'Brake' Warning Light Always on - What'd I Miss? - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 13 (permalink) Old 06-24-2018, 08:30 AM Thread Starter
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'Brake' Warning Light Always on - What'd I Miss?

1995 F-350 CC Longbed Powerstroke, 170k miles. This one's got me stumped. The dash 'Brake' warning light is always on when key in 'Run'. It started with fuse #13 in dash Fuse Panel repeatedly blowing and locking truck in 'Park'. Traced that to a wiring fault in the trailer. Disconnected trailer, replaced fuse, no longer blows.

The brake fluid level is right where it should be--jumped the green and tan wires together to confirm. Parking Brake is off--disconnected Parking Brake Pedal Switch wire to confirm, Brake wrng light still always on when key in 'Run' position. It's off when key is off. Brake lights and turn signals work normally. Can move gear selector lever in & out of 'Park' normally now.

Disconnected RABS Diagnostic Connection, grounded BK/O wire, 16 flashes on ABS light. ABS light goes on and off as it should when using ignition switch. What's wrong with this SoB Brake wrng light? Thanks.
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post #2 of 13 (permalink) Old 06-24-2018, 12:59 PM
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Re: 'Brake' Warning Light Always on - What'd I Miss?

Odds are your vacuum pump is on its way out.

96 F250 XLT 4x4, 4:10 gears, 5 speed, Tymar intake and HX hose, downpipe, pyro and boost guages, coolant filter, Luc clutch
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post #3 of 13 (permalink) Old 06-24-2018, 01:59 PM
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Re: 'Brake' Warning Light Always on - What'd I Miss?

yup. low vacuum will turn the brake light on.

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post #4 of 13 (permalink) Old 06-24-2018, 06:20 PM Thread Starter
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Re: 'Brake' Warning Light Always on - What'd I Miss?

Thanks both for that. Where would that vacuum switch be? I have a vacuum tool to test it with.
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post #5 of 13 (permalink) Old 06-24-2018, 06:28 PM Thread Starter
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Re: 'Brake' Warning Light Always on - What'd I Miss?

Even simpler would be to jump the switch, or isolate it from ground, I guess?
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post #6 of 13 (permalink) Old 06-24-2018, 07:12 PM
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Re: 'Brake' Warning Light Always on - What'd I Miss?

put a vacuum gauge on a vacuum port. if you have less than 19 inches vacuum, the pump is bad.

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post #7 of 13 (permalink) Old 06-24-2018, 07:52 PM Thread Starter
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Re: 'Brake' Warning Light Always on - What'd I Miss?

Ok, pulled the Low Vacuum Warning Switch connector and the Brake warning light finally extinguished. So it's a bad switch or a bad vacuum pump. I'll do the check tjctransport mentions, thanks, and go from there. I attached a page from the EVTM that shows the various parts that can cause a 'Brake' warning light to come on. What threw me off was the trailer short blowing fuse #13 AND the vacuum issue happening at the same time--weird.

Thanks for the help!
Attached Images
File Type: jpg What Turns On Brake Wrng Lt.jpg (23.3 KB, 10 views)
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post #8 of 13 (permalink) Old 06-24-2018, 07:57 PM Thread Starter
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Re: 'Brake' Warning Light Always on - What'd I Miss?

The file size of the camera was plenty big enough to read the wiring diagram. Must be reduced a lot when attached to a post, can't be read. Sorry, tried to contribute.
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post #9 of 13 (permalink) Old 06-28-2018, 11:31 PM
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Originally Posted by WilST View Post
Ok, pulled the Low Vacuum Warning Switch connector and the Brake warning light finally extinguished. So it's a bad switch or a bad vacuum pump. I'll do the check tjctransport mentions, thanks, and go from there. I attached a page from the EVTM that shows the various parts that can cause a 'Brake' warning light to come on. What threw me off was the trailer short blowing fuse #13 AND the vacuum issue happening at the same time--weird.

Thanks for the help!
Had the same issue on my 96. Go ahead and replace your vacuum pump. Try to find OEM and not reman. Itll go out again lol
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post #10 of 13 (permalink) Old 06-28-2018, 11:42 PM
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Re: 'Brake' Warning Light Always on - What'd I Miss?

X 4 on the pump.

Charlie
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