Planning to install a 3rd LED brake light?
05-13-2009, 08:02 AM
#1
Moderator (Ret.)
Thread Starter
Planning to install a 3rd LED brake light?
If you have, or are thinking of installing an LED 3rd brake light assembly, be warned that your cruise control will not operate (assuming you have cruise control, or are planning to install it in the future). It was discovered and verified that after the LED 3rd brakelight unit is installed, the cruise does not activate. Apparently, Ford uses this circuit to detect the bulb filiments "resistance" to determine that the brakes are not applied, and so the cruise can activate.
In my findings, the reason for using the 3rd brake light rather than the rear brake/tail lights is that there is only one bulb to detect in the 3rd brakelight assembly, rather than the 2 bulbs the rear circuit would have. Plus, the rear circuit is tapped into when the trailer wiring is added, which would add more resistance to the circuit and affect the cruise detect circuit.
What some folks have done to keep their LED 3rd brakelight assembly AND cruise is to solder in the filiment bulb in parallel with the wiring of the LED bulb, then tuck this bulb inside the cab (be carful that the hot bulb does not rest on the inner roof liner!!!). This way you get your LED brake light, and the cruise circuit still "sees" the filiment resistance. If you have a multimeter, you could measure the resistance of a 921 bulb (bulb I believe that is used for the 3rd brake light), then go to Radio Shack and buy a resister with this same resistance and solder this across the two wires that the LED/bulb plugs into. It would think a bulb is plugged in, and would not heat up like a bulb would do, as long as you get a high wattage resister..
Just a heads up for you, and should this come up again in the future, this thread will show up when a search (ha ha ha) is done. Yea, right.
In my findings, the reason for using the 3rd brake light rather than the rear brake/tail lights is that there is only one bulb to detect in the 3rd brakelight assembly, rather than the 2 bulbs the rear circuit would have. Plus, the rear circuit is tapped into when the trailer wiring is added, which would add more resistance to the circuit and affect the cruise detect circuit.
What some folks have done to keep their LED 3rd brakelight assembly AND cruise is to solder in the filiment bulb in parallel with the wiring of the LED bulb, then tuck this bulb inside the cab (be carful that the hot bulb does not rest on the inner roof liner!!!). This way you get your LED brake light, and the cruise circuit still "sees" the filiment resistance. If you have a multimeter, you could measure the resistance of a 921 bulb (bulb I believe that is used for the 3rd brake light), then go to Radio Shack and buy a resister with this same resistance and solder this across the two wires that the LED/bulb plugs into. It would think a bulb is plugged in, and would not heat up like a bulb would do, as long as you get a high wattage resister..
Just a heads up for you, and should this come up again in the future, this thread will show up when a search (ha ha ha) is done. Yea, right.
Last edited by Mod (Ret.); 05-13-2009 at 08:05 AM.
05-13-2009, 08:43 AM
#2
Senior Member
I was considering doing this down the road some after I install the other 50 mods I want. 
The addition brake lights I have seen are down by the rear tailgate. I assume this is the brake like you are talking about installing? If so, I am not electrical expert, but couldn’t you tap into the wiring of the one or both rear tail lights?
The addition brake lights I have seen are down by the rear tailgate. I assume this is the brake like you are talking about installing? If so, I am not electrical expert, but couldn’t you tap into the wiring of the one or both rear tail lights?
05-13-2009, 09:25 AM
#3
Senior Member
I believe Bucko is referring to the high mount cab 3rd brake lite. Its a wiring issue that is very interesting to read, and also wierd enough to be correct. Its opened my eyes on a couple of issues.
05-13-2009, 10:47 AM
#6
Moderator (Ret.)
Thread Starter
If the "RECON" unit provides an LED brake light, and the cruise still works, then they must have a "load resistor" in their circuit. I looked at the one (not a recon) I removed last night from my truck (again, it prevented my cruise from activating). While there is a tiny circuit board to operate the LED's, it non the less disabled the ability to set my cruise.
