The Electronic Control Unit on my FJ Cruiser is not relaying the signal properly to the rear differential lock actuator when I switch it on.

I want to bypass the ECU altogether and make the switch lock and unlock the diff on-demand. How do I do this?

I have the circuit diagrams but some parts I can't make sense of. Does anyone have experience with Toyota circuit diagrams?


Diagrams, see page 200: http://www.lastgreatroadtrip.com/wp-content/uploads/2008/03/2007-toyota-fj-cruiser-electrical-wiring-diagram.pdf

Also, the shop at the Toyota dealer confirmed the ECU failure. According to them the actuator is working fine.

Edit 2 and happy conclusion:

So much for the shop's diagnosis. Very recently I took it off road again and after a couple of bumps I decided to push the REAR LOCK button once more. It worked fine. Bought my '07 FJ used in '14 and it took almost 3 years of use to shake it loose. Seems the previous owner never even tried to use the locker and it was seized.

Show your truck some love and take it off road for some exercise!

  • Would you want to comment farther as to your issue with A-TRAC? I found this online: I talked to one of the guys on the FJ Trail Team earlier this summer and he said that A-Trac only engages when the ABS sensors detect that one wheel is not rotating at the same speed as another. If one would engage the locking rear differential, both rear wheels would always be turning the same speed and A-Trac would only function on the front. ie. The computer would "think" that the lack of traction was at the front wheels only and that the back wheels must have great traction. It may be your (non-)issue. Mar 20 '15 at 22:21
  • This is independent of A-TRAC. They are separate systems. My A-TRAC is working fine and although A-TRAC does connect to the ECU this bypass would make it irrelevant to the rear locker process.
    – tempoc
    Mar 20 '15 at 22:23
  • 2
    It looks like there may be a little more to it than just a switch. The searching I did turned up a motor with travel stops. This would mean that control would be a SPDT momentary switch rather than a SPST rocker (think window switch). My best guess is (after grounding) you'll hit either of the + relay points to drive the motor each direction and the other two are either a single travel stop sensor or a pair for each end of travel using ground to complete a circuit. You may have to crack it open to learn anything more. Mar 21 '15 at 2:08
  • The ECU could be a big part of it IF it controls it. I know Palmer Performance offers some 3rd party scripts on it's Scan XL software, that for instance could change the idling RPM's, to save gas mileage. You have to buy the software license, (lifetime serial numbers via email), and an OBD/OBDII connector, which vary depending on whether you just need a simple 60 dollar connector or a wireless bluetooth, turn your car off and on from your phone connector. Apr 8 '15 at 7:10

From page 200 for the 'Rear Differential Lock' diagram... The 'RR Diff Lock' switch simply passes ignition voltage (+12) to an input on the ECU (R) pin. When the ECU sees this signal it controls an electric motor to lock the differental via the (M1) (M2) pins. This happens via an h-bridge (inside the ecu), which is capable of reversing the current through the motor to turn it in one direction or the other. (RLY1) and (RLY2) are limiting switches to stop the motor from turning too far in either direction. It appears the switches close to ground when the limit is reached.

I'm not too familiar with this differential specifically, but my educated guess is that it is an 'adjustable' slip differential. As the motor turns it can increase or decrease the amount of 'slip' between the two wheels speeds. So one limit is 'open', the other is 'locked', and a variable 'limited' in between.

The ECU can be bypassed, but the solution would require some circuitry to do so (more than just a simple switch). If you're interested in going down this path let me know and I'll sketch something out.

SAFETY NOTICE - You'll also notice in this diagram that 'SPD' or speed is an input to the ECU. The ECU won't allow the driver to lock the Differential if the speed is over a certain preset. This is because fully locking a differential at high speeds can be dangerous and unpredictable, especially while in a turn/curve.

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