Wiring Issue/Brake Lights On after replacing PCM - 5.0

[ShoeSlinger]

I had to replace my PCM on my 5.0 conversion after water got to it. Wiring harness and PCM from EFI Guy. Motor runs fine but my check engine light is now on along with my brake lights. Brake lights stay on after disconnecting brake light switch. I have to remove the alternator fuse from the Power Distribution Box to get the brake lights to turn off. I am really lost. Any ideas on where to start? Appreciate any help.

 

[Steve83]

Contact EFIGuy about a wiring diagram. Since it's not stock, we can't guess how it's wired, or why those might be related.

But water alone doesn't hurt an EEC. Was it saltwater, or did it stay wet for a long time? Were there any symptoms? Do you still have it? I'd try putting it back in.

 

[EPB72]

Knowing the year could help but looking at a 97 explorer there is a brake on off input to the PCM on pin 92 other years as well ,, If yours has this set up or not IDK but thinking it would be spliced in after the brake light switch ,, so a wire goes back to brake lights through turn signal switch and guessing before TS it’s spliced and goes to PCM ,, so guessing as well would have to isolating parts of the circuit and possibly but very uncommon if the PCM is defect and sending out voltage on circuit ,

 

[EFI Guy]

Some of the later Explorer computers put out a trace amount of voltage on the brake input wire, this is normal. Regular bulbs draw enough current that you would never see them light up from the trace amount of power present. However, it will cause some very low power LEDs to illuminate.

The best solution is to buy a set of the resistor devices used to eliminate "hyperflash". (Google it)
You can probably just add in resistors but I'm unsure of the value required and resistors create heat so I don't want to tell you to do anything potentially dangerous. It's probably best to use a commercial product designed for this purpose.

Having said that...The trace voltage should only be present when the key is on. The alternator fuse is not related to this circuit in any way. So unless it is one of the EFI fuses being pulled that shuts it down, there may be another issue at play here,

 

[ShoeSlinger]

Some of the later Explorer computers put out a trace amount of voltage on the brake input wire, this is normal. Regular bulbs draw enough current that you would never see them light up from the trace amount of power present. However, it will cause some very low power LEDs to illuminate.

The best solution is to buy a set of the resistor devices used to eliminate "hyperflash". (Google it)
You can probably just add in resistors but I'm unsure of the value required and resistors create heat so I don't want to tell you to do anything potentially dangerous. It's probably best to use a commercial product designed for this purpose.

Having said that...The trace voltage should only be present when the key is on. The alternator fuse is not related to this circuit in any way. So unless it is one of the EFI fuses being pulled that shuts it down, there may be another issue at play here,

Unfortunately the brake light stays on even with the key turned off. Painless harness and in great shape. I am lost on this one since when I pull the alternator fuse, the brake light (among other things) goes off. Not looking forward to tracing everything back, but that might be the only way. I even have the brake switch at the pedal disconnected.

 

[EFI Guy]

Try unplugging the PCM completely and see if it is still there.

 

[DirtDonk]

Which alternator fuse? The small exciter fuse in the fuse panel? Or the large mega/midi style in the charge wire to the battery?

 

[ShoeSlinger]

Try unplugging the PCM completely and see if it is still there.

Well after finishing building my new home and moving, I started back on this. I unplugged the PCM and the brake lights function correctly. They appear not pully powered with the PCM plugged in, but they are lit up. I had it at a shop and this problem arose instantly after they worked on it and they couldn't find the cause (owner is no longer there due to major health issues). I assume I need to find after the PCM where they might have tied into the brake wiring? Or could it be power feeding back from backfeeding from an improper ground?

 

[samamarshall]

The PCM should be tapped into the brake light switch to receive hot/signal when the brakes are applied, if wired correctly. Check the wiring near your brake switch...maybe you'll see out going OUT to brake lights and one toward the PCM/harness. Disconnect there with PCM hooked up and see what happens?

 

[ba123]

EFi guy mentioned in post 4 that the PCM can backfeed.

Put in a diode on that Brake Input line and you'll be good. Current will only flow towards the ecu if you put it the correct direction (The side with the solid highlight facing away from the brake lights).

I have a few of this one in my build and has worked flawlessly:
https://www.amazon.com/dp/B07Q7YNZZF/?tag=classicbroncos-20

 

[Steve83]

I unplugged the PCM and the brake lights function correctly. They appear not pully powered with the PCM plugged in, but they are lit up.

They must be LED brake lights. You need to add a ~180 Ohm 2W shunt resistor from the BOO circuit (LG) to ground so that the PCM's tiny current isn't enough to light the LEDs. It also allows the PCM to properly detect when the brake is applied so it doesn't keep the TCC unlocked all the time, burning up the trans.

 

[ShoeSlinger]

EFi guy mentioned in post 4 that the PCM can backfeed.

Put in a diode on that Brake Input line and you'll be good. Current will only flow towards the ecu if you put it the correct direction (The side with the solid highlight facing away from the brake lights).

I have a few of this one in my build and has worked flawlessly:
https://www.amazon.com/dp/B07Q7YNZZF/?tag=classicbroncos-20

Just ordered it yesterday. I appreciate the help. So want to get past this issue. Thanks.

 

[EFI Guy]

I don't recommend the diode method, some people that have tried have reported a DTC in the computers from using it. It appears that the issue is that the PCM needs to sense ground when the brakes are not being pressed. This is done through the filaments of the bulbs normally. Steves method of using resistors is correct, but I can not vouch for any resistance value. Resistors create heat which could be dangerous, so I have always recommended using a commercial product designed to prevent hyperflash.

Another option would be to use a newer (5 wire) brake light switch. You would wire it so that the PCM wire is closed to ground normally, and closed to power when the brakes are pressed. This also takes it off of the light circuit so that there is no feedback.

 

[ba123]

I used a 2 circuit brake switch…that’s a great idea. I used one side for power and the other side is a ground signal for my push button start and eStopp brake.

Here is my post showing that:
Post in thread ''71/'76 Badass Bronco Bronco Build'
https://classicbroncos.com/forums/threads/71-76-badass-bronco-bronco-build.318124/post-3524558

 

[Big Slim]

the brake input to the PCM needs to be on its own circuit. the explorer brake switch has the 2 circuits on it. the one for the pcm is a three way switch. is normally to ground so knows that brake is off and then switches to 12v when brake is pressed. i had this issue. is also the same for the cruise control. that is also why people have problems with LED lights. i installed a secondary brake switch for the PCM and cruise circuits.

 

[Steve83]

...the issue is that the PCM needs to sense ground when the brakes are not being pressed. This is done through the filaments of the bulbs normally.

Correct, for incandescent bulbs. That's why LED brake light bulbs create this issue - they have no filament, so they aren't a ground path for the weak signal that the PCM puts into the BOO circuit to detect the state of the lights.

That's also why another diode isn't a solution.

Steves method of using resistors is correct...

It only takes 1 resistor for the BOO circuit, per vehicle.

Resistors create heat which could be dangerous...

That's why the size (wattage) of the resistor is important. A resistor capable of dissipating more Watts (heat) doesn't get as hot as a smaller one would. But it will only get hot when the brake lights are on. At all other times, it won't produce any detectable heat.