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Old 08-12-2020, 12:42 PM   #1
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1994 ford b700 large brake lights not working

I have a 1994 ford b700 and the large brake lights are not working. there are two lines that run into fuse panel and are connected with fuse. There is no power going to that connection. I connected a jump wire from positive on fuse panel to brake wire and they light up. My question is there power sent to fuse when stop lamp switch is activated and why power not at brake fuse location otherwise.

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Old 11-04-2020, 04:56 PM   #2
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I have the exact same bus/problem!

I have the same bus and am encountering the exact same problem and I can’t figure it out! Closest I’ve gotten is assuming some fuses need to be replaced because I did find some that were blown and I’m waiting to receive them from shipping.
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Old 11-04-2020, 05:02 PM   #3
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Probably a brake light switch. Some vehicles have two. Could also be loose or out of adjustment.
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Old 11-04-2020, 05:59 PM   #4
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Just googled it, thanks! Didn’t even know those things existed 😅
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Old 11-04-2020, 06:00 PM   #5
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They are different for air brake vs hydraulic, however.
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Old 11-04-2020, 07:08 PM   #6
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Interesting. Mine is hydraulic.
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Old 11-04-2020, 07:46 PM   #7
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Hydraulic will have an on-off pushbutton type switch mounted where the pedal's forward movement activates the lights. Air systems have an air-pressure-activated switch in the air lines, usually around the treadle valve.
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Old 11-05-2020, 07:17 AM   #8
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Thank you so much! Literally just woke up and I’m going to inspect it right now!
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Old 11-07-2020, 03:41 PM   #9
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Okay, so it looks like the fuses were the case for getting lights on in the back which is awesome. But the breaks still don’t turn on when pressed, so it’s definitely gotta be the brake switch. But for the life of me, I swear I see nothing anywhere around the brake pedal. This is the only thing that is coming from the break pedal, but there isn’t a switch or anything like it. I just unplugged it and replugged it.
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Old 11-07-2020, 03:51 PM   #10
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That probably is the switch. It screws into the threaded insert on the other side, and if you unscrew it or press the brake pedal, there is a button on the threaded end that rests against the body plate, which activates the lights when released as the brake pedal is pressed.

Most I've seen were not mounted on the pedal arm itself, but it works as long as something is holding the contacts open when the brake is not being pressed, and holding them closed when it is.

It is what is known as a normally closed switch, meaning the contacts are normally closed without intervention. An ordinary push button switch that passes current when pressed is a normally open switch, this is the exact opposite of that. If that's not it, it's possible it may be further up the brake pedal lever toward where it actuates the master cylinder. I'm pretty sure that's it, though.

You may want to check the wiring with a test light or multimeter, just to be absolutely sure it is the switch. One wire will be constant 12V, another should have voltage coming through the switch ONLY when the switch is not resting against anything. There may also be a ground. Usually lighter colored wires are 12V, darker colors are ground unless they have a stripe, which also indicates 12V.

And now that I think about it, it is possible with this type of pedal arrangement that the switches might be the same for either brake system.
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Old 11-09-2020, 03:03 PM   #11
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You were right! That totally is the brake switch. Looks like it might be damaged, so I’m going to order a replacement. The wires were reading good so hopefully it’s just that! I will keep you updated!
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Old 11-13-2020, 01:29 PM   #12
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Well, I don’t think it was the brake switch. I replaced it and still no brake lights. All of the back lights work including parking and turning, but not brake. I think I’m just going to have to take it to a shop and have them butcher me with costs, but I need to be back in Texas from Ohio this week. I learned a lot and I appreciate your help! Luck was not on my side this time lol
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Old 11-30-2020, 03:51 PM   #13
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Found the problem

Finally found the cause of this brake light problem with the Ford 1994 B700. Apparently, it’s a diode that controls the brake light and the way to fix it is replace it, which they don’t make this part anymore, or wire a new brake light system. It cost me about $700 ($75/hr and the mechanic said it took him 8hrs to find it, oof). He showed me it was located way under/inside the dashboard. So if you take it to a shop tel them that to save time! Hope this helps!
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Old 11-30-2020, 04:11 PM   #14
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So much for Ford and their better ideas....
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