Tuesday, April 3, 2012

Brake Light Switch

Next on the agenda was sorting out the brake lights. See, they only come on about 60% of the time, and since this bus was engineered long before Ralph Nader ever thought about publishing 'Unsafe at Any Speed", and it is likely therefore to explode into a bazillion pieces were it to be involved in an accident from the rear, brake lights would be good, as opposed to not having them at all.

So under the bus I go, under the front this time, to right below the pedal assembly. There, there is a belly splash shield that covers the underside and inner workings of the pedals. That pan needs to be removed so I can get to the master cylinder to get to this...

This sucker is the brake light switch. It literally threads right into the rear end of the master cylinder and functions as a simple pressure switch. Push on the pedal, and the increased fluid pressure in the master cylinder closes the switch, makes the contact to close the light circuit, and activates the brake lights. When it fails, the pressure in the master cylinder doesn' succeed in closing the circuit and the lights don't work. You can tell that the problem is in the switch by shorting across the two terminals with a screwdriver and calling back to a buddy at the rear of the bus and confirming that the lights do work when you bridge the terminals. If they don't then it aint the switch. It usually is though.

Of course, because this is a pressure switch, you are opening the brake master cylinder and introducing air to a closed system. Not good. Usually you will need to bleed the brakes after opening the system, but here's a short cut...

Take the switch and fill the end of the switch with fluid so that there is no air in the switch, then loosen the old switch from the master cylinder. Then when the old switch is barely hanging on, have the new switch in the other hand to swap them one for the other, like Indiana Jones swapping the bag of sand for the golden idol in the opening scene of Raiders. The faster you are the better, and if you do it right, you will only lose a few drops of fluid, and therefor only introduce a minimum of air into the system. Minimum is good.

Tighten the switch onto the master cylinder, dress the contacts of the switch with dielectric grease, and reattach the two contacts. Have your buddy confirm the lights work again, and then also check the pedal to make sure it is still firm. If it goes to the floor, you did something horribly wrong.

If you really did something wrong, your car will careen out of control like the big stone ball that chased Indiana Jones down the mountain when he screwed up the sand-for-idol switch.
A shot of the switch, here threaded into the end of the master cylinder, in this case, an mc from a Type 3 VW.

Brakes, sorted.

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