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Old 08-11-2013, 08:29 AM   #1
matteo_g
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Default Restore short bursts to your 2007 Ranger OEM Horn

The horn response on my 2007 Ranger Sport has always been delayed, making it difficult to control the horn duration. I later found that many other model Ford vehicles in the same and surrounding model years all had the same problem. The issue seemed to start with the introduction of the Smart Junction Box (SJB). After doing some investigation, I found that the cause is due to a circuit on the Low Current Board (LCB) in the SJB that drives a relay that actually turns on the horn itself. This circuit can be activated from at least a couple sources: 1) the horn switch on the steering wheel of course, and 2) the OEM security system and keyless entry. The horn switch from the steering wheel grounds an input to a microcontroller on the LCB. Unfortunately, the designers introduced a digital electronic delay between when you press the horn switch and when the horn relay actually turns on. So as a result, you can't gently pound the on the horn to get a couple short "beeps" without making it seem like your having road rage against the driver in front of you. It also doesn't work well when you're trying to let the cute chica on the sidewalk know that she's hot!

Below is a procedure I developed to modify the horn circuit to restore the rapid response like that of an older vehicle. The Ford part number of the SJB on my 2007 Ranger Sport is 7L5T-14B476-CG. This procedure probably applies to most, if not all of the SJBs with different letter suffixes, although I am not certain if it applies to SJBs without keyless entry; I suspect that this method does apply since it’s unlikely the microcontroller & relay circuit got eliminated in systems without keyless entry, even if there is only one source to turn on the horn.

Step 1 – Wiring diagram and a technical description of the modification itself:
This is the wiring diagram for the circuit being modified. The change is very simple. The connection to the LCB sense input gets disconnected and a wire gets run from the horn switch itself to the relay that turns on the horn. Because the output of the LCB that turns on the horn relay is an open-collector/drain type circuit, grounding this output to turn on the relay from a different source (the steering wheel horn switch) is a safe thing to do; it’s called a “wired-OR” circuit in electronic terms. Second, since the relay’s coil circuit is low current, there is no issue with overloading the steering wheel horn switch and its wiring. Third, disconnecting and leaving the horn switch sense input floating is okay because this input has a pull-up resistor at the microcontroller input where it leads, so the horn won’t be turning on when you don’t want it to. Lastly, this modified wiring still allows the output from the LCB to turn on the horn whenever it needs to for the keyless entry and security system.


Step 2 - Locate the Smart Junction Box in the passenger side kick:


Step 3 – Remove the entire kick panel covering the SJB:
The access panel door is not large enough through which to do this job. The entire kick panel must be removed. This photo shows the two pins that hold it in. Two ends of the panel are tucked under two adjacent panels, one on the door threshold and one on the hinge jamb.


To remove the panel, first take out the large gray retaining pin towards the front of the vehicle, slide the foot of the panel out from under the door threshold trim, then slide the entire panel down to get it out from under the door jamb panel (not shown here). Sliding the panel down also disengages it from the hidden white retaining pin that’s underneath along the door jamb. This white pin can then be removed and put back into the panel before re-installation later on (as shown in the above photo).


Step 5: Disconnect all the plugs to the SJB and remove the three 10mm hex bolts to completely remove it from the vehicle
The plugs have special release handles that pop out the plugs. Unlatch them from one end, and swing them across, and the plug pops out. Leave the handle where it is because it will swing itself back across the other way when you put the plug back in later on.


Step 6 – Dissemble the SJB:
This is your starting point. Take a picture of your own so you have a record of where all the fuses go in case it is different.


Remove all the fuses and arrange them for reinstallation later on. Also remove the two plug-in relays (window and accessories) and the big 30A fuse (pink one).
[/CENTER]

Pop the cover off the back by starting in this corner. The plastic is kinda brittle, so be careful. Carefully use a couple screwdrivers to gently release the catches, and work your way around the back cover.


Once the back cover is off, when you turn over the SJB the entire board assembly will slide out.


Step 7 - Locate the points where the modification will be done:
These are the points to be worked on.


This is the horn relay itself and its associated coil bleed resistor. (Nothing to be done here, so this is just FYI.)


Step 8 –Remove the pin that connects the horn switch to the sense input on the LCB:
All of the pins that sandwich the various SJB boards together are soldered on both ends to create the connections back and forth. Remove the pin shown since we want to disconnect the horn switch connection from the steering wheel from the LCB sense input. Use a very hot, large tip soldering iron to do this if you have one. I used my Weller with the temperature turned up all the way and added more solder for good heat transfer, and I was able to keep both solder joints melted while I pushed out the pin with another metal pin (paper clip or a pick).


Step 9 –Thread a #18 wire inside the board sandwich to both locations:
The plastic housing for the SJB fits around the board sandwich very tightly. Therefore, it’s best to run the jumper wire that much be installed inside the board sandwich so it doesn’t interfere with the housing.


Step 10 –Solder the white wire to the horn switch input pin connection:
Once threaded, solder the end shown here into the white PCB and pull the wire through. The connection to the green PCB (LCB) remains open.


Step 11 – Prepare and solder the connection on the relay coil connection pin:
There is not a lot of room in the board sandwich to push back in any extra slack in the wire. So, strip the end as close to the pin as possible, tin it with solder, and make a small hook that you can wrap around the pin.


Solder the hooked connection to the pin.



Step 12 – Reassemble and reinstall:
You’re done! Reassemble the SJB in the reverse order, replacing all the fuses and relays to their exact same locations, and snap on the back cover. Put it back in your vehicle, go for a test drive, and toot (nicely) at the pretty girls!

Last edited by matteo_g; 07-26-2015 at 07:03 AM.
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Old 08-11-2013, 12:26 PM   #2
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Nice write up, I was wondering what the deal was with that delay. Now I know.
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Old 08-12-2013, 12:48 AM   #3
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Cool...what little I know about electronics and PCBs was really outdated...never heard of some of the components mentioned, but your writeup was very informative and well done!
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Old 08-12-2013, 01:10 AM   #4
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^^^ I agree.

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Old 08-12-2013, 07:48 AM   #5
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Thank you all for the compliments.

The guts of the SJB didn't appear to be covered anywhere on the internet, so hopefully this helps someone out somewhere. The SJB does a lot and it can be a convenient access point for other custom electrical modifications.
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Old 08-12-2013, 09:03 PM   #6
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Quote:
Originally Posted by matteo_g View Post
The SJB does a lot and it can be a convenient access point for other custom electrical modifications.
Excellent and YES thanks for sharing this.
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Old 08-16-2013, 10:40 AM   #7
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Added this to the tech library:

http://www.therangerstation.com/tech...horn_mod.shtml
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Old 08-16-2013, 03:46 PM   #8
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Damn, I wanna do this on our work van! I work work the disabled and the new Dodge Caravan we just got will NOT do a quick couple honks. And like you mentioned, I sound like a colossally impatient dick when I pull up and just want to let them know I'm there...
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