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Old 09-15-2020, 03:52 PM   #1
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Re-wiring Flashing Lights?

I'm buying a 2006Thomas HDX, and it currently has all the standard school bus lights and flashers. They are all incandescent bulbs. Removing the stop sign arms is easy, but I'm curious about the wiring for the flashing stop and caution lights which, as you know, flash alternately front and rear. What I'd like to do is rewire the rear roof red signals in such a way that they become high brake and tail lights, and the amber signals on both front and rear become turn signals only. The front red signals will be removed entirely, replaced with spot or flood lights. I'd also like to switch over to LED lighting. Anyone here with experience in these electrical systems who can give an old man a clue? Thanks!
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Old 09-15-2020, 04:17 PM   #2
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An electrical schematic is your friend. Search long and hard to find one. To convert a wire to a different purpose, you need to trace it back to it's relay and reroute it to your intended source. To do that, you may need to route it to a relay that is fed from the battery and triggered by your intended source.

In other words, take the signal that triggers the flashing relay and replace it with power from the flood light switch to use the wires for flood lights.

The rear roof red lights will need to be fed from a relay that is triggered by your rear turn signal lights and fed from constant 12V power. You may not be able to just parallel the rear roof lights with your rear turn signal lights because the circuit may not be able to handle the extra load. Switching to LEDs may make adding them to the turn signal circuit possible. Just be careful about changing wiring willy nilly and keep a notebook to document changes in detail for future reference.

Having an amp clamp handy may help you determine what load you can or cannot add to a circuit safely. You cannot exceed the amp capacity of the wire or the value of the fuse that feeds the circuit. Generally, you want to keep the load at 80% or less of the rated fuse.
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Old 09-15-2020, 04:30 PM   #3
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I just spliced into the wiring for the lower turn and brake lights. You would have less draw with LEDs, but just splicing into the lowers will work.
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Old 09-15-2020, 06:09 PM   #4
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Thank you, Texan and o1marc! I will be looking for the wiring diagram; I may be able to get it from Thomas. The HDX has really good outside access to most fuses, breakers, and relays below the driver's window. I already have a clamp-type multimeter, which I usually use for house wiring, so I'm familiar with its use. I look forward to the diagram, though, because some manufacturers (cars, too) will splice wires or have a junction box or module in some hidden spot.
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Old 09-15-2020, 06:31 PM   #5
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If doing a simple splice, I would highly recommend swapping at least the upper bulbs, if not all of them, for LEDs to reduce draw on the circuit. Twice the bulb load could overheat the wiring and burn it up.
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Old 09-15-2020, 07:58 PM   #6
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Quote:
Originally Posted by CHEESE_WAGON View Post
If doing a simple splice, I would highly recommend swapping at least the upper bulbs, if not all of them, for LEDs to reduce draw on the circuit. Twice the bulb load could overheat the wiring and burn it up.
I would agree with this. The complete housings with LEDs are not that pricey. I change mine to brake and back ups in the rear and turn and driving spot lights in front, drivers on a separate switch.
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Old 09-19-2020, 10:43 PM   #7
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I did it a little differently but it wasn't too hard. And then I decided it was terrible so I undid it.

I wired up all four back lights to be brake lights, but people driving behind me complained because they were obnoxiously bright so I disabled them. It seems like a good idea but didn't really turn out to be.

For the front I purchased replacement reverse lights so they're all white and I used the old switch to turn all four on as flood lights when I'm in a dark place and need exterior light. That I kept, and I like having them up there.

Figuring this out was actually pretty easy but I added regular $5 30A 12v car relays to do so.

In my bus wiring panel on the far left were terminals that were labeled and I could find brake lights, turn signals, etc there. A wire from the appropriate 'input' terminal there to the coil of the relay will get the relay to turn on, or you can trace the wire from a switch down to the relay if you want to turn it on manually. You can grab the flasher switch wire that goes to the yellow flasher unit to use that switch, which made it easy. Ground the relay coil to a ground screw right there in that electrical panel.

The power to the main relay contacts can come from one of the bigger wires leading to a fuse block in that electrical panel (or from your house batteries which I did for the front lights) but should be probably 10 awg wire if you are still with incandescent bulbs.

The output of the relay I connected to the wires that came off of the school bus flasher unit yellow box for the appropriate lights. This was kinda easy to figure out because all of the light wires on my bus were fatter pink wires and I could just hook one up to 12V and see what turned on.

If that doesn't make much sense let me know and I can take some pics and draw a diagram but if you're relatively electrically inclined and understand the basics of relays it's pretty easy.
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Old 09-20-2020, 12:50 AM   #8
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Makes sense. Most vehicles only have two brake lights for a reason. I do recommend rewiring the top reds and ambers to be high-mount brake lights and turn signals. Also recommend swapping for LED replacements. All of which can help prevent being rear-ended by something bigger, taller and heavier than you.
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