LJ71 Prado Automatic – Swapped from 2L-TE to 3RZ-FE with 2WD Hilux ECU, White Paint, Maxxis Tires – Need Help with RPM & ECT Issues A343F Transmission

[rakan100]

Hi everyone,

I wanted to share the latest updates on my Toyota Land Cruiser Prado LJ71 (automatic) project. I’ve done a lot of work on it recently — from engine swap to paint and tires — but I’ve run into several issues that I hope to get some feedback on.

What I’ve Done So Far:


1. Engine Swap – From 2L-TE to 3RZ-FE:


The Prado originally came with a 2L-TE turbo diesel. I swapped it for a 3RZ-FE (2.7L petrol) engine to improve power, especially for city and occasional off-road use.


2. Hilux 2WD ECU:


I’m using an ECU from a 2WD Hilux to run the 3RZ-FE. I’m wondering if this could be the reason for some of the problems I’m experiencing, particularly with idle and transmission-related electronics.


3. Transmission – A343F (Mechanical, No ECU):


The automatic gearbox is the factory A343F, a mechanical transmission without an ECU. It shifts fine, but some of the functions like ECT aren’t working anymore (more on that below).


4. White Paint Job:


I repainted the Prado in clean white, giving it a much fresher and more classic look. Really happy with how it turned out.


5. Maxxis Tires:


I installed Maxxis off-road tires. They’ve been excellent so far, with great grip and road stability. They also give the Prado a tougher stance.

Problems I’m Facing:


1. Unstable RPM at Idle:


The RPM isn’t stable — it fluctuates, especially at idle. This might be due to vacuum leaks, idle control valve issues, or maybe ECU mismatch since it’s from a 2WD Hilux.


2. ECT Button Not Working:


The ECT (Electronically Controlled Transmission) button no longer does anything. Since the A343F transmission is mechanical, I’m not sure if the ECT is even relevant anymore, or if something in the wiring can still make it function partially.


3. 4WD System Issues:
• The 4WD (light-duty / high range) switch does not work. Pressing the button doesn’t engage anything.


• The front hub lock switch activates (I can hear/see it respond), but it doesn’t actually lock or send power — nothing changes when it’s turned on or off.


• I noticed that the front driveshaft is currently disconnected, so I assume this is part of the issue, but I’m not sure how it’s all linked together or what should be checked to fully restore 4WD functionality.





Questions for the Community:


• Could using a 2WD Hilux ECU affect the behavior of the 4WD system or other electronics?


• Is it normal for the hub lock to “click” but not engage anything if the front driveshaft is disconnected?


• How do I properly restore function to the 4WD button and make sure the front axle and hubs are engaging correctly?


Any help would be greatly appreciated — I know many of you have gone through similar builds, and your advice is always valuable. Thanks in advance for your time and feedback!

Best regards,

 

[JuanJ]

Hello,

I have not gone through such a swap. However, there are things to consider.

Engine ECUs tend to be specific for manual and automatic transmissions. The microprocessor checks for the transmission it was programmed to work with. I understand there are ways around this without having to hack the software. This may or may not include disabling the ECT function.

Your truck engages 4WD with a vacuum switch. It may be disconnected, or it may ave a leak. The actuators are on the right side, and it is good to check them out.

70 Series trucks have a 4WD ECU that controls front drive (as well as electric hubs and differential lockers if available) operation. It is located above the pedals. Make sure this ECU is connected.

The procedure to test the front drive and 4WD is in the factory manual.

Hope this helps.





Juan

 

[rakan100]

Hello,

I have not gone through such a swap. However, there are things to consider.

Engine ECUs tend to be specific for manual and automatic transmissions. The microprocessor checks for the transmission it was programmed to work with. I understand there are ways around this without having to hack the software. This may or may not include disabling the ECT function.

Your truck engages 4WD with a vacuum switch. It may be disconnected, or it may ave a leak. The actuators are on the right side, and it is good to check them out.

70 Series trucks have a 4WD ECU that controls front drive (as well as electric hubs and differential lockers if available) operation. It is located above the pedals. Make sure this ECU is connected.

The procedure to test the front drive and 4WD is in the factory manual.

Hope this helps.





Juan

Thank you very much for your kind and informative response — I really appreciate it.
Honestly, I wasn’t sure whether my transmission had a separate ECU or not. Your explanation helped me understand things much better, especially about the 4WD ECU and the vacuum system.
I’ll start checking the areas you mentioned and see what I can find. Your advice gave me a clear direction to follow.
Thanks again, Juan

 

[rakan100]

Hello,

I have not gone through such a swap. However, there are things to consider.

Engine ECUs tend to be specific for manual and automatic transmissions. The microprocessor checks for the transmission it was programmed to work with. I understand there are ways around this without having to hack the software. This may or may not include disabling the ECT function.

Your truck engages 4WD with a vacuum switch. It may be disconnected, or it may ave a leak. The actuators are on the right side, and it is good to check them out.

70 Series trucks have a 4WD ECU that controls front drive (as well as electric hubs and differential lockers if available) operation. It is located above the pedals. Make sure this ECU is connected.

The procedure to test the front drive and 4WD is in the factory manual.

Hope this helps.





