4WD Flashing Light – TL2/TL3 Circuit Issue – Traced to Wiring, Need Help with 12V Feed Source

Hey all,
Like many, I’m dealing with the dreaded flashing 4WD light. I’ve read through a bunch of threads and haven’t started blindly replacing parts — I’ve been methodically testing everything with a multimeter to trace the issue.

I’ve confirmed the truck is in 2H, and the front driveshaft spins freely, so the transfer case isn't engaging.

Here's where I'm at:

• The break seems to be at the TL2 and TL3 position sensor circuits
  
  
• In 2H, the ECU should see 12V at both TL2 and TL3
  
  
• Instead:
  
  • TL2 is showing 0V at the ECU
    
    
  • TL3 is showing ~7V at the ECU
    
• When I probe the sensor plugs directly, both TL2 and TL3 are only getting ~7V
  

So I’m seeing:

1. Low voltage across the board to the TL sensors (they should be getting 12V)
   
   
2. A broken signal wire between TL2 sensor and the ECU (7V at the sensor, 0V at ECU)
   

This strongly points to an electrical issue — not mechanical — likely a bad splice or broken wire in the harness. I followed the loom up along the transmission and across the firewall but haven’t visually found a break yet.

Also worth noting:

• No relay clicks
  
  
• Nothing happens when attempting to switch modes
  

My questions:

• Where exactly does the 12V feed to the TL position switches come from?
  
  • Is it tied into the 4WD relay, ignition circuit, or something else?
    
• Has anyone had a similar issue with a broken TL2 signal wire? Any known failure points along the harness?
  

Any insight would be seriously appreciated — I’m close, but hitting a wall tracking this last part down.

Thanks!

 

Have you read through the thread below?

https://www.tacomaworld.com/threads/4wd-system-operation-help-thread.742420/

The 12v is a pull-down type sense circuit, the 4wd ECU sends out the 12v and the limit switches pull it to ground (0v) when they close.
With the Transfer Case Actuator unplugged you should see 12v on TL1, TL2, and TL3.
If you don't then you need to check the circuits for a short to ground.

 

Yes, I've looked at that thread, and as mentioned, I'm getting 7v at TL2 and 3 plugs on switches, but at the ECu, I'm getting 0v and 7v, so it is 100% an electrical issue. From my understanding, it's actually the opposite. The sensors are connected to a 12v source, and the sensors send the signal to the ECU; the ECU doesn't send the 12V to the switches.

 

Interesting, it kind of looks like the 4wd ECU has failed but I'd verify none of the limit switch wires have continuity to ground and verify power and ground to the 4wd ECU before condemning it.
Also curious that you have 0v at the ECU but 7v at the actuator unless your on the wrong terminal or do you have the actuator plugged in when checking this?

Nope the limit switches are Grounded at Ground Point EE on the back of the left cylinder head, the ECU sends out the 12v reference on the sense circuits and the switches pull it to Ground when they close thus it's a pull-down type sensing circuit. When the circuit is open it should be right at 12v.