CTD NUT

Well, on the weekend, I noticed my brake lights (tail lights only - NOT the 3rd cab light) come on with the ignition. The pedal still activates the 3rd cab light properly but the tail lights stay on and are unaffected by the brake pedal. All other tail light functions work properly. The brake lights work properly with the ignition off. I can't figure it out. I pulled every fuse in the PDC one at a time until they went off and the only fuse that killed the lights was and ignition feed fuse that appears to be an internal feed circuit for the IPM/FCM. Through the body builder schematics, I have traced the circuit back to the FCM brake light drivers circuits but cannot figure out the flow of that circuit past that. As near as I can guess, the FCM appears to have somehow failed and is turning the ground side of the relays on for some reason.

Am I on the right track? I can hear the relay for the lights click on when the ignition comes on. There is a 2 second delay after the ignition is turned on and the brake lights come on. I can clearly hear the relay click on and watch the lights come on at the very same time.

I'm baffled. Help!

Lil Dog

Do you have a trailer brake controller? That may be tripping from some other fault in it. Unplug it and see what happens.

CTD NUT

^ Yup, I have a P3. I did unplug it and it did not change anything and there are no DTC's present. The P3 appears to function normally and is unaffected by this issue.

Lil Dog

Darn.. Thats the only thing I've got..

Foxborough

I seem to recall that the brake switch is not just (1) single pole single throw but (2) poles in it. May be as simple as a new switch. It was that way on my sisters Ford, I would think for safety there would be (2) poles on it. Just my guess.

CTD NUT

I hear you but the brake lights function just fine with the ignition off. The 3rd brake light, tails and controller all funtion properly. The only thing that changes is the tail lights come on and stay on when the ignition is turned on. It really does appear to be something other than the brake switch turning the tail brake lights on. Even with the ignition on, the brake controller circuit and 3rd brake light function properly.

CTD NUT

Swapped FCM's and it didn't solve the problem...anyone have any ideas? Is there a chance it could be IPM/PDC related instead of the FCM? I'm lost now.

Fisherguy

Brake light switch is my guess, they're only bout 20 bucks.

CTD NUT

^ Hard to imagine when the brake lights work fine when the ignition is off and the 3rd brake light and brake controller continue to function properly off of the brake switch when the ignition is on.

The tail brake lights stay on even with the brake switch disconnected when the ignition is on!

Lil Dog

Is it full on brake circuit or are they on park circuit? Would be weird to have both of them on without anything else. The hazard light circuit is like that where the CHMSL will work with the brake pedal but the tails are on flash.. Would the hazard flasher be kaput or that switch?

CTD NUT

Hazards and signals use the same bulb circuits and the brakes have another all their own. It is the brake lights that are coming on with the ignition. Not the parking lights or signals. Signals and parking lights continue to work fine.

Colo_River_Ram

Are you throwing any codes and does the cruise control work?

CTD NUT

No codes. As for the CC, I'm fairly sure it does work. The problem is, I'm not sure when my brake light issue started. I may have noticed it right away, or it may have been like that for a while and I did not notice - not too often that I am looking at the back of the truck with the ignition on to realize this particular problem existed in the first place. So, I believe the cruise works BUT I am NOT 100% sure of that. That is a good point, though. That might go a long way to deciding if the brake switch or IPM is the cause? The thing is, the brake lights stay on even with the switch disconnected and the ignition on. That really makes me think it is not the switch....but I am far from an electrical expert on these fricken trucks.

Nascarcamper

Canadian truck has DRL (daylight running lights) this could be a culprit, I think it is controlled by the cluster but there may be a relay under the hood. Other things to look at would be the trailer light connections, 7 way and 4 pin flat somehow back feeding the system (no 3rd light lit). Just my 0.02$ for what it's worth.

CTD NUT

Well, something is energizing the relay and ignition on only. This pretty much makes a back feed impossible since the load side and trigger side (energize) of a relay is two separate circuits. Also, there is no trailer circuit on my truck that has ignition on only power.

I'm really hoping an electrical guru will chime and point me in the right direction...I'm pretty much down to the PDC but it is so much freaking money and non-refundable that I want to be sure of the diagnoses.