Brake Pressure Switches/Warning Light
10-19-2010, 04:39 AM
#16
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Sorry about the newbie mistake:
Dr. Bob, ... 1980 5Spd.
I noticed the scanned document, ref: disconnecting the battery for the 'reset'. This is the procedure I performed ( I generally disconnect the wing nut neg-pole, located by the tool kit).
Wally, ... Thanks for the theory of operation, very helpful.
Don
Dr. Bob, ... 1980 5Spd.
I noticed the scanned document, ref: disconnecting the battery for the 'reset'. This is the procedure I performed ( I generally disconnect the wing nut neg-pole, located by the tool kit).
Wally, ... Thanks for the theory of operation, very helpful.
Don
Up thru 1983, the brake lights are operated by twin pressure switches on the bottom of the brake master cylinder. From 1984 - up, the brake lights are operated by a simple plunger switch mounted on the brake pedal under the dash.
The earlier brake light switches do double-duty, operating both the brake lights and the brake pressure warning signal. These switches can cause some problems.
The switches are identical - one in each hydraulic circuit. They are single-pole, double-throw switches. This means that you have only one circuit (single pole) thru each switch, and that the switch chooses one of two outputs (double throw).
Each switch has three connections: power, central warning computer, and brake light.
Not Activated
There is a power feed into each switch. When the switch is not activated (no pressure in the brake circuit) the power isn't hooked to anything.
When the switch is not activated, the central warning computer and the brake lights are connected inside the switch. Nothing happens, since there is no power applied on either circuit.
Activated
When the switch is activated by brake hydraulic pressure in one of the circuits, the central warning computer connection is broken, and the brake lights are hooked to the power feed. This removes the central warning computer from the circuit, and puts power on the brake lights.
(This might be easier to visualize if you think of three wires: one for 12 vdc, one to the brake lights and one to the warning computer. The lead that goes to the brake lights is connected to the one to the warning computer until brake pressure pushes it off of that wire and over to the one going to power.)
All three leads go to both switches.
When you apply the brakes, both switches should activate. Both switches disconnect the warning computer and connect the power to the brake lights. The brake lights come on, and no signal is sent to the warning computer, so it is happy.
If only one switch activates (either no pressure on one side of the brakes, or a bad switch), then things change.
On the switch that is not activated, the brake light circuit is still hooked to the warning computer.
On the switch that activates, the brake light circuit is hooked to 12 vdc.
The brake lights come on.
In addition, power flows on the brake light circuit back to the non-activated switch. Since in this switch the brake light circuit and warning computer are hooked together, 12 vdc is applied to the warning computer circuit. This set the Brake Pressure Warning light.
If you have a bad brake light switch, I strongly suggest that you replace both switches.
(P/N 113 945 515 G)
The earlier brake light switches do double-duty, operating both the brake lights and the brake pressure warning signal. These switches can cause some problems.
The switches are identical - one in each hydraulic circuit. They are single-pole, double-throw switches. This means that you have only one circuit (single pole) thru each switch, and that the switch chooses one of two outputs (double throw).
Each switch has three connections: power, central warning computer, and brake light.
Not Activated
There is a power feed into each switch. When the switch is not activated (no pressure in the brake circuit) the power isn't hooked to anything.
When the switch is not activated, the central warning computer and the brake lights are connected inside the switch. Nothing happens, since there is no power applied on either circuit.
Activated
When the switch is activated by brake hydraulic pressure in one of the circuits, the central warning computer connection is broken, and the brake lights are hooked to the power feed. This removes the central warning computer from the circuit, and puts power on the brake lights.
(This might be easier to visualize if you think of three wires: one for 12 vdc, one to the brake lights and one to the warning computer. The lead that goes to the brake lights is connected to the one to the warning computer until brake pressure pushes it off of that wire and over to the one going to power.)
All three leads go to both switches.
When you apply the brakes, both switches should activate. Both switches disconnect the warning computer and connect the power to the brake lights. The brake lights come on, and no signal is sent to the warning computer, so it is happy.
