SJB Issue: Brake Lights Inop

AnotherS197GT

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On Sunday I was informed that my brake lights weren't working by another driver on the road. I checked the fuse in the SJB and it had popped. I replaced it with another 10A fuse and it popped as soon as I hit the pedal. I tried unplugging the brake lights and tested again to see if there was a problem with the tail light harnesses. The fuse still popped.

Yesterday I spent some time trying to diag the problem and this is what I came up with:

I have power in the BEC, power at the SJB, but no power at the brake switch.

I tested continuity on the wire that feeds the brake switch and it's good. Jumped voltage to it and got a full 12V+ on the other end.

Tested continuity on the fuse for the brake switch, it's good.

Pulled the SJB apart and tested continuity on the brake light circuits. All good. (pins 10, 11, 16)

I reassembled everything and the lights were working fine last night. This morning, they're not working anymore. The last time I tested the brake switch with the SJB plugged in, there was no voltage at the brake switch. I'm going to test again tonight, but I doubt anything has changed. At this point, I'm leaning towards a dead or dying SJB, but it seems like it should work or at least send 12V to the brake light switch.

Turn signals, hazards, parking lights all work just fine. Only the brake circuit is not working.
 
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408Stang

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I agree SJB is looking like the culprit. Do you have a clogged cowl drain?

Sent from my A0001 using Tapatalk
 

AnotherS197GT

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I usually stay on top of keeping crap out of the cowl. The car hasn't gotten wet in weeks either.
 

redfirepearlgt

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I also suggest you try this before dropping the wallet on an SJB. With Key off and Fuse removed from the brake light circuit try the following.

Remove a Brake lamp bulbs from their holders. Don't forget the third brake light.
Disconnect the Connector from the SJB that feeds the lamps.
This should isolate the 12vdc feed side to the brake lamps.
Using an ohmmeter, clip the black lead to ground. Probe each brake wire feed on each brake lamp socket should all be a red/white wire). IN each case if the wiring is good you should read very high if not infinity resistance. If all readings are high resistance or infinity this will confirm you have no short to ground in the wiring harness between the SJB and the brake lamps. Be careful you don't touch the probe on the red lead to the case of the lamp socket or you could get a false indication as that is the path to ground via the bulb when it is inserted.
 

AnotherS197GT

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Thanks, I'll try that after work!

I measured voltage at the fuse box and I'm getting 12V on the fuse, so thats good. Measured at the brake switch and it's reading in millivolts. I'm going to test continuity on that wire again or maybe just run a different wire to the switch and see if the circuit works. I'm also going to try to use a jumper to go over the switch, even though it tests good.

I do have an SJB on the way (got a smoking deal on one), so that will let me rule that out once it shows up. According to the Ford pinpoint tests, voltage at SJB, no voltage at switch, and continuity in the wire means that the SJB needs to be replaced.

I also ordered a tool that can read and clear body codes, so maybe I'll be able to see any codes relating to this issue. I find out tomorrow.

http://iihs.net/fsm/?dir=389&viewfile=Stoplamps.pdf
 
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redfirepearlgt

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My luck would be there was a short to ground in the wiring harness if I did not isolate and check before replacing an SJB for that problem. Look forward to hearing the resolve.
 

AnotherS197GT

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I need to test that wiring anyways. As far as I can tell, this all started after I swapped the MT82 into my car. The only wiring I touched was for the reverse switch (which no longer goes through the SJB on my car). All I can think is that having a transmission in my trunk may have damaged some wiring on the brake lights and shorted something or this just happened at a very strange time.
 

redfirepearlgt

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I found the prints when I got home and able to see them on a 24 inch monitor. I need to make a correction. Left rear Lamp number 2 feeds from a wire color coded light blue/orange (not red/white like the other three for some reason). Also the high mounted stop light feed wire is tan/light blue. Since you suggest you had a big transmission in the trunk (I read that right didn't i?) you may wish to spend plenty of time inspecting the wiring back there as a harness may have gotten crushed. The high mounted stop light harness may be the first one to check as I believe it is exposed since it has to be routed along one of the trunk hinges if memory serves correctly. I'm attaching the link to the stop lamp schematic link in case you need it.

http://iihs.net/fsm/?dir=40&viewfile=Turn Signal, Stop, Hazard Lamps.pdf

As you stated, if there is no power coming out of the SJB from the 10 amp fuse and it is good chances are that unit is bad. The switch as you mentioned should be fine as long as it changes continuity state and reads less than 2 ohms when made.
 

AnotherS197GT

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Now there is voltage at the brake switch and the brakes work fine lol. I think the SJB is just on its way out, I have voltage at the rear lights and I don't see any shorts on the multimeter when testing the pins on the taillights.
 

redfirepearlgt

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That's good. I agree then that its the SJB going flaky on you. I wish I could have an internal on that SJB to see the strategy inside. And what common components are shared by what systems, etc.
 

AnotherS197GT

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Pulled the codes for the SJB today and the only lingering codes I have are for the reverse lights, which is to be expected. My reverse lights are on their own circuit after the swap. I cleared everything and will keep this thread updated if anything else happens.
 

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