So now this gets interesting: some manufacturers of a 3rd brake light (cab mount) work with cruise control, and some don't.
I just got my new LMC truck parts manual, and it advertises some LED cab mounted, LED brake assemblies. I'm going to call them tonight to see if the ones they sell are compatable with the cruise function.
I can tell you this for a fact: the "LED 3rd brake" unit I bought off of a classifieds post on another forum PREVENTED my cruise from activating. As soon as I put the stock unit back on, the cruise works.
Two more tests I'm going to do today: remove the stock 921 bulb in the stock 3rd brake light assembly, and try the cruise. If it fails, it would be good to know! Second test is to re-install the LED bulbs into my rear taillight assemplies and try the cruise. I'm pretty sure this will work.
Last edited by Mod (Ret.); 05-13-2009 at 10:59 AM.
05-13-2009, 10:49 AM
#7
Notorious Thread Editor
If you have, or are thinking of installing an LED 3rd brake light assembly, be warned that your cruise control will not operate (assuming you have cruise control, or are planning to install it in the future). It was discovered and verified that after the LED 3rd brakelight unit is installed, the cruise does not activate. Apparently, Ford uses this circuit to detect the bulb filiments "resistance" to determine that the brakes are not applied, and so the cruise can activate.
In my findings, the reason for using the 3rd brake light rather than the rear brake/tail lights is that there is only one bulb to detect in the 3rd brakelight assembly, rather than the 2 bulbs the rear circuit would have. Plus, the rear circuit is tapped into when the trailer wiring is added, which would add more resistance to the circuit and affect the cruise detect circuit.
What some folks have done to keep their LED 3rd brakelight assembly AND cruise is to solder in the filiment bulb in parallel with the wiring of the LED bulb, then tuck this bulb inside the cab (be carful that the hot bulb does not rest on the inner roof liner!!!). This way you get your LED brake light, and the cruise circuit still "sees" the filiment resistance. If you have a multimeter, you could measure the resistance of a 921 bulb (bulb I believe that is used for the 3rd brake light), then go to Radio Shack and buy a resister with this same resistance and solder this across the two wires that the LED/bulb plugs into. It would think a bulb is plugged in, and would not heat up like a bulb would do, as long as you get a high wattage resister..
Just a heads up for you, and should this come up again in the future, this thread will show up when a search (ha ha ha) is done. Yea, right.
In my findings, the reason for using the 3rd brake light rather than the rear brake/tail lights is that there is only one bulb to detect in the 3rd brakelight assembly, rather than the 2 bulbs the rear circuit would have. Plus, the rear circuit is tapped into when the trailer wiring is added, which would add more resistance to the circuit and affect the cruise detect circuit.
What some folks have done to keep their LED 3rd brakelight assembly AND cruise is to solder in the filiment bulb in parallel with the wiring of the LED bulb, then tuck this bulb inside the cab (be carful that the hot bulb does not rest on the inner roof liner!!!). This way you get your LED brake light, and the cruise circuit still "sees" the filiment resistance. If you have a multimeter, you could measure the resistance of a 921 bulb (bulb I believe that is used for the 3rd brake light), then go to Radio Shack and buy a resister with this same resistance and solder this across the two wires that the LED/bulb plugs into. It would think a bulb is plugged in, and would not heat up like a bulb would do, as long as you get a high wattage resister..
Just a heads up for you, and should this come up again in the future, this thread will show up when a search (ha ha ha) is done. Yea, right.
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05-13-2009, 12:00 PM
#9
Moderator (Ret.)
Thread Starter
Keep them coming. We can make a list when this thread finally craps out for the search (or a sticky) of all the LED 3rd brake light units that work with cruise.
Where the heck was this list when I delt out 40 bucks for the one I have (and does not work with cruise)?!?
Where the heck was this list when I delt out 40 bucks for the one I have (and does not work with cruise)?!?
