Found Another Way to Break the Supra

[Supraman40]

After doing some extensive engine work and having the battery disconnected I'm having an electrical issues for the past couple days

Battery was disconnected for about two months

Symptoms
Car wakes immediately when battery is connected
Dash lights flicker
Headlights repeatedly self-level / adjust
Fuel pump runs
Car won't not turn off unless unplugged or remove and replug BDC Con4
DME not discoverable by MHD (cannot connect)
No crank
Symptoms temporarily resolve after unplugging/replugging BDC Con4, but return after sitting
Key Observations



All fuses verified at ~12.5 V
Engine block → battery negative: 0.0 V (ground confirmed good)
DME constant power pin: ~12.6 V (normal)
Troubleshooting Already Done
Verified all fuses (no blown fuses)
Confirmed engine and chassis grounds are good
Tested starter power and grounds
Isolated multiple circuits by unplugging modules
Disconnected:
Headlights
Fuel pump control module
Rear junction box (JBR)
Multiple BDC connectors
Identified BDC Con4 (wiper-related connector) as repeatable reset point
Believe the issue is not permanent backfeed, but a wake/logic latch

Ruled out:
Headlights
Ground faults
Fuel pump hardware
DME power feed failure
Strong suspicion of failing BDC (Body Domain Controller) with internal wake/ignition latch fault

The next thing I'll do is remove Reflex wiring.

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[itzTang]

Sure the battery still good? Try disconnect all dme harness and make sure no pins bent

 

[razorlab]

Did you check the actual battery?

 

[Supraman40]

Did you check the actual battery?

I replaced the battery when this first started happening, it was bad. But replacing battery didn't change anything.

 

[Evolution]

I replaced the battery when this first started happening, it was bad. But replacing battery didn't change anything.

did you code the car to tell it that it got a new battery?

 

[Supraman40]

did you code the car to tell it that it got a new battery?

I can't code the car because MHD, Bimerlink, AND Bimercode need the DME to respond. When I attempt to connect the apps say the DME could not be found.

 

[razorlab]

So what work did you do on the car while the battery was out?

 

[Supraman40]

So what work did you do on the car while the battery was out?

I'm the guy that was doing the engine build in the garage. I finally got everything put back together, but I'm having this issue now.

 

[tracer bullet]

Strong suspicion of failing BDC (Body Domain Controller) with internal wake/ignition latch fault

FWIW, something like this seems to make sense. The DME gets inputs from a number of other controllers, and if one of them is flaky, then it seems like it could have a chain reaction. Flickering headlights, dash lights... I'm trying to think if they could share a common thing like thinking that the door is being opened then closed then opened again or something.

On the other hand I'm doubting that because it doesn't seem like that should prevent MHD from at least finding and communicating with the DME to start with. For the wiper connector and the issues temporarily resolving do you mean that you can for a while connect to the DME or actually start the car? That's kind of crazy but if so it certainly seems to set you down a path of finding out if other symptoms can be tied to it.

With all the other work you did, maybe a wire accidentally got pinched or pulled on too hard?

Just thinking out loud of course.

 

[Supraman40]

I've looked for physical damage to wires and can't find anything. When I unplug Con4 of the BDC the symptoms stop (flashing dash lights, headlights, fuel pump running, vehicle won't turn off) but I still can't connect to the DME, also, the symptoms stop when I unplug any one of the DME connections.

Next I'm going to remove the Reflex wires from the BDC.

 

[itzTang]

Did you unplug or plug any module while battery was connected? If so may have bricked any one of them. But a scan tool should be able to still connect thru obd port and tell you which module isn’t communicating. If not start testing the obd port continuity

 

[Supraman40]

I went back through the wires and found 3 wires cut, 2 for the intake cam sensor and 1 for the pcv sensor. I crimped them but no change. Not sure if crimping those wires would actually be good enough.

 

[Supraman40]

Fix: I found the root cause, I missed the wiring harness ground that connect to the rear of the block.

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