Brake Light Fuse Blows

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whc03grady
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Location: Livingston Montana
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Brake Light Fuse Blows

Post by whc03grady » Mon Jun 24, 2013 9:42 pm

Okay, this is Electricity 101. Or maybe Remedial Electricity. Whatever.
What's the difference, if any, between the brake light fuse blowing when the key is turned on vs. it blowing when the brake pedal is pressed? What does the one indicate as opposed to the other?
My problem is the fuse blowing when the key is turned on fwiw.
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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Amskeptic
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Re: Brake Light Fuse Blows

Post by Amskeptic » Tue Jun 25, 2013 8:45 am

whc03grady wrote:Okay, this is Electricity 101. Or maybe Remedial Electricity. Whatever.
What's the difference, if any, between the brake light fuse blowing when the key is turned on vs. it blowing when the brake pedal is pressed? What does the one indicate as opposed to the other?
My problem is the fuse blowing when the key is turned on fwiw.
Consult your Bentley wiring diagram.
The power for the brake light circuit comes from a fuse (#12?) that is only powered via the ignition switch. If you look at the upper end of the fuse in the diagram (letter "s" with a number) you will see its path eventually to a #15 coming off the ignition switch.

Blowing the fuse *before* actuating the pedal/brake light switches, is an indication that the electrical path between the fuse and the brake light switches has a short somewhere. This path goes through the warning light in the dash too. Carefully inspect the rat's nest between the fuse box and the warning light in the dash. Remember how we pulled the connectors off the master cylinder then re-plugged them in one at a time? Do that again with an experimental ignition-on at each re-plug to see if one or the other connector is responsible for blowing the fuse.

If you blow a fuse only upon pressing the brake pedal, start with the taillamp/stoplight bulbs and in the totally unlikely event that the problem is not there, work up towards the brake switches on the master cylinder.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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whc03grady
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Location: Livingston Montana
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Re: Brake Light Fuse Blows

Post by whc03grady » Fri Jun 28, 2013 10:20 am

Amskeptic wrote:
whc03grady wrote:Okay, this is Electricity 101. Or maybe Remedial Electricity. Whatever.
What's the difference, if any, between the brake light fuse blowing when the key is turned on vs. it blowing when the brake pedal is pressed? What does the one indicate as opposed to the other?
My problem is the fuse blowing when the key is turned on fwiw.
Consult your Bentley wiring diagram.
The power for the brake light circuit comes from a fuse (#12?) that is only powered via the ignition switch. If you look at the upper end of the fuse in the diagram (letter "s" with a number) you will see its path eventually to a #15 coming off the ignition switch.

Blowing the fuse *before* actuating the pedal/brake light switches, is an indication that the electrical path between the fuse and the brake light switches has a short somewhere. This path goes through the warning light in the dash too. Carefully inspect the rat's nest between the fuse box and the warning light in the dash. Remember how we pulled the connectors off the master cylinder then re-plugged them in one at a time? Do that again with an experimental ignition-on at each re-plug to see if one or the other connector is responsible for blowing the fuse.

If you blow a fuse only upon pressing the brake pedal, start with the taillamp/stoplight bulbs and in the totally unlikely event that the problem is not there, work up towards the brake switches on the master cylinder.
Colin
Or, reflect upon the fact that you horn also doesn't work, and that it doesn't work because it's not there. Wonder about how the horn and the brake lights share a fuse. Speculate as to what the wires that would supply an existing horn might be doing in their master's absence. Take a look and notice that there they are with their bare connectors jammed up into the body work near the bumper. Wind a bunch of electrical tape around them and marvel at how now the fuse doesn't blow when the key is turned on, and how one brake light now works fine.

Now move on to that other light to see what's the deal....
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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Amskeptic
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Re: Brake Light Fuse Blows

Post by Amskeptic » Sat Jun 29, 2013 11:17 pm

whc03grady wrote: Or, reflect upon the fact that you horn also doesn't work, and that it doesn't work because it's not there. Wonder about how the horn and the brake lights share a fuse. Speculate as to what the wires that would supply an existing horn might be doing in their master's absence. Take a look and notice that there they are with their bare connectors jammed up into the body work near the bumper. Wind a bunch of electrical tape around them and marvel at how now the fuse doesn't blow when the key is turned on, and how one brake light now works fine.

Now move on to that other light to see what's the deal....
Excellent Deductive Reasoning
=D>

Well? Does the other lamp now work?
:bounce:
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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