[T3] DOT 5 and brake light switches

Jim Adney jadney at VWType3.org
Wed Nov 18 13:08:32 PST 2015


On 18 Nov 2015 at 15:29, Sean Bartnik wrote:

> One other possibility for the apparent failure I guess would be the
> possibility that sometime after installation, an air bubble worked
> itself into the switch and was preventing the proper transfer of
> hydraulic pressure to the diaphragm in the switch. That's the only
> other thing I could think of that might cause the same symptoms if
> indeed the switch itself was good. 

An air bubble would not have the effect you describe. I'm afraid this 
is a common misunderstanding.

The pressure at both "ends" of a bubble is the same, so whatever 
pressure is applied to one end will be transferred to the other end.

The problem with air bubbles is that they are compressible, so it 
takes more stroke of the pedal to develop a given pressure in a 
system that has bubbles in it. If the total volume of bubbles is 
large enough, the pedal will go all the way to the floor before 
significant pressure is raised. In your case, the brakes worked, so 
you know the pressure was sufficient.

In case there's any question, here's how to test the dual brake 
switches in a tandem system:

Get a second person to watch your taillights, or back up to a wall so 
you can see their reflected light. Remove the connecting plug from 
one switch and apply the brakes. If the brake lights come on, the 
switch that's still connected is good.

Reattach the removed connector and remove the other one. Repeat the 
test. If the brake lights come on, the connected switch is good.

The switches are redundant, so if either one is good the brake lights 
will work. So if the brake lights don't work on a tandem system, both 
switches are bad.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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