4WD transmission system operation

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Introduction

• The operation of Jimny 3'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 3's 4WD transmission system works when it works, and how it works when it does not work.

This article was written in the best faith and with the outmost care by a single person, who has had personal experience with only about 30% of the content which is explained here (based on some of his experience with a push-button Jimny 3). He wrote the other 70% of the content based on theoretical knowledge and various Internet hearsay. Therefore, there might be errors here, and the contents of this article are still pending verification by other knowledgeable forum users!

Types of Jimny 3's 4WD transmissions

Main characteristics

First of all, there are some rare "el cheapo" Jimnys 3 (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 3 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.
• Dedicated transmission box / transfer case (separate unit from the gear box) with the following characteristics:
  • User selectable high & low (reduction) range gear set;
  • With (dis)engagement mechanism for the front propeller shaft;
  • Without center (inter-axle) differential;
• Free wheeling hub heads on the front wheel hubs, pneumatically (vacuum) operated.

The only operational differences among different Jimny 3 editions / revisions are in the design of the control of the transmission system.

General 4WD system architecture

Jimny's 4WD system consists of two physically unrelated subsystems:

1. Pneumatic (vacuum) system for (dis)engaging the free wheeling front wheel hub heads;
2. Transfer box with its embedded status sensors + wiring, and with a mechanical selection lever (in the older design) or with a electro-mechanical actuators (in the newer design).

These two subsystems are then functionally joined/operated together by a dedicated computer called the 4WD controller, so they all together functionally form the 4WD transmission system.

Operating principle of Jimny's pneumatic front wheel hub system

General theory

Manually operated free-wheeling wheel hub head - the black thing in the middle of the wheel

• Most vehicles with part-time 4WD transmission (like the Jimny) have (un)lockable free wheeling front wheel hub heads / caps, which can (dis)connect the road wheel hub assembly from/to the transmission (the CV joint).
• The hubs heads / caps sit on the ends of the axle - they visibly stick through the hole in the center of the wheel.
• With them unlocked, the wheels are free to rotate on their own, without turning all the bits and pieces in the (front) axle - that's the "free wheeling" part of the name!
• This saves fuel, saves on mechanical wear of the disconnected components and improves performance when the vehicle is in 2WD (RWD) transmission mode, as it is pointless to turn a lot of gears and metal if they are not required.
• However, in 4WD transmission mode, the hub heads lock the wheels to the drive shafts in order to let the engine turn the wheels.
• There are three operational types of free wheeling hub heads - manually operated, pneumatically operated and automatically (un)locking.
• Jimnys have pneumatic (vacuum) (un)locking free wheeling front wheel hub heads / caps.

• To clear any misconceptions - free wheeling wheel hub heads have no relation to differential lockers or with the action of locking a differential.
• They also have no relation to reduction gearing, nor with any form of traction control.
• And they generally don't bring bad nor good luck!
• On the other hand, if you paint them to look sexy, they might attract some females (depending on the species - wolf, bear, snake, skunk, etc.).

Wheel hub heads / caps are usually just shortened in naming to "hubs", but a "hub" is actually an entire wheel hub assembly - the casing which contains the wheel bearing with its seals, the steering knuckle surround casing, the king pin bearings and (in the case of the Jimny) the pipes and passages which transport the air / vacuum through the wheel hub assembly to/from the free wheeling wheel hub heads.

Pneumatic (un)locking wheel hub heads

The most common causes (by far) of 4WD transmission system operation issues in Jimnys 3 are related to the vacuum system for the pneumatically operated free wheeling front wheel hub heads.

Advantages

Pneumatically (un)locking free wheeling hub heads is the most convenient design - it combines the best operational attributes of manual and automatic locking hubs heads, and eliminates the issues of both:

1. Unlike manually operated free wheeling hub heads, it is not required that the driver exit the vehicle to fiddle with anything when switching between 2WD<->4WD transmission modes;
2. Unlike most automatic free wheeling hub heads, vacuum operated free wheeling hub heads can (dis)engage "in situ" - without requiring that the vehicle/wheel makes at least one full wheel turn during the (dis)engagement process.
   • This attribute makes vital difference if 4WD engagement is about to be performed after the vehicle had already got stuck in 2WD.

• 

  Automatically operated wheel hub head on a wheel

• 

  Vacuum operated wheel hub head on a wheel

• 

  Vacuum (top) and manual (bottom) free wheeling wheel hub head for a comparison

Disadvantages

However, pneumatically operated free wheeling wheel hub systems are the most delicate, with the highest risk of failure.