Juan

Do you know where I can find the factory workshop manual or wiring diagram for the 70 Series in PDF format? I’ve been searching online but haven’t had much luck

 

[JuanJ]

Do you know where I can find the factory workshop manual or wiring diagram for the 70 Series in PDF format? I’ve been searching online but haven’t had much luck

Hello,

Check the Resources section here on 'Mud. There is also a sticky thread about online manuals.

The manual you need is RM183E.





Juan

 

[rakan100]

Hello,

Check the Resources section here on 'Mud. There is also a sticky thread about online manuals.

The manual you need is RM183E.





Juan

Thank you so much

 

[AirheadNut]

1. I don't know anything about the 3RZ, so unfortunately I cannot help with that issue.

2. The A343f is actually an electronically controlled automatic transmission, on automatic versions of the Prado the engine ECU also controlled the transmission. The transmission can be "manualized" or run off a standalone ECU, or if the Hilux you got your ECU from came with an A343 or even an A340 the appropriate ECU could probably make it work like factory. Even without an ECU the transmission can still be shifted through each gear using the shifter, but functions like the ECT or Power button won't work.

3. Like Juan said, the 2WD/4WD shift is controlled via a pair of VSV's mounted on the RH side of the firewall. It is not unlikely that the vacuum source or the lines from the VSV's to the diaphragm on the transfer case were disconnected during the swap.

The hub lock system is a common failure point on these vehicles, likely the switches and relays are present and functional, but the brushes, slip rings or motors in the hubs themselves are dirty, corroded or worn out. Sometimes the system can be functioning but the hubs themselves don't engage right away, I usually slowly rolled the vehicle back and forth a couple of times to help the hubs engage or failing that, would reach underneath and manually rotate the front driveshaft until I felt both hubs lock in. I have since converted to manual hubs, a much more failsafe and reliable system. The electric hubs can be taken apart, cleaned and lubricated, which may or may not restore functionality, or replaced altogether. Pretty sure the entire hub assemblies are still available new.

Obviously not having a front driveshaft will preclude having 4WD, but its absence will not prohibit any of the other parts of the system from functioning (like the electric hub locks or 4WD engagement in the transfer case).

 

[rakan100]

1. I don't know anything about the 3RZ, so unfortunately I cannot help with that issue.

2. The A343f is actually an electronically controlled automatic transmission, on automatic versions of the Prado the engine ECU also controlled the transmission. The transmission can be "manualized" or run off a standalone ECU, or if the Hilux you got your ECU from came with an A343 or even an A340 the appropriate ECU could probably make it work like factory. Even without an ECU the transmission can still be shifted through each gear using the shifter, but functions like the ECT or Power button won't work.

3. Like Juan said, the 2WD/4WD shift is controlled via a pair of VSV's mounted on the RH side of the firewall. It is not unlikely that the vacuum source or the lines from the VSV's to the diaphragm on the transfer case were disconnected during the swap.

The hub lock system is a common failure point on these vehicles, likely the switches and relays are present and functional, but the brushes, slip rings or motors in the hubs themselves are dirty, corroded or worn out. Sometimes the system can be functioning but the hubs themselves don't engage right away, I usually slowly rolled the vehicle back and forth a couple of times to help the hubs engage or failing that, would reach underneath and manually rotate the front driveshaft until I felt both hubs lock in. I have since converted to manual hubs, a much more failsafe and reliable system. The electric hubs can be taken apart, cleaned and lubricated, which may or may not restore functionality, or replaced altogether. Pretty sure the entire hub assemblies are still available new.

Obviously not having a front driveshaft will preclude having 4WD, but its absence will not prohibit any of the other parts of the system from functioning (like the electric hub locks or 4WD engagement in the transfer case).

Thank you so much for the detailed reply, I really appreciate your help and the information you shared — it gave me a much clearer picture of how the system works.

After going through all this work, I’m actually starting to consider getting a 1998 Prado (J90) with the factory 6-cylinder engine and automatic transmission, since that model is very common and affordable here in Saudi Arabia.

Do you think it’s a good idea to use one of those as a donor vehicle for parts?

Thanks again and much respect!

 

[Eurasiaoverland]

I have a 3RZ-FE in a Hilux Surf. When I got the vehicle it was very neglected and I had issues with high idle (and many others, which led to a full engine rebuild). I changed the idle sped control valve (which was a Made in China copy) for a genuine one, cleaned the MAF sensor and still had the issue. Eventually it was I think the air temperature sensor in the air filter box which cured the issue. There is also a coolant temperature sensor on the back of the cylinder head (or block, can't remember now). All these things need to be working correctly in my experience to get the idle to sit correctly.

 

[rakan100]

I have a 3RZ-FE in a Hilux Surf. When I got the vehicle it was very neglected and I had issues with high idle (and many others, which led to a full engine rebuild). I changed the idle sped control valve (which was a Made in China copy) for a genuine one, cleaned the MAF sensor and still had the issue. Eventually it was I think the air temperature sensor in the air filter box which cured the issue. There is also a coolant temperature sensor on the back of the cylinder head (or block, can't remember now). All these things need to be working correctly in my experience to get the idle to sit correctly.

Thank you so much for sharing your experience — I really appreciate it!

My issue is a bit different though. The engine feels like it's struggling or under heavy load even at low speeds, like it's "out of breath" or being held back somehow. So it might not be just an idle control issue in my case.

Still, your advice is very helpful and I’ll definitely check those sensors too. Thanks again!