4WD Flashing Light Saga – Now Stuck in 4WD After ECU Swaps

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.

 

Appreciate that, I picked up 2 4wd Ecus from a junkyard, and can finally hear the relay clicking in the unit. Sometimes when I turn ignition on lights are off. But second I move to 4h lights come back on. Other times at start it keeps clicking and lights keep flashing. Not consistent. The ecu is getting 12v but I am still getting low voltage around 7v across the board where pins that are on at 2H should be 12, hard to drill down issue when all pins on b plug are showing low voltage. I’m stumped at this point. No way all 3 existed are bad. Any other suggestions would be extremely grateful

 

Very interesting, I can't see what would damage a 4wd ECU in this way. The sense circuits are simply a 12v supply that goes through resistors and is pulled to ground when the switches close.

You have verified you have correct battery voltage between Pin 5 (Violet) and Pin 6 (White/Black) with the key on and ECU plugged in?

 

Correct, i get the battery voltage, between 12-14, depending on whether it's running or not, between 5 & 6. When I take the actuator cover/motor off, is the limit switch supposed to spin freely or have resistance? I noticed it was pretty difficult to spin, I am wondering if the resistance of it turning is causing the ECU to fritz. And in this case took 2 ecus to slowly get it engaged. the contacts looksed pretty clean but all that grease in there was pretty hard from being old and probably original

 

Your referring to the gear?
It's not gona turn too easy but it's designed with a spring mechanism so that the motor can move it to the desired position and when things line up the spring moves the shift fork.

I can't see anything that would harm the ECU, if one or more sense circuits were shorted that would just cause flashing lights it wouldn't cause all of the limit switch voltages to get pulled down.
If a motor circuit was shorted I would expect it to blow the fuse.

Wouldn't hurt to unplug the ECU and both actuators and check all the circuits for continuity to ground though.

 

a head scratcher, that's for sure. Are you familiar with what the clicking is from the ecu when the ignition is turned on? That was a symptom that was interesting, as there were times it would keep clicking til I turned the truck back off.