• There are multiple points where an air (vacuum) leak can occur, and it's not just in the conducting pipework.
• In most pneumatic designs (Jimny's included), the air ("vacuum") passes through a significant part of the wheel hub assembly (which consists of the wheel bearing and various shims, bushings, seals, spindles, washers, circlips, gaskets, nuts, bolts, pins, casings, bowels, nerves, bones, skins, veins, arteries, capillaries, nails, hairs .....).
• Even a slight wear out or improper seating of some of those components (example: wheel bearing or wheel bearing seals) can cause an air leak, rendering the pneumatic system impotent.
• The system also consists of some valves and switches in the engine bay (which control when and in which direction the air ("vacuum") goes). They can also be a point of failure.
• Last but not least, there are some computers and electronics, which run the system.
• Another vehicle which uses pneumatically operated (un)lockable free wheeling wheel hub heads is SSangYong Musso, and it is notorious because of the issues with its pneumatic hub system.

Control designs of the 4WD transmission system

Jimnys 3 exist in two editions / generations regarding the 4WD transmission system:

• Older Jimnys 3 with mechanically (lever) operated transfer boxes;
• Newer Jimnys 3 with electro-mechanically ("push-button") operated transfer boxes;

• 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 itself is different in the older and the newer Jimny 3's 4WD system, and they operate differently.

• One of the more controversial changes that Suzuki made to the Jimny 3 was to fit electronic selection of 4WD transmission mode.
• The main issue that people have with the newer system is that complete control is handed over to a computer device, which (after receiving a request from the driver through a button press) makes the decision on whether to (dis)engage 4WD or not.
• This is opposed to the earlier system, where the driver operated a mechanical 2WD/4WD selection lever in the transfer box at will, and the computer just operates the vacuum system for the (dis)engagement of the front wheel hub heads upon sensing the change in the transfer box.

• The change of the transmission system occurred around 2005.
• These two types of transmission systems can be easily discerned by looking if the Jimny 3 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 3 edition. That Jimny 3 has no 4WD transmission at all.

Operational design of the older 4WD system

Short description

• In the older (lever operated) transmission system, the 4WD controller just operates the vacuum system for the (dis)engagement of the front wheel hub heads and informs the ABS computer upon successful completion of the entire process - all this after sensing that the driver has manually shifted the mechanical bits in the transfer box.
• The driver manually operates the 4WD selection mechanisms in the transfer box at will.

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.

Operational design of the newer 4WD system

Short description

• In the newer (push-button) 4WD transmission system, the 4WD controller does everything - commanding the electromechanical transmission mode shifting mechanisms in the transfer box, commanding the vacuum system for the (dis)engagement of the front wheel hub heads, informing the ABS system, and (most importantly) deciding when and if to operate the transfer box.
• The driver just invokes the 4WD controller by pressing one of the buttons, and then can only sit and watch.
• The 4WD controller checks a number of inputs / conditions before it will change the transmission mode, and these can all contribute to it not working.

The video example

• The video below shows the sequence for normal selection.

The sequence is shown on a 2006 Jimny 3. Later models (cca 2008 and newer) permit skipping of some steps in the sequence in the video - read the subchapter "Shifting "directly" from 2WD to 4WD-L and vice versa".

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.

• A simple cause can be a loose connector or broken wiring between the 4WD controller and the transfer box - check the wires and connectors under the car and around the transfer box.

Exception - vacuum system fault

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

1. The green 4WD light will remain in the "flashing" state, flashing twice per second;
2. The 4WD 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), the front wheel hubs not being engaged, and the green 4WD light flashing.
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 immediately after every vehicle restart, and will sit idle for the remainder of the vehicle run, ignoring any button presses.
7. The only way to break out of this situation is either to eliminate the issue in the vacuum system (then the 4WD controller will complete the 4WD-H 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 detect the successful completion of the 4WD-H 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 essentially the reverse of the engagement.

To "fake" the signal to convince the controller it is in 4WD, look under the battery tray where you will see two valves (VSV1 and VSV2). UNDERNEATH these is a vacuum sensor with a white connector and two wires. Short out the connector with a bit of wire to make the controller think the sensor has operated. This fakes that the vacuum is good and the 4WD system will complete its cycles ok.

Exception - Internal transfer box actuator and position sensor

The transfer box can also get "stuck" in 4WD if one of the internal sensors to the transfer box fails. The 4WD controller reports this by rapidly flashing the 4WD light (5 times per second)

1. Check the wires on the outside of the transfer box are not damaged. 2. Unplug the connector and thoroughly clean the contacts 2. Hit the actuator on the outside of the transfer box (yes - really!) 3. Unbolt the actuator, inspect it and manually operate it checking that it moves. 4. If this does not cure it then the cheapest way is to replace the transfer box as its cheaper than getting the parts.

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

First of all, a successful shift into 4WD-H mode is the prerequisite for the 4WD controller to initiate the shift into 4WD-L mode.

1. The driver presses 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 a 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".

Transfer box out of sequence issue

• