4WD transmission system operation

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Introduction

• The operation of Jimny's 4WD transmission system is a mystery to most owners, especially to those who have newer "push-button" operated transmissions.
• This is all fine when the transmission works fine.
• But once it stops working, it is hopeless to diagnose the fault without understanding its operating principles, especially since the vehicle's instrument panel does not say much!
• To aid confusion to the mystery, there is a lot of wrong information around on various places on the Internet (including some topics in this forum!).

• The purpose of this article is to explain in detail how Jimny's 4WD transmission system works when it works, and how it works when it does not work.

Types of Jimny's 4WD transmissions

Main characteristics

First of all, there are some rare "el cheapo" Jimnys (mostly from early year 1998-2001) which have no 4WD transmission mechanics - they are 2WD (RWD) only. This article is not for them.

All Jimnys with 4WD transmission have the same overall transmission design:

• User selectable part-time 4WD transmission which defaults to RWD 2WD (real wheel drive two wheel drive) mode.
• Transmission box with user selectable high/low range gear set, without center differential, with (dis)engagement mechanism for the front propeller shaft.
• Free wheeling hub heads on the front wheel hubs, pneumatically (vacuum) operated.

The differences are in the design of the control of the transmission system.

Control designs of the 4WD transmission system

Jimnys exist in two editions / generations regarding the 4WD transmission system:

• Older Jimnys with mechanically (lever) operated transfer boxes;
• Newer Jimnys with electro-mechanically ("push-button") operated transfer boxes;

• The change of the transmission system occurred around 2005.
• These two types of transmission systems can be easily discerned by looking if the Jimny has an additional gear-box-like lever between the gear box lever and the hand brake lever, or not.
• If it has the lever, it has the older mechanically operated transfer box.
• If it does not have the lever, and it has the newer (post-2005) interior design, than it has the newer "push-button" operated transfer box (there are the "2WD / 4WD / 4WD-L" buttons on the center console, above the ash tray).
• If it does not have the lever, and it has the older (1998-2005) interior design, than it is the rare el-cheapo "2WD-only" Jimny edition. That Jimny has no 4WD transmission at all.

• Both the older and the newer 4WD system have a dedicated computer - a 4WD controller.
• That device is an important operational element of the entire vehicle.
• The device is different in the older and the newer Jimny's 4WD system, and they operate differently.

Operational design of the older 4WD system

Example when shifting from 2WD to 4WD-H

1. The driver operates the mechanical transmission lever from 2WD to 4WD, which physically moves the bits in the transfer box, which connect the front propeller shaft to the transfer box.
2. The 4WD controller senses this change in the lever position and monitors (by a dedicated sensor in the transfer box) if the mechanical engagement in the transfer box has completed successfully.
3. If the 4WD controller detects that the transfer box has completed the change successfully, then the 4WD controller changes the condition of the green 4WD light on the instrument panel from "off" to "flashing", and sends the signal to the vacuum system (valves and switches) to send the vacuum to the front wheel hub heads, in order to engage them.
4. The 4WD controller waits a bit to get a return signal from the vacuum system if the vacuum is achieved (this is officially considered an indication that the hub heads should have engaged).
5. If the 4WD controller receives a return signal from the vacuum system before time-out, the 4WD controller changes the condition of the green 4WD light on the instrument panel from "flashing" to "ON" and also sends the signal to the ABS controller (another computer in the vehicle - if the vehicle has ABS) that 4WD transmission is engaged, so that the ABS controller can adjust the operation of the ABS system accordingly.
6. The process of shifting the transmission from 2WD to 4WD-H is formally complete.

Exception - transfer box fault

If the 4WD controller does not receive a positive signal from the transfer box in step 2 of the above process:

1. The 4WD controller will change the state of the green 4WD light on the instrument panel from "off" to flashing" and will not even attempt to engage the vacuum system.
2. The green 4WD light will remain in the "flashing" state until the driver manually disengages the transmission lever back to 2WD mode.
3. Only then will the 4WD controller turn off the green 4WD light.

