Diode in brake light circut

Vehicle 1995 F250 W/PSD and automatic

Someone told me to put a diode in my brake light circuit to stop T/C hunting. I've lost the info. A repost of the directions or a link would be appreciated.

Reply to
Peter Arnold
Loading thread data ...

Interesting idea. But would not blocking the BOO from the PCM also effect some other engine control functions too?

DC

Reply to
Deadcarnahans

You are being mislead. You simply need to relight the pilot light in your turbocharger and put a 1 can of lighter fluid in the fuel tank.

Reply to
GW Bush

You were complaining about the turn signals causing the tc to unlock. Put the diode between the brake switch and the brake lights to prevent current from the blinkers from coming back down the line to the brake switch and energizing the line going to the engine that normally disables the tc lock when the brake is pressed. I'd probably put the diode as close to the lights as possible so I'd be sure of it.

Reply to
Chris Hill

The real answer is to correct the problem and don't worry about diodes. If using the turn signal is causing the TC to unlock, it is because you have lost the ground in one of the turn signals. It will then feed back through the brake light. When the brake light circuit has voltage applied, it will disengage the TC.

Reply to
GW Bush

Since the brake lights and turn signals use the same bulb element, what prevents it from feeding back no matter the state of the ground?

Reply to
Chris Hill

Oooh, oooh, let me answer this one. The turn signal switch prevents that. When you're giving a signal, the bulb that's signaling no longer functions as a brake light. Have you noticed that? There are contacts in the turn signal switch for that.

Reply to
Joe

OK, I think I've got it now. As soon as the weather warms up I will:

1) Get my old body under the rear bumper. A bigger task than you may appreciate.

2} Find the brake Light hot wire. This will involve wife stepping on brake, another old person in the mix.

3} Cut afore mentioned wire.

4} Solder in diode, current allowed to flow to lights.

5} Tape /shrink wrap the whole mess together.

Question, I will probably resort to Radio Shack for the diode, is there a specific value? Any clues would be helpful.

Reply to
Peter Arnold

The bulb in use is a 3157K. At

formatting link
it is the lastitem listed on the page. It is a dual filament miniature. Bothfilaments have a return to battery via a connection to ground. When theground is intact, current flows through the turn signal filament toground, when the turn signal is operated. This is because the currentfollows the path of least resistance. When the ground is lost, anywhereother than immediately at the bulb socket, the only path available afterthe turn signal filament is the brake light filament. At this point,turn signal voltage is now also applied to the downstream side of thebrake switch, through the brake light filament. That is the same sideof the switch that when minimal voltage is applied, will disengage theTC. I hope this clears this up for you. PS: Another oddity of this circuit is that no cruise control will be available if the filaments of both brake lights are broken or burned out.

Reply to
GW Bush

Maybe the guy's real problem, then, is a bad turn signal switch?

Reply to
Chris Hill

Interesting, but not exactly how electricity works. There may still be a voltage on the brake line even with a decent ground, that's why a relay is sold to keep the turn signals from energizing trailer brake controllers.

>
Reply to
Chris Hill

given the following conditions:

intact filaments good ground non-faulty brake switch

please tell us how there can be 'a voltage on the brake line even with a good ground'

Reply to
TranSurgeon

Or better yet, fix the faulty ground! Why would you take that Jerry-Rigged approach to fixing your problem? Your diode fix is like adding another valve under your kitchen sink instead of fixing the leaking faucet... If your old body and/or mind can't do it, take it to a competent automotive electrical repair shop.

Reply to
351CJ

Define good. In modern vehicles, there really isn't one unless it goes directly back to the battery.

Reply to
Chris Hill

'good ground' = at same voltage potential as battery negative

now...........can you explain that bit about 'in modern vehicles...........' ??

Reply to
TranSurgeon

I'd qualify that with 'at the same potential at operating conditions' ie with normal current present. Since voltage is a function of current and resistance, it can change. A good ground needs to have a near-zero resistance path to battery negative. Even the frame will show a potential difference from battery negative when turning the starter over.

As far as the modern vehicle comment, I don't have a clue what that means.

Stephen

Reply to
Stephen

"Modern vehicle?" I don't think Chris Hill has a clue either. He got an excellent explanation of what was causing the problem. In the process, it was pointed out that the taillight/brakelight and turn signal bulb has a dual filament, contrary to his belief. At this point, I think he is simply trying to direct attention away from his improper description of the circuit and how it works. Some people just have a hard time admitting they were wrong.

Reply to
Tyrone

Nothing like a good strawman to give noodle like strength to an argument! Admittedly though, some wierd things can happen with that dual filament light system.

Stephen

Reply to
Stephen

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.