Blowing ECU B 7.5A Fuse Immediately

Help Tacoma Family! 2012 Tacoma TRD Sport, 4.0L automatic, 129K miles, well maintained, no recent electrical additions... Blowing ECU B 7.5A Fuse immediately, there has to be a short somewhere.

Getting the VSC, SRS (Airbag), center seatbelt flashing lights, no Wireless Key Entry, and the 7.5 Amp ECU B fuse blowing every time a new fuse is plugged in.

To my knowledge this is the 12 volt feed to the Body ECU/Driver side fuse box in cabin. So I pulled all the other fuses that feed the ECU B fuse, it still blows the second I snap it in. The Body ECU is integrated into the fuse box. If I remove the Fusebox Connector 1C, no fuse is blown. So it is in that circuit. I looked at the circuit diagrams, broke it down leg by leg. Any ideas?

That fuse box is expensive and the only other thing I can think it could be would be the steering wheel clock spring or steering angle sensor in the steering wheel. Can I isolate that circuit without pulling the steering wheel? I really don't want to buy the fuse box if there is a real problem that could damage a new fuse box. Do Tacoma fuse box/Body ECU go bad like that?

Here is the pin-out, Inside Fuse Box, Wire Loom 1C

Lower Right to Left (catch down)

1 - (Gray) To A13 Ambient Temp Sensor Conn #1
2 - (Green) RH Parking Lamp Connector P3-1 (Located Front Left Fender)
3 - (Yellow) To A13 Ambient Temp Sensor Conn #2
4 - (White) (7.5A) From Dome Lamp Fuse Internal to Room Lamp Connector 1E-2/7 (Map Light)
5 - (Green/Yellow) BECU #24 (PKB) to Parking Brake Switch <14-6><15-4?
6 - (Light Blue) to ECM Connector DD-5/Direct Tied to Connector IC-15
7 - (Violet) BECU 19 (TRLY) to Connector 2-1 Towing Tail Relay
8 - (Orange) (15A-IGN) To S10 Stop Lamp SW <9-4> Connector 3
9 - (Light Blue/Blue) To Towing Convertor Relay <7-1><7-2><26-4> (BECU Internal to 1D-11 Rear Combo Lamp LH)

Upper Right to Left (catch down)
10 - (Green/Red) Horn Relay Connector 2-2
11 - (Green) LH Parking Lamp Connector P2-1 (Located Front Right fender)
12 - Not Used ?
13 - (Orange) (15A-IGN) Goes to Connector 2-1 C/OPN Relay
14 - (Light Green) (10A-IG1 No.2) to Connector DC-13 Ignition Rheostat/DC-12 Stop LP CTRL/1K-13 Comb Meter AT Oil Temp/1C-16 Combo Meter/DC-3 S1(B) IG1
15 - (Green/White) Skid Control ECU W/ Actuator <14-8><15-3>/Direct Tied to Connector IC-6
16 - (Red) BECU ECUB #6/Goes to Connector 1J-14 Transponder Key ECU/To 7.5 Fuse Conn 2-2
17 - (Orange) (15A-IGN) To Connector
C-1 (IG2) S1(A), S2(C) Skid Control ECU w/ Actuator
18 - (Light Blue/Yellow) To Towing Convertor Relay <7-1><7-2><26-4> (BECU Internal to 1D-4 Rear Combo Lamp RH)

 

I'll poke around on some wiring diagrams here in a bit and see what I can come up with, it would be helpful to have a headlight bulb in place of the fuse so we're not just wasting fuses.

https://youtu.be/oKCLzf4va7M?si=qcmBnmuhRU7_SgGn

 

Thanks Dm93, added the wiring diagram for you on the post. I am still pouring over it. Really appreciate any assistance!

 

I have access to service info/diagrams.

I don't think your problem is in the Body ECU/Junction Box, the reason it doesn't blow when 1C is unplugged is that is the power feed from the fuse into the junction box. The junction box then distributes it to other terminals.

Do you have a light bulb you can plug in place of the fuse and a clamp on amp-meter? That would help us narrow this down pretty quick.

I'm working on a list of stuff it feeds.

 

Here's everything it feeds and the connector/pin on the Junction Box, all are Red wires.


