I had no brake lights a week ago and changed the front ( easier to get to ) switch under the master. Brake lights worked great. Now I am back to NO Brake lights again. All other lights work fine.

Will ONE bad switch not allow Brake lights. i was thinking the 2 were redundant ( a back up ). The back one looks to be a bear to replace. I did find a short adjustable wrench made changing the front one easy.

 

They are a redundancy system, but instead of replacing them, you can install a later switch on the pedal and rerun the wires inside the car

 

The two brake light switches are not for redundancy...

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.

 

crap.


Thanks Wally...I will be removing the rear %$@# switch tomorrow....somehow.

 

Be verry, verry careful...

If you screw the switches in too hard, you will break the boss of the master cylinder.

 

Crap to the second power.

Will do. Thanks for the explanation.

 

LOL! Dean, have faith, man. After all you've been through, this is a very small bump in the road.

 

Thanks Bill....I will get through it. Just don't know what they were thinking putting those switches there in the first place. Way too close quarters. Almost as bad as the Hall sensor. My s4 is needing attention ( may be TT bearings ) now that Marvin is close.

 

Dean, keep us posted and good luck.

Wally, thanks for that great explanation. Are the switches prone to failing?

 

Replaced rear switch today...that makes both switches new...still no brake lights.

On a side note. I used a SHORT adjustable wrench and even the rear switch was easy to remove.

Where should i go from here. Fuses are good.

 

Test the switches with an ohmmeter and someone pressing on the pedal to confirm that they're operating.

 

Check the brake light power into and out of the light check unit. Don't forget that both bulbs could be bad...

 

Well crap to the third power....

Rechecked fuse 10 and while I was checking bottom to top the top dropped out. Went from 13 volts to 0...I moved it around and starting getting 13 volts consistently... Started and pressed brakes and the brake light in the dash went out like it should. SORRY folks....thanks for the help again but it looks like I just did not have a consistent good connection at fuse 10.


Now on to the last item...no speedo.

 

Clean the connectors in the spare tire well...