Exception - vacuum system fault

If the 4WD controller does not receive the return signal from the vacuum system that the vacuum is achieved (in step 4 in the above process):

1. The green 4WD light will remain in the "flashing" state;
2. The ABS controller will receive no signal about the engagement of 4WD operation;
3. The transfer box will still remain in 4WD mode, because it is operated by a mechanical lever, until the user shifts the lever back to 2WD mode.

Example when shifting from 4WD-H to 4WD-L

1. Since the shift is being performed with a mechanical lever, it is physically not possible to shift directly between 2WD and 4WD-L (and vice versa) - it has to be intermediately shifted in 4WD-H.
2. The driver shifts the transmission lever from 4WD-H to 4WD-L position - under the condition that the vehicle is stationery and that the clutch pedal is pressed (for vehicles with a manual gear box) or that SOMETHING IS (for vehicles with an automatic gear box). If those two conditions are not fulfilled, it will not be possible (or will be very hard) to shift the lever. - TRUE?
3. The 4WD controller senses this change in the lever position and monitors (by a dedicated sensor in the transfer box) if the mechanical shift in the transfer box has completed successfully.
4. If the 4WD controller detects that the transfer box has completed the change successfully, then the 4WD controller changes the condition of the yellow "L" light on the instrument panel from "off" to "on" state, and simultaneously sounds a single beep sound.

Conclusion

• In the older (lever operated) transmission system, the 4WD controller just operates the vacuum system and informs the ABS module upon successful completion of the entire process - after sensing that the driver has manually shifted the mechanical bits in the transfer box.
• The driver manually operates the transfer box at will.

Operational design of the newer 4WD system

Example when shifting from 2WD to 4WD-H

1. The driver pushes the "4WD" button on the dash board.
2. The button sends the signal to the 4WD controller, which then sends a signal to the electrical actuator in the transfer box.
3. The actuator moves the mechanical bits in the transfer box, connecting the front propeller shaft to the transfer box.
4. A dedicated sensor in the transfer box informs the 4WD controller if the mechanical engagement in the transfer box has completed successfully.
5. If the 4WD controller detects that the actuator in the transfer box has completed the change successfully, then the 4WD controller changes the condition of the green 4WD light on the instrument panel from "off" to "flashing", and sends the signal to the vacuum system (valves and switches) to send the vacuum to the front wheel hub heads, in order to engage them.
6. The 4WD controller waits a bit to get a return signal from the vacuum system if the vacuum is achieved (this is officially considered an indication that the hub heads should have engaged).
7. If the 4WD controller receives a return signal from the vacuum system before time-out, the 4WD controller changes the condition of the green 4WD light on the instrument panel from "flashing" to "ON" and also sends the signal to the ABS controller (another computer in the vehicle - if the vehicle has ABS) that 4WD transmission is engaged, so that the ABS controller can adjust the operation of the ABS system accordingly.
8. The process of shifting the transmission from 2WD to 4WD-H is formally complete.

Exception - transfer box fault

If the 4WD controller does not receive a positive signal from the transfer box in step 4 of the above process:

1. The 4WD controller will turn the green 4WD light to "on" just for a fraction of a second, and then leave it in the "off" state.
2. An audible click will be heard the the same time when the green 4WD light goes back to the "off" state.
3. The 4WD controller will simultaneously command the actuator in the transfer box to return the bits inside back to "2WD" mode (front propeller shaft disengaged from the transfer box).
4. The 4WD controller will not even attempt to engage the vacuum system.

