R100RS frnt brake switch problem

Hi guys,
New to me '81 R100RS - it got through the tough Mass inspection, but barely. The front brake switch was sticking on periodically. Now it's stuck on all the time. I'm guessing it's the switch, but there's a bit of equipment that isn't like my R90/6 - hoping someone can explain before I go messing it up.

It has twin front rotors, so two hoses from the right grip into the junction under the tank. Then a switch (two leads grn/bk and grn/red) and two metal hoses out to the brake calipers. If you pull the leads the light goes out (of course). If I put the multi meter on the leads, I expect to see a switch on/off from the brake. But it just shows a solid signal. Not sure why.

And I think the piece above is the cable (?) that activates the hydraulic system. I need to take that off to get the switch. I hope brake fluid doesn't come squirting out.

Rear brake works as normal.

Any suggestions welcome.
thanks

plehman said:

It has twin front rotors, so two hoses from the right grip into the junction under the tank.

Click to expand...

It should be a single hose from the right grip to the junction.

plehman said:

Then a switch (two leads grn/bk and grn/red) and two metal hoses out to the brake calipers.

Click to expand...

The green/black is the fused switched power. the green/red goes from both switches (front and rear) to the brake light. The two switches are wired parallel. See schematic here...

https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxibXdyMTAwN3R8Z3g6NDc1YWI2M2NlMmQzNDJhNw

plehman said:

If you pull the leads the light goes out (of course). If I put the multi meter on the leads, I expect to see a switch on/off from the brake. But it just shows a solid signal. Not sure why.

Click to expand...

Not sure what you are checking. You should not be checking across the leads (wires) for continuity. If you short the green/black and green/red together, the brake light should energize if the ignition is on. You can check continuity with the meter and test to see that the switch closes properly (with the two wires removed) when the front brake is applied.

plehman said:

And I think the piece above is the cable (?) that activates the hydraulic system. I need to take that off to get the switch. I hope brake fluid doesn't come squirting out.

Click to expand...

When you change the pressure switch, you will lose a bit of fluid and possibly entrain a bit of air. So, you should take care to capture the leakage so it doesn't ruin paint and be prepared to bleed the system if necessary. Is should not be under pressure and start "squirting out" unless the brake system is still under pressure. In this case, your front brakes would be dragging - possibly because the master cylinder wasn't venting the pressure properly. That could also keep the brake light energized but I would expect that you would have noticed the condition.

Yes, the grn/red shows on the meter only when I step on the rear brake. That seems correct.

The grn/bk however, is always on (positive probe in the connector, grounded, key on). I expected power to that lead only when pulling on the front brake (unless it's a switched off situation).

Now I see the single hose in (duh). I just don't understand what the two things out the back of the brake junction do? There's a big connector (that's what I didn't want to remove).

I'll order a switch from Max tomorrow.

thanks a lot for the response.

plehman said:

Yes, the grn/red shows on the meter only when I step on the rear brake. That seems correct.

The grn/bk however, is always on (positive probe in the connector, grounded, key on). I expected power to that lead only when pulling on the front brake (unless it's a switched off situation).

Click to expand...

The grn/blk should be hot all the time. The switch is normally open and brake pressure closes it, continuing the power to the brake light. The grn/blk and grn/red are wired parallel to both brake light switches. Closing the rear switch powers the entire grn/red wire - including at the front switch. This is correct.

Now I see the single hose in (duh). I just don't understand what the two things out the back of the brake junction do? There's a big connector (that's what I didn't want to remove).

Click to expand...

A close-up pic with a pointer please. Not sure what you are looking at.

beemerphile said:

The grn/blk should be hot all the time. The switch is normally open and brake pressure closes it, continuing the power to the brake light. The grn/blk and grn/red are wired parallel to both brake light switches. Closing the rear switch powers the entire grn/red wire - including at the front switch. This is correct.



A close-up pic with a pointer please. Not sure what you are looking at.

Click to expand...

It's just above the switch.

That looks like the ignition control unit and is unrelated to your repair. The switch you need is under it. If it has been more than a couple of years since the brake fluid was flushed, this would be a good time to do it. If you take a syringe and empty the fluid reservoir before opening the switch, it will reduce leakage. Place a heavy towel over the frame rail. Get a new sealing washer if one does not come with the switch. Work quickly. Don't pull the lever while the reservoir is empty. If some air gets in when you change the switch, then refilling the reservoir and tapping the hose with a wrench is usually sufficient to bubble the air up and out of the line.

BTW: If the rubber hoses are original, they are probably in need of replacement as well. Lots of joy bleeding the air out of new hoses.

Switch on order.

To confirm I messed with the switch and noticed if you give it a tap it shuts off. And the fluid is dark and not nice looking. So thanks for the advice.

And as you say the mystery thing is just an unrelated control unit (unscrews to get out of the way).
Will be changing the fluid and the switch (which sure acts faulty) using your method.

Nice to know going in to the project.
Thanks again.

plehman said:

Switch on order.

To confirm I messed with the switch and noticed if you give it a tap it shuts off. And the fluid is dark and not nice looking. So thanks for the advice.

And as you say the mystery thing is just an unrelated control unit (unscrews to get out of the way).
Will be changing the fluid and the switch (which sure acts faulty) using your method.

Nice to know going in to the project.
Thanks again.

Click to expand...

That mystery thing is your electronic ignition unit. It gets a signal from the bean can hall sensors and the output of that unit fires the coils. I have an 82RS with identical setup. While you are there it would be a very good idea to remove the unit itself from the large heatsink it sits on and clean up the interface between the unit and the heat sink. Once clean, apply some heat sink compound between the two. This is very important to do as a maintenance item as the stuff dries up and that unit requires good cooling.

As for your dirty brake fluid; it may very well be the reason your switch is failing or has failed. Brake fluid should be completely flushed every season to get rid of the water it absorbs from the air. Yes, it's a "sealed" system but moisture ingress occurs slowly anyway. I find sucking the dirty fluid out of the master cylinder and pushing new fluid from a new sealed container using a 60ml syringe with some tygon tubing shoved on the end works the best.

- Push the brake pistons all the way back into the calipers.
- Prime the syringe with fresh fluid and get it right to the tip then slip it over the bleeder screw.
- Open the bleeder and _slowly_ push new fluid up into the master.
- Close off the bleeder screw while still pushing to expel any air in the bleeder screw and threads.
- Suck out the master reservoir again.
- Repeat until that circuit runs clear fluid up to the master.

Now do the other side the same way. I find this way works better than trying to pump new fluid down to the brakes. Why? Bubbles like to rise in the lines. They do not like to be pumped downwards and also love to hide in the top of calipers. Pushing it up avoids this. Sometimes you still have air. If so, gently squeeze the brake handle and tape it or zip tie it and just leave it overnight. When the master cylinder is pushed in it allows some air to rise out of the lines. Gently pumping the lever will also help air rise sometimes. Do not pull hard or fast or brake fluid will squirt back up at you and all over the place if not careful.

Always have a spray bottle of water around or better yet do it outside and keep the hose handy. Brake fluid is water soluble and rinsing it off immediately from painted surfaces especially will save your bike from damage. Clean any spills immediately after bleeding. I rinse everything off just in case.

Oh yeah, if you don't have Spiegler teflon braided lines on your front RS brakes, get you some. Your front brakes will be rock hard. ;-)