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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?

Edit:

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!

tempoc
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1 Answers1

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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.

Nick G
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