Body ECU Pin 6 (Internal to Junction Box)

Connector 1I
Pin 1: Steering Angle Sensor
Pin 5: Occupant Classification ECU (Bench Seat Only)
Pin 16: Door Control Receiver

Connector 1J

Pin 4: AutoDimming Rearview Mirror
Pin 14: Transponder Key ECU

Connector 1K

Pin 6: A/C Amplifier
Pin 13: Combination Meter

 

@Dm93 Really appreciate the pointers! Here is what is done so far:
Complete- Made a 5 AMP headlight test fixture, could not find 10 or 15 Amp bulbs at Auto Store.
Need to- Disassemble dash to get to back of BECU Fuse/Junction Box for Connectors 1I, 1J, and 1K.

So if I am following you correctly, attached diagram to help others that may have this issue.

Steps to follow, relating this to the video you sent:

1. Un-plug these 3 connectors (1I, 1J, 1K)
2. Insert ECU B Fuse (7.5A), it should not blow as these three connectors are out of the circuit.
3. If fuse does not blow immediately, we know it is on one of these legs.
4. I proceed to 1I, connect my test fixture to ground.

BECU Connector 1I
5. Test Connector 1I pins 1, 5, and 16 (All red), if light goes off that is the circuit to isolate (half step)
6. If Connector 1I checks, plug it back in proceed to Connector 1J

BECU Connector 1J
7. Test Connector 1J pins 4 and 14 (All red), if light goes off that is the circuit to isolate (half step)
8. If Connector 1J checks, plug it back in proceed to Connector 1K

BECU Connector 1K
9. Test Connector 1K pins 6 and 13 (All red), if light goes off that is the circuit to isolate (half step)
10. Plug it back, should be one of those circuits shorting out system

Thoughts? I will be ripping apart dash, will let you know.

 

@Dm93 Well...
1. I disconnected battery again.
2. Tore apart dash, exposed back of BECU.
3. Disconnected Connectors 1I, 1J, and 1K.
4. Replaced ECU B Fuse (7.5A), reconnected battery.
5. ECU B fuse did not blow...
6. Decided to reconnect each connector to see which would blow the circuit.
7. ECU B did not blow a fuse...
8. Started Tacoma with dash torn apart...
9. No codes, no lights, no blown fuse...

Should I proceed with test fixture?

 

Sounds like you are on the right track, let me know what you find or if you need any other help.

 

Interesting so it would seem whatever was shorted must have gotten moved in the process and is no longer shorted.

 

@Dm93 please see my new post above. We simultaneously posted. Should I still do the test fixture? Intermittent faults are the worse.

 

Yes, I agree, we had a 4 day hot spell for Washington State (80 degrees lol), maybe things got more pliable in the dash?

 

@Dm93 spoke too soon, ECU B fuse blew again. Proceeding with the steps. Will advise, thanks!

 

Yea they are, that's why you need your test light in place when your moving stuff around so when it goes out/gets dim you know what you moved.

 

@Dm93 - I did tests on BECU Connectors 1I, 1J, and 1K, no headlight illumination or probe light in ignition in off position. Fuse is good in off position. I also got a C1432 code from my SRS scan tool, but dismissed it due to ECU B fuse being blown. Any more ideas? Does that code matter?

Added...
All connected and off no blown fuse. Maybe steering wheel related?

 

I think the best course of action here would be to put the headlight in place of the fuse and plug everything in, then when the headlight lights up (short occurs) then you can unplug/wiggle stuff to find out what is shorted.

Stuff is gona set codes when it's missing a power feed so I would ignore the code for now.

 

@Dm93 - Can do.

1. Will my headlamp (5 amps) be enough for fuse circuit? Or do I need to get another one to get to 10 amps?

2.I started the vehicle, everything connected, no fuse blow. I actuated locks, turned steering wheel, used wireless fob, no blown fuse. It sure seems to be in the loom.

 

It's plenty, I doubt that circuit ever draws more than 3 amps under normal conditions. It's esdentially just keep alive power for the modules.

 

@Dm93 - I found it... You will never guess what was grounding out whole Body ECU (BECU).

I checked every red wire system. It is the door control circuit.

I found it by accident. My Tacoma has an ARE Z-Series Canopy/Topper, installed 4 years ago. The electronic canopy lock is tied to the Tacoma Door Lock System. It failed after one year, had ARE dealer fix it under warranty. It lasted 1 more year. Now out of warranty, I said I will just use my key to lock. It never effected the Tacoma until now. It is powered on a 10 Amp circuit. When I pulled the fuse to that circuit, no ECU B fuse blown. When I install the 10A fuse to canopy, blows ECU B immediately. Joy.

@Dm93 I owe you a cold one next time I am in the Great State of Texas, the Last Great Republic. Thank you!