[Jerry Miller]

I put some LED's in my cargo and 3rd brake light. Now I just need to wire up the load resistor so the cruise will work. I bought the 6 ohm resistor from Putco (they make the LEDs) Now, I'm wondering....I did the math and the stock bulb is 18 watts which is 1.5 amp load, which means to match properly, the resistor should be an 8 ohm. With the 6 ohm resistor, the load is 2 amps.
Since the PCM monitors this circuit, will the 6 ohm resistor work properly? Will the extra load hurt the PCM? Has anyone here been running the 6 ohm resistor?
Thanks,
Jerry

 

[RonKZ650]

I could very well be wrong, but my thoughts. The PCM uses this circuit to determine when the brakes are applied to unlock the torque converter and turn off the cruise. The only way volts get to the third brake lamp is when the brakes are applied. The multifunction switch on your column is the only other way volts can get to the bulb. So in normal operation, you don't need a load because no volts are ever there unless the brakes are applied regardless of load. The way volts can get there without load is a flaky multi function switch so a good switch you don't need load. A flaky switch may leak volts confusing the PCM, so then the bulb or load would minimize this. This is my theory why a burnt out 3rd brake lamp causes all these problems. The missing load from the bulb being burnt seemingly causes the problem, but the real problem is the switch leaking volts to the bulb. Reason I think this is several guys here have completely removed the third brake lamp for various reasons as adding a camper ect and it caused no ill effects removing the load. If you do add a 6 ohm resistor of course it's going to disapate about 25w of power.

 

[lucasd]

I wired up an exhaust brake on my truck, and the drivers side brake lights don't work. Any ideas? Sounds like a similar problem. I used a wire on my brake controller for the hot wire when the brake pedal is pushed.

Thanks!!

 

[nashvillebill]

For adding additional lights or for an exhaust brake, don't tap into the light itself or brake pedal. There is a plug under the hood, in the engine compartment, for the Center High Mount Stop Lamp, it may be labelled CHMSL. It was a gray plug I believe, with a single wire to it, and it's used to relocate or extend the brake light. I used it for the brake light on my camper shell. It could also be used to power a relay for an exhaust brake.

Google QVM Bulletin Q28.pdf for the Ford document with pictures even....Note, this worked on my '94, no guarantees it's pertinent to later models.

 

[Phil G.]

I put some LED's in my cargo and 3rd brake light. Now I just need to wire up the load resistor so the cruise will work. I bought the 6 ohm resistor from Putco (they make the LEDs) Now, I'm wondering....I did the math and the stock bulb is 18 watts which is 1.5 amp load, which means to match properly, the resistor should be an 8 ohm. With the 6 ohm resistor, the load is 2 amps.
Since the PCM monitors this circuit, will the 6 ohm resistor work properly? Will the extra load hurt the PCM? Has anyone here been running the 6 ohm resistor?
Thanks,
Jerry

I would recommend using a EP-27 flasher from Napa or similar. This flasher will deal with the smaller loads of LED's without building in extra resistance to the circuit.
Electronic LED Flasher EP27 - Automotive LED Lights - LEDLight

Sorry, when I re-read your post, I see you were talking about 3rd brake light which does not involve the signal light circuit so disregard this post. I will leave it here for reference for anyone adding LED tailights.

 

[Jerry Miller]

Thanks everyone. The theory about a bad multi-function switch on some trucks, does make sense. I have never tried running my cruise without the bulb in, so I don't even know if mine would have an issue.
After all of this, I decided not to run the LEDs. Even with brand new sockets the bulbs were a very loose fit, and so were the correct 921 bulbs. If you gently tap the housing you can get the bulbs to tilt down. Great design:thdown:
So, I tried something. I installed smaller 194 bulbs. They stayed in much better and even though they are only about 6 watts, as opposed to the stock 18 watts, the third brake light and cargo lights seem to light up plenty.
Hopefully, if there is any truth to the PCM and 3rd brake light relationship, the 194 bulb will meet the need. I'll see once I get my truck back together from the paint job.
Thanks guys,
Jerry

 

[gnxtc2]

I would install the LEDs and drive it to see if you really need the resistors.

Billy T.
[email protected]

 

[Jerry Miller]

I would install the LEDs and drive it to see if you really need the resistors.

Billy T.
[email protected]

Billy,
If I could get the LED's to stay in place, I would. I'm just going to use the 194 bulbs and be done with it.
Jerry