If only one switch activates (either no pressure on one side of the brakes, or a bad switch), then things change.
On the switch that is not activated, the brake light circuit is still hooked to the warning computer.
On the switch that activates, the brake light circuit is hooked to 12 vdc.
The brake lights come on.
In addition, power flows on the brake light circuit back to the non-activated switch. Since in this switch the brake light circuit and warning computer are hooked together, 12 vdc is applied to the warning computer circuit. This set the Brake Pressure Warning light.
If you have a bad brake light switch, I strongly suggest that you replace both switches.
(P/N 113 945 515 G)
Last edited by pcarsrule; 10-19-2010 at 05:06 AM.
10-22-2010, 11:55 AM
#18
Chronic Tool Dropper
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You'll want to use the correct flare nut wrench on the tube fittings. A standard open-end wrench will round the nuts before they come loose. Otherwise it's pretty straightforward with normal sockets/wrenches. Oh-- get a dedicated garage-only turkey baster from the grocery store if you don't have one already. Use it to pull the fluid out of the reservoir before you disconnect anything. Tape a plastic garbage bag over the fender before you start work, and stuff a garage bath towel underneath the work area to catch parts and dribbles. Brake fluid eats paint. (!!!)
06-22-2011, 10:29 PM
#19
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do yo really need to remove the resevoir and or master cylinder to replace the brake light switches?or can you just remove switches while master is in place?if so is there a tool that will do the job correctly.mine is an 82 model
06-23-2011, 01:34 AM
#20
Rennlist Member
I have replaced the switches in place. 24mm box wrench is best, with electric connectors off. Seal the reservoir cap with gladwrap or similar - if air cant get in, it minimises loss of fluid out the bottom. Do one switch at a time, only tighten lightly (as above - casting is brittle). It can be difficult to get the wire connectors back on, but keep trying. Once fitted, check reservoir (remove gladwrap), kick the pedal around some if soft - I did mine and didnt need to bleed the system, just let the air bubbles work their way to the top.
jp 83 Euro S AT 53k
jp 83 Euro S AT 53k
06-23-2011, 11:47 AM
#21
Track Day
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I did not remove the MC when I replaced the switches in my 79. It's been a while, but what I do remember is that the wrench swing was so small, that I put pillows on the fender to lie on because it took so long. It went well, but took a lot of patience. The electrical connectors were more than a little annoying too. With the vent blocked, little if any fluid comes out and bleeding is not needed. On old machines, I tend to do things the hard way if it means less things get disturbed, even if it takes longer, because I don't want to break something else. Removing the MC might be quicker, if all goes well. (I've heard don't use a pressure bleeder if you have an original blue hose.)
06-23-2012, 04:42 PM
#22
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I was driving in a thunderstorm yesterday, and pulled over to fill up with non ethanol fuel.
When I started up, the Brake Pressure light and central warning light ! started to flash. Brake fluid was topped up and brake pedal felt normal.
I disconnected battery as suggested here and in the owners manual. Started up again and warning lights were not active any more...
This solution lasted 24 hours and last night while driving both warnings began flashing again...guess I will be buying a couple of these switches!
When I started up, the Brake Pressure light and central warning light ! started to flash. Brake fluid was topped up and brake pedal felt normal.
I disconnected battery as suggested here and in the owners manual. Started up again and warning lights were not active any more...
This solution lasted 24 hours and last night while driving both warnings began flashing again...guess I will be buying a couple of these switches!
Last edited by david_alford; 06-24-2012 at 10:10 AM. Reason: More info
06-29-2014, 09:27 AM
#24
Team Owner
06-29-2014, 09:35 AM
#25
Rennlist Member
They are a VW/Audi part , quite common and cheap. Do a search here for the part no....I bought a pair on ebay recently - the boxes have 'SLS87' hand written on them; the switch is stamped 113 945 515 8 as best I can make out its pretty small ! Maye from Standard Motor Products - they are often on ebay. Some seem crazy cheap - check with a VW parts place in Ca maybe.
jp 83 Euro S AT 55k
jp 83 Euro S AT 55k