Exception - vacuum system fault

If the 4WD controller does not receive the return signal from the vacuum system that the vacuum is achieved (in step 6 in the above process):

1. The green 4WD light will remain in the "flashing" state;
2. The ABS controller will receive no signal about the engagement of 4WD operation;
3. The transfer box will still remain in 4WD mode, even after restarting the vehicle.
4. Pressing the 2WD button on the dash board (in order to cancel everything and return the transfer box back to 2WD operation mode) will have no effect - the 4WD controller will not do anything.
5. The system will effectively be permanently "stuck" in the state where the transfer box is in 4WD mode (front propeller shaft connected to it) and the front wheel hubs are not engaged.
6. This situation will remain even after restarting the vehicle and pressing the 2WD button afterwards - the 4WD controller will just keep on trying to finish the 4WD engagement process once after every vehicle restart.
7. The only way to break out of this situation is either to eliminate the issue in the vacuum system (so the 4WD controller will complete the engagement process upon next vehicle start-up), or to fake the return signal coming from the vacuum system to the 4WD controller upon next vehicle start-up, in order for the 4WD controller to formally proclaim the successful completion of the 4WD engagement process.
8. It's only then possible to shift back to 2WD by pressing the 2WD button (and faking the return signal from the vacuum system again).
9. This time the controller will command the actuator in the transfer box to disengage the front propeller shaft - the procedure is the reverse of the engagement.

Example when shifting from 4WD-H to 4WD-L

• Successful shift into 4WD-H mode is the prerequisite for the 4WD controller to initiate the shift into 4WD-L mode.

1. The driver pushes the 4WD-L button on the dash board.
2. The button sends the signal to the 4WD controller.
3. The controller checks if the clutch pedal is pressed (for vehicles with manual gear box) or if SOMETHING (??) (for vehicles with automatic gear box), and also if the vehicle is moving (or maybe not?).
4. If the conditions above are not fulfilled, the 4WD controller sounds a repeating buzzing sound, while rejecting the initiated shifting procedure.
5. If the conditions above are fulfilled, the 4WD controller then sends a signal to the electrical actuator in the transfer box.
6. The actuator moves the mechanical bits in the transfer box, engaging the low ratio gear inside.
7. A dedicated sensor in the transfer box informs the 4WD controller if the mechanical engagement in the transfer box has completed successfully.
8. If the 4WD controller detects that the actuator in the transfer box has completed the change successfully, then the 4WD controller changes the condition of the yellow "L" light on the instrument panel from "off" to "on", together with a single beep.

• The shifting procedure back from 4WD-L to 4WD-H is essentially the same with the same conditions.
• The only difference is that the actuator is doing the opposite job in the transfer box, and that the "L" light on the instrument panel changes its condition from "on" to "off".

Shifting "directly" from 2WD to 4WD-L and vice versa

• When shifting from 2WD(-H) to 4WD-L (or vice versa), the 4WD controller in earlier "push-button" transmission vehicles (2005 - cca 2008) requires it to be done gradually, first from 2WD to 4WD-H and then from 4WD-H to 4WD-L (or vice versa all the way).
• This means that the controller will refuse to do anything when being in 2WD and pressing the 4WD-L button, or when being in 4WD-L and pressing the 2WD button.
• In other words, the driver first has to press the 4WD button, wait for that shift to complete, and then press the final button (2WD or 4WD-L, depending on which desired shift is being performed).

• However, the 4WD controllers in Jimnys made in cca 2008 and newer allow a "direct" shift from 2WD to 4WD-L and vice versa.
• Technically, the exact same procedure is being performed as when shifting first from 2WD to 4WD-H and then from 4WD-H to 4WD-L (and vice versa), just automatically sequentially.
• Essentially, the 4WD controller first performs the standard shift from 2WD to 4WD-H (as when pressing the 4WD button), and immediately after that is successfully complete, automatically sequentially performs the shift from 4WD-H to 4WD-L.
• It's just a bit faster and a bit more convenient.
• Therefore, there is nothing different in the technical sense.

Conclusion

• In the newer (push-button) 4WD transmission system, the 4WD controller does everything - commanding the electromechanical shifting mechanisms in the transfer box, commanding the vacuum system and informing the ABS system, and (most importantly) deciding when and if to operate the transfer box.
• The driver just invokes the controller by pressing one of the buttons, and then can only sit and watch.

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Page last edited on 11/09/2017 by user Bosanek