Master Cylinder wiring harness bypassed
07-29-2012, 01:16 PM
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Master Cylinder wiring harness bypassed
While putzing around on my new-to-me '81, I found some random wires connected to the unused relay ports in the fuse block. As the wiring in this thing appears to be modified by a previous owner, (the seller blamed it on the guy before him..whatever) I decided to trace it and was a little concerned when I found it's destination. The first pic. is a homemade plate attached to the brake pedal arm from which the red wire goes to the relay panel. From there I traced the white wire to where it is splice into a harness that should be attached to the master cylinder. Both harnesses are disconnected as indicated in the 2nd pic. There are no warning lights, and the brakes and brake lights work. Any idea what I have here?
07-29-2012, 08:16 PM
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Those switches went out on mine a few years ago. Brake warning light went on with no problems. I think about $35 a piece. Replaced them. No more warning light. Maybe previous owner didn't want to spend the money?
07-29-2012, 09:48 PM
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That's the thing, I have no warning light. I'll try disconnecting and see what happens. Hopefully my brakes will still work. If not, know where I can get a good deal on a front bumper?
07-30-2012, 12:13 AM
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Your PO has replaced the brake light/brake pressure warning switches with a simple brake light switch, as used on the later cars, albeit a crude implementation.
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 switches do not affect whether (or how) the brakes work. They only turn the brake lights on, and set a brake pressure warning if on brake circuit has more pressure than the other.
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 switches do not affect whether (or how) the brakes work. They only turn the brake lights on, and set a brake pressure warning if on brake circuit has more pressure than the other.
07-30-2012, 04:15 AM
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I can understand why someone did this. The are always fail. They were so bad that porsche changed the design to operate directly off the brake pedal. You could come up with a hybrid design that might be a better solution than the stock one and still give you the warning system.
