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Fixing a faulty brake light switch

6 posts in this topic

Poor design is reason why the brake light switches keep failing.

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Old brown switch opened out. Fails due to housing or internal plastic bits swelling or deforming.

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Newer white brake light switch opened out. The white switch has spring with ground ends but no gaitor seal. There is sufficient clearance between internal plastic bits when switch is new but spring is not properly centered. Switch will malfuntion when spring becomes uncentred and starts touching internal wall of housing. Switch will also malfunction if housing deforms or internal bits swell.

Suggested fix:

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There is sometimes too much radial play between spring and above gray plastic piece causing spring to become uncentred. Adding two wraps of electro tape centres spring. Note locating pin at side. This means outer diameter of the gray piece can not be turned on a lathe.

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Spring is not centred at all where it contacts the black piece. A special 8.4 mm ID x 12.45 mm OD x 0.7 mm thick washer glued to the black piece centres spring. Kindly note that this washer is not readily available at Canadian Tire or other shops. Also recommended is turning down outside diameter of black piece by 0.2 mm. It may also be necessary to reduce outside diameter on the gray piece. Can only be done with a file since there is a locating pin at its side.

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Repaired switch fully assembled. Plunger should be free to move and there should be about 0.5 mm axial play. Plunger should be heard when shaking switch in longitudinal direction.

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How to fit and adjust brake light switch.

The switch comes with rather poor English language fitting instructions. You are better off following the German text or this guide.

1) Depress brake pedal about 2 cm. I use a special wooden T-stick not available at Lidl or other shops. See photo.

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2) Make sure switch is fully extended. Fit switch.

3) Fit a 3.6 mm thick setting gauge (Maßlehre) between switch and brake pedal lever.

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4) Remove wooden stick and slowly release brake pedal. A ratchet sound may just be audible. The sound is from the ratchet mechanism inside the switch indicating that the mechanism has self adjusted.

5) Remove Maßlehre and test operation of switch. Brake lights should turn on when brake pedal is depressed 7 - 12 mm.

Special 3.6 mm thick soft wood setting gauge (Maßlehre) is available upon special request for C$ 2 plus delivery. I wonder why Smart cannot not provide a setting gauge with the switch.

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Are we to activate the switch again when the removing and / or change it ?

There is no diagnostics action whatsoever. These switches are extremely poorly made. Its a shame they are all made by Bosch!Proves Bosch is synonymous with bad quality.
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Poor design is reason why the brake light switches keep failing.

Suggested fix:

Posted Image

There is sometimes too much radial play between spring and above gray plastic piece causing spring to become uncentred. Adding two wraps of electro tape centres spring. Note locating pin at side. This means outer diameter of the gray piece can not be turned on a lathe.

Plunger should be free to move and there should be about 0.5 mm axial play. Plunger should be heard when shaking switch in longitudinal direction.

It appears that this is one of those preventive maintenance things to do when convenient. Is it possible to turn a piece that replaces the locating pins by a step i.e do the "locating pins" do more than stop the spring from sliding higher on the grey piece? What is the red material? I don't see it on the other pictures.

Raymond

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It appears that this is one of those preventive maintenance things to do when convenient. Is it possible to turn a piece that replaces the locating pins by a step i.e do the "locating pins" do more than stop the spring from sliding higher on the grey piece? What is the red material? I don't see it on the other pictures.Raymond

The red material is electro tape. Just applied to centralize that end of spring. Most switches probably won't need this minor modification.I believe the locating pin can be cut off. It does not seem to serve any important function.The switch contacts themselves are pretty reliable. Main problem is plunger mechanism seizing or jamming. Switch contacts may burn out when plunger jams and preload on contacts is reduced.

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