1998 ranger brake lights come on with the key on, led swap

Hi, so I'm in the middle of an led upgrade on my truck. I haven't had any issues yesterday when I changed the brake lights out yesterday. Today, I got all the blinkers and their resistors installed. No issues there. I pull out the third brake light to swap out that brake bulb, and with the bulb off when pulling it, I insert the LED bulb and it comes on immediately. I'm thinking that's odd, but I had the bed lights on, along with the key (so I have some tunes) at the time, so I go and turn the bed lights off and all of the brake lights stay lit. So I figured the bulb wasthe issue, so I pull it back out, and the lights are still lit.

So here's my current symptoms. With the key off, brake lights only come on with the brake pedal being pressed. With the key on, brake lights and third brake light is on, but dim. With the lights on but key off, the red marker lights come on, and the brake lights come on with the pedal being pressed like normal. With the key on and the lights on, brake lights are fully illuminated, and they do not get any brighter when the brake pedal is pressed, however they do flicker as the switch engages and disengages. As I was cataloging my symptoms, I noticed a beeping coming from my dash, that is five beeps long. The abs light is not illuminated.

Now, I'm leaning towards this being an issue with the ignition wiring, but I'm not so sure because it happened when I changed that last bulb out, instead of when the ignition was turned.

Or could this be an issue with me installing LED modules?

Any help would be appreciated. I'm a decent mechanic and even know some stuff about wiring, but I cant wrap my head around how this could've happened. Thanks

Edit: I'm realizing this should be in the wiring section if there is one. Kinda frantic here. I put the old bulb back in and the issue went away. Looks like theres a voltage when the bulb is removed which goes away when an incandescent bulb is installed. The led bulb doesnt seem to make it go away. Is there some sort of diagnostic mumbo jumbo here that's getting confused by the LED bulbs?

It is common for just enough power to bleed through an "open" switch to let LEDs light up dimly because of the low load. An incandescent is enough resistance to drop that voltage and not light up or drain the battery.

I'm not sure why key on or off would have an effect since it seems it is the brake lights, and not the tail lights, and those are always hot.

Interesting. I wonder if a resistor would allow me to finish the c conversion. Looks like I'm gonna have to tear one out of something.

Usually when a weird electrical condition exists... that has no reasonable explanation... it's most likely caused by a bad ground.

Brake lights illuminate with a switched 12 volt source. There shouldn't be any voltage on a brake circuit until the pedal is depressed.... but it appears there is... so some voltage source is finding it's way into the brake circuit and finding it's way to ground through the LED third brake light and completing the circuit which will illuminate all of the brake lights.

Like I said... weird.

But when you key on... somehow the switched ignition source is not being pulled to ground as it should be and is finding it's way to the brake circuit and going to ground through what the engineering world refers to as a "sneak path".

Perhaps it's just me trying to make sense of something that doesn't make sense... make any sense?

A lot of people have had issues with LED modules and LED bulbs in automotive circuits for brake lights.
A friend of mine has a Harley Trike and he had problems with LED retrofit for the brake lights too.

Uncle Gump said:

Usually when a weird electrical condition exists... that has no reasonable explanation... it's most likely caused by a bad ground.

Brake lights illuminate with a switched 12 volt source. There shouldn't be any voltage on a brake circuit until the pedal is depressed.... but it appears there is... so some voltage source is finding it's way into the brake circuit and finding it's way to ground through the LED third brake light and completing the circuit which will illuminate all of the brake lights.

Like I said... weird.

But when you key on... somehow the switched ignition source is not being pulled to ground as it should be and is finding it's way to the brake circuit and going to ground through what the engineering world refers to as a "sneak path".

Perhaps it's just me trying to make sense of something that doesn't make sense... make any sense?

Click to expand...

Oh it definitely doesn't make sense.

The brake lights should be isolated from everything but ground. My guess has always been that it has something to do with the super small gap between the contacts in the brake light switch. That small amounts of power can jump it when given a low resistance path to ground, something an LED can provide but an incandescent bulb can't.

If you think about it, the air gap there has to be pretty small since in most vehicles just resting your foot on the pedal is enough to trip the switch.

adsm08 said:

If you think about it, the air gap there has to be pretty small since in most vehicles just resting your foot on the pedal is enough to trip the switch.

