder is a brake light switch... so is the one on the handlbar unit. I've no idea why you have two... maybe the master cylinder or the handlebar unit has been replaced at some time.

As far as I can tell from the parts lists (and I accept that they're not always 100% accurate), the /6 models with disc brakes used the hydraulic switch on the master cylinder while the drum braked /5 and /6 models used the handlebar mounted switch... where a front brake light switch was fitted at all.

It's possible that the front handlebar unit remained the same for all models and that no switch was fitted to disc braked models, in which case the switch might have been retrofitted following a failure of the hydraulic switch... but there I'm, theorising well beyond the available data.

Rob

[RPGR90s]

"My 74 R90 project has no wiring running to the hydraulic switch on the master cylinder. I assumed the P.O. removed it at some point. The front brake lever has a small mechanical switch built into the perch (the 74's have old style /5 controls).
The front brake light works just fine. So what is the purpose of the hydraulic switch mounted on the M/C ?

Sounds like the M/C was changed since /6 disc brakes (including the dual disc R90s) only have the float switch on the master cylinder. Later models had the brake switch and float switch on the M/C. Unless you're looking for 100% authenticity, and the brake light is working, probably no need to change it.

[jagarra]

I agree with RPGR90S. The original 14mm under tank on my bike did not have a switch on the M/C, just the switch on the handlebar. The handlebar controls changed in 1975, so possibly later M/C had the switch moved from the handlebar to the M/C. When I converted to dual disks, I got a M/C from a 1974 S model, it too did not have the switch on the M/C and was 16mm not the later 17mm.

[mschmitz57]

I noticed on one of Brooks videos that the brake pressure switch wiring comes out of the main harness at the same point as the m/c float switch wiring, but my harness doesn't have any wiring for the pressure switch. I thought I might have some remaining wiring cutoff from this point. This is clearly a transitional model year. My original m/c had a pressure switch which I think was original to the bike. I replaced the switch with new during the restoration. I was thinking the dual front brakes switches might just be redundant. I'm surprised BMW didn't just install a block-off plug and save a few deutschmarks on the switch. Part of the appeal of this bike for me is the /5 switch gear with it's minimal buttons. Wondering if the later bikes still had front brake switch built into the brake lever perch and only used the pressure switch.

h2lYKM63S2KnrIskMaQwZw.jpg (293.72 KiB) Viewed 495 times

[melville]

I noticed on one of Brooks videos that the brake pressure switch wiring comes out of the main harness at the same point as the m/c float switch wiring, but my harness doesn't have any wiring for the pressure switch. I thought I might have some remaining wiring cutoff from this point. This is clearly a transitional model year. My original m/c had a pressure switch which I think was original to the bike. I replaced the switch with new during the restoration. I was thinking the dual front brakes switches might just be redundant. I'm surprised BMW didn't just install a block-off plug and save a few deutschmarks on the switch. Part of the appeal of this bike for me is the /5 switch gear with it's minimal buttons. Wondering if the later bikes still had front brake switch built into the brake lever perch and only used the pressure switch.

h2lYKM63S2KnrIskMaQwZw.jpg

No switch on the lever on 1975 R75/6, but one could be bolted and wired right up.