Ok so one of the factory headlamps was full of water and kind of wrecked, so I had the guy that was working on my truck order new headlights, talking lights, signals, and 3rd brake light, all led.

when I picked it up, it wasn't locking the torque converter. Or it would lock and unlock then just stay unlocked. With the lights on, like pulling the light switch on the dash, it wont even try to lock up the tq converter. If I shut lights off, it will try but won't work but sometimes will for a bit.

so of course I hit the internet and instantly realize this is a common led light issue with these trucks (it's a 1994 7.3idi turbo from factory).

I've read that led bulbs can cause no Tc lockup, abs fault codes, and cruise control doesnt work. I also found out the headlight brights indicator lamp in the dash also doesnt work, but that's minor.

is there a fix for this other than installing a bunch of 6k 50 watt resistors all over the place? Seems like a lot of wasted power.

 

https://youtu.be/xirZ_DYYoaw

also for some reason we ordered white turn signals 🤔 but even if I order amber, I dont feel like there enough change from bright to dim to be noticeable and safe. Anyone know how to fix that?

 

The only other fix that I know of is to put the original bulbs back in.

 

First off, I'd swap in a few load resistors(or original bulbs) in specific things. I'd start with the rear tail lights, and see if that fixes the problem.
Then, once the circuit is nailed down, we should be able to look at the wiring diagram, figure out where the computer is tied in, and add a load resistor right there.

 

well that would be too simple 😅

 

well from reading it appears that theres not much mystery, the running light circuit seems to be the culprit. I know if my lights are on, it disables the Tc lockup entirely and cruise control. Lights off, cruise works and it tries to lockup the converter but doesn't, so I don't drive with lights off.

what I'd like to do ideally, is know exactly what load needs to be on the running light circuit for things to run normal, then maybe I could put something useful on that circuit instead of just a resistor. At very least I could find a 12 volt to 12 volt battery charger, and hook it back up to the battery so that it draws 100 watts of power, loses 10 watts in conversion, but puts 90 watts back into the battery. That should put a load on the circuit and not have huge electric net losses

 

An interesting diagram

 

Found a random tidbit of possibly useful information:

The way the 3rd brake light works, is by a resistance module. when the resistance of the brake bulbs lessen (i.e. when you hit the brakes and the bulbs light up passing current through them) the module tells the 3rd brake light to come on. Since the LED bulbs had a lower resistance than stock, the module thought the brake lights were on all the time, thus keeping my 3rd brake light on all the time.

 

Ok I'm making a note after reading all evening...

there are multiple possible issues here. First, we have the 3rd brake light operating apparently on a resistance sensing setup.
secondly, depending on what kind of tail lights you got, it may have crossed the running light circuit with the flash/turn/brake circuit.
with a normal bulb theyd be separate filament. With led, I'm not sure what goes on.
I know that when I turn my running lights on, the tail lights turn on. If i step on the brakes they get brighter. If i hit the turn signal, the corresponding light will flash on and off, not bright and dim. Also, the 3rd brake light comes on with the running lights. I will take a better look tomorrow and see what's going on better

 

All i can add, and its probably obvious, but i know hitting the brakes and or turning the flashers on unlock the converter. Regular bulbs are old school, just get a trans controller.

 

Just thinking out loud again....
wires going to rear lights should be
running lights
left turn
right turn
reverse
Is there a convenient location under the dash or hood where this bundle is loacated? And do we know if the 3rd brake light connects to this under the dash or or under the truck somewhere?

logic is telling me that maybe some light duty resistors and a zener diode needs to be on the current sensing circuit for the 3rd brake light.

 

Then something is wrong. The running lights should not affect cruise or TC lockup. The brake light circuit certainly does, but not the running lights. I think you have a short somewhere.

 

More than likely the bulbs aren't installed correctly/not the correct bulb. I'll bet that instead of the diodes being from running lights to ground and turn signal to ground, it's running lights to turn signals.
So when you turn on the running lights, the turn signal circuit becomes the "ground" pathway. The flashing happens "backwards", with an "on" flash turning the light off(as it has voltage on both sides), and an "off" flash turning the LEDs ON(as it has something to ground through).

I know that I had to specially look for LED bulbs that had two circuits in them(two sets of LEDs), because most cheap ones didn't.
(The cheap ones I got worked perfectly well, but were no brighter than the original incandescent bulbs, so I just put them back in).

 

The circuit that affects the torque converter lockup is definitely the brake lights/blinkers not the running lights.
It's made to when you let off the gas the converter stays locked to provide engine braking, but if you step on the brakes it will unlock it so the motor doesn't die.

If your parking lights are causing the issue something isn't wired right.

You can either add led load resistors, which Napa used to carry, or modify the tail light housings to use traditional bulbs.

Had this problem on a 91 that didn't even have a third brake light so you may have to add a resistor there as well.

Personally I'm not a fan of led's, the cheap ones don't last any longer than an incandescent, the good ones are too expensive, and you can't tell when there burned out.

Most incandescent bulbs I change out aren't even burnt out, there main failure is a broken filament caused by excess vibration. Mainly from rough roads and bad body bushings

 

There is engine braking with or without the torque converter locked. There is approximately 10% more engine braking with the converter locked that with it unlocked.