smart junction box/headlight issue...
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More Turbo
Joined: Jul 2007
Posts: 523
Likes: 1
From: Indiana
smart junction box/headlight issue...
on 08 F350 psd, had the smart junction box replaced at dealer two years ago when i began to notice my low beams would not come on in autolamp mode without cycling the headlight switch(cycling to any position would kick them back on). ford said it was likely my boss snowplow wiring harness and that the SJB doesn't alway "play nice" with plow wiring. of course my smart *** rebuttle was "so what value is the plow prep package?" i went ahead and had boss look it it, they could find nothing out of the ordinary. so the SJB replacement worked fine until i put the plow on for the first time earlier this week. instantly the problem came back and is now worse in that the headlights will go off (in any multi-switch position) driving down the road and cycling the switch will only give me a few seconds of lights before they go back off. my only saving grace is that the fogs stay on regardless. anyone else seen this? i just can't see paying ford another $500+ when i'm confident they won't make a long-term fix. what really sucks is that we have a bunch of snow coming tomorrow and the little fogs don't too too well behind the blade 
Cargo Master
Joined: Oct 2012
Posts: 2,333
Likes: 35
From: South Carolina
Not sure this will help.
It appears the SJB is connected to the battery though a pair of fusible links. The SJB has fuses, logic circuits, and some FETs (field effect transistors) to direct power to various items, including the headlamps.
My guess is that with the plow attached, the load may be more than the FETs can handle.
One work around would be to wire some aux lights with a fuse and switch directly to the battery, and bypass all the logic and transistors of the SJB.
When I had my 09 wired to provide a trailer connector in-bed for my slide in camper, the shop doing the work used a premade harness that plugged into the Ford harness. The boss would not the tech tie in with "taps" and wire his own connector. The reason cited was that the computer is sensitive to the connections (I'm guessing voltage drop associated with a poor tap connection).
Wish I had a magic bullet for ya, but that was not to be.
It appears the SJB is connected to the battery though a pair of fusible links. The SJB has fuses, logic circuits, and some FETs (field effect transistors) to direct power to various items, including the headlamps.
My guess is that with the plow attached, the load may be more than the FETs can handle.
One work around would be to wire some aux lights with a fuse and switch directly to the battery, and bypass all the logic and transistors of the SJB.
When I had my 09 wired to provide a trailer connector in-bed for my slide in camper, the shop doing the work used a premade harness that plugged into the Ford harness. The boss would not the tech tie in with "taps" and wire his own connector. The reason cited was that the computer is sensitive to the connections (I'm guessing voltage drop associated with a poor tap connection).
Wish I had a magic bullet for ya, but that was not to be.
Thread Starter
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More Turbo
Joined: Jul 2007
Posts: 523
Likes: 1
From: Indiana
I think you are on to something here. Although I think the plow installer did a pretty professional job, other things like a less than professional install of my ziebart remote start (on third system) could have the effect you have mentioned.
Elder User
Joined: Jan 2008
Posts: 529
Likes: 0
Just to add:
The clicking you're hearing is a non-serviceable relay in the SJB for the high beam.
The SJB monitors for over current and can be reset by the dealer, the thing is that the SJB can also disable the circuit and can't be reset and the SJB has to be replaced.
Their are two different circuits for the Left and Right low beams. Each are controlled by a separate FET (Transistor) and 10 Amp Fuse (Left F7, Right F8).
Up here they install relays to take the higher current or they use a upfitter's switch.
ADDED FYI: Member Post this on another Thread.
https://www.fleet.ford.com/truckbbas...tml/Q155R1.pdf
The clicking you're hearing is a non-serviceable relay in the SJB for the high beam.
The SJB monitors for over current and can be reset by the dealer, the thing is that the SJB can also disable the circuit and can't be reset and the SJB has to be replaced.
Their are two different circuits for the Left and Right low beams. Each are controlled by a separate FET (Transistor) and 10 Amp Fuse (Left F7, Right F8).
Up here they install relays to take the higher current or they use a upfitter's switch.
ADDED FYI: Member Post this on another Thread.
https://www.fleet.ford.com/truckbbas...tml/Q155R1.pdf
Last edited by MPD56; Mar 4, 2015 at 12:32 AM. Reason: Added Link
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