Click to expand...

You could be on to something... that theory could be easily proven by disconnecting the brake light switch. They don't illuminate... proven theory. If they do... keep looking elsewhere.

Seriously.... I have seen bad grounds cause some really strange symptoms. When things get weird... it's always the first thing I consider.

I've actually thought about this probably more then I should have today. I would first ensure that any ground that was added for this LED upgrade was confirmed solid.

Secondly... with the symptom this particular vehicle has... symptom only with the key ON... there has to be a common point where switched 12V ignition and the brake light circuit meet. I'm thinking within a controller that has these two inputs. I think the basic module list would include.. ECM/TCM/PCM... ABS... Body controllers... and the possibility of an aftermarket remote start. To isolate the fault... find the brake light circuit at the the modules and disconnect them one at a time and see if the fault goes away. If you do find where switched ignition is leaking into the brake light circuit... the fix would be to install a blocker diode to "block" the leaking voltage in the brake light circuit.


Lastly... is there really a problem many people experience when doing LED conversions? I've never converted anything I've owned or done it for someone else... so I have no knowledge of it.

My theory could be all washed up... again... just trying to make sense of it.

I converted all the exterior lights on my 08 Corolla. Not a single glitch. Just LED tho, not hid or anything that need extra hardware, just plug in stuff.

Uncle Gump said:

I've actually thought about this probably more then I should have today. I would first ensure that any ground that was added for this LED upgrade was confirmed solid.

Secondly... with the symptom this particular vehicle has... symptom only with the key ON... there has to be a common point where switched 12V ignition and the brake light circuit meet. I'm thinking within a controller that has these two inputs. I think the basic module list would include.. ECM/TCM/PCM... ABS... Body controllers... and the possibility of an aftermarket remote start. To isolate the fault... find the brake light circuit at the the modules and disconnect them one at a time and see if the fault goes away. If you do find where switched ignition is leaking into the brake light circuit... the fix would be to install a blocker diode to "block" the leaking voltage in the brake light circuit.


Lastly... is there really a problem many people experience when doing LED conversions? I've never converted anything I've owned or done it for someone else... so I have no knowledge of it.

My theory could be all washed up... again... just trying to make sense of it.

Click to expand...

It is a somewhat common issues, maybe 10-20%.

The only place I can think of where the two circuits meet at a key-switched point is at the multi-function switch. The brake light circuit is routed through there, so the light flashes while braking.

This one is unique in my experience in that key position plays a role. All the other ones I know of had the lights on key on or off.

I know my 94 has a single element bulb for turn signals... the brake and tail lights share a two element bulb. So no sorting out turn vs. brake light signals in the multi function switch there... I was thinking the 98 was the same.

Uncle Gump said:

I know my 94 has a single element bulb for turn signals... the brake and tail lights share a two element bulb. So no sorting out turn vs. brake light signals in the multi function switch there... I was thinking the 98 was the same.

Click to expand...

Yes, that would be a year dependent thing.

So I think I pretty well got it figured out. The brake switch itself was not the culprit. Every component that was drawing power from fuse 13 was leaking voltage. So I start with 8 volts on the line, pull the socket for the brake switch, still 8 volts. Pull fuse 13, 2.5 volts. Turn off the switch for my aftermarket cruise control, 0.1v. Just a bunch of stuff leaking voltage. I tapped a tail brake light with a load resistor and dropped it down somewhere where it won't do too much damage if it catches fire, and that seems to have solved the problem. It doesn't even get hot while grounding the leaking voltage.

Well it appears that old Uncle Gump was onto something after all.

Just a heads up... if it catches fire... the damage was already done.

Not sure exactly what is on fuse 13... but a safer way to correct your condition would be to put a blocker diode in the brake light circuit going to whatever module is on fuse 13. Probably zero chance of fire.

I think the top suspect for me would be the aftermarket cruise control module. Most all factory modules go through vigorous bench validation from the supplier and internally for these types of things. After production of modules... and vehicles were built they would be validated again... I know because vehicle validation was part of what I did in engineering for several years.

I think one of the moderators should move this post over to the lighting section... seems like this very topic could be useful to at least a few people doing led conversions and would be easier to find there.