Date:         Mon, 9 Jun 2014 05:18:12 -0700
Reply-To:     Steve Williams <sbw@SBW.ORG>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Steve Williams <sbw@SBW.ORG>
Subject:      Re: Brake Light Troubleshooting Tips?
Comments: To: Mike B <mbucchino@CHARTER.NET>
In-Reply-To:  <5394FC14.3010901@charter.net>
Content-Type: text/plain; charset=UTF-8; format=flowed

On 6/8/2014 5:13 PM, Mike B wrote:
>    If one brake light switch fails and the other one still works, the
> brake warning light will not come on.

I studied the Bentley schematic for my model year carefully and
concluded that it will, depending on the switch failure.

The schematic shows the switches are SPDT, with their outputs wired so
the brake warning light will come on if one switch stays at its NC
(normally closed) position, presumably because it's not seeing hydraulic
pressure, while the other switch goes to its NO (normally open) position.

So if one switch failed so it stayed NC even with pressure, the brake
warning light would come on.  If one switch failed so as to leave both
outputs open, I agree there would be no warning light until the second
switch failed.

>    Jumpering one switch would not trigger the dash brake warning light.

I think it might, but I'd have to study the schematic again.

> The warning light on the dash is driven by the third terminal on the two
> switches ... The third terminal goes to ground when the master cylinder
> piston moves farther in the bore due to one or the other piston seal
> failing ...

I see nothing in the schematic about the brake light switches going to
ground.  Each switches 12V between their two outputs.

Hope I'm looking at the right schematic.

>    It's really not hard to reach the master cylinder and brake light
> switches, if you pull the entire instrument cluster, which is not very
> hard to do.

Thanks for the encouragement.

>    Start at the fuse and check for voltage that goes into one side of
> each switch, then out of the other side when you apply brake pressure.
> If that all checks out ok, then the wire traveling back to the
> tail-lights is open somewhere, possibly due to corrosion in a connector
> at some point.

That's what I was thinking.  The schematic shows another connector
somewhere between the switches and the taillight assemblies.