What could this be?
New User
Joined: Aug 2005
Posts: 22
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As a joke a couple of guys at work jumped the fuse from the brake lights to the horn fuse on this other guys van. It was great. He went to drive away from work and everytime he hit the brakes his horn would blast. He had no idea what was going on 
. It was way to funny
. Try it sometime, you'll get a kick out of it. Sorry, no advice on the prob. Looks like they have it covered. Just reminded me of that.
. It was way to funny . Try it sometime, you'll get a kick out of it. Sorry, no advice on the prob. Looks like they have it covered. Just reminded me of that.
Thread Starter
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Elder User
Joined: Jan 2004
Posts: 706
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So basically most of you are saying this: Change the bulbs first (cheapest route)...if that doesn't work start searching the ground wires. Would it only be a brake bulb or could it be a parking/running light as well?
Elder User
Joined: Dec 2002
Posts: 624
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From: Georgia
I'll try to answer without getting out the wiring diagram. I believe you will find that there is a dual filiment bulb used for the brake light and tail light. I have never seen it but I have heard that one filiment can break and short to the other filiment causing this. Also if you having the wrong bulb in there it can cause it.
The third possibility is that the ground wire from one (or both) of the brake/tail light sockets has broken or come loose. Normally when you press the brake pedal 12v is applied by a switch on the pedal to the brake light. It passes through the filiment and then to ground completeing the circuit.
What happens when the ground is not there is that the 12V goes back up the other filiment finding a ground somewhere else. Both the brake and tail filiments light up. Now think of where the tail light wire goes and what it connects to. When you turn on the lights the tail lights and the dash lights light up. That 12V comes back up the wire and light up the dash light and then find a ground.
That is the simple version. You could check for voltage at all three wires from the back of the socket when someone presses the brake pedal. Anyway, start simple and cheep. Check for the correct bulb and verify it's a good bulb. Move on the grounds next.
The third possibility is that the ground wire from one (or both) of the brake/tail light sockets has broken or come loose. Normally when you press the brake pedal 12v is applied by a switch on the pedal to the brake light. It passes through the filiment and then to ground completeing the circuit.
What happens when the ground is not there is that the 12V goes back up the other filiment finding a ground somewhere else. Both the brake and tail filiments light up. Now think of where the tail light wire goes and what it connects to. When you turn on the lights the tail lights and the dash lights light up. That 12V comes back up the wire and light up the dash light and then find a ground.
That is the simple version. You could check for voltage at all three wires from the back of the socket when someone presses the brake pedal. Anyway, start simple and cheep. Check for the correct bulb and verify it's a good bulb. Move on the grounds next.
Last edited by tom_foreman; Feb 21, 2006 at 07:52 PM.
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Junior User
Joined: Dec 2004
Posts: 84
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From: w.v.
hi i have the same question i have an 89 f150 4x4 and it is going the same thing now every thing works fine till i hit the brakes every thing still works but the left turn signal gets briter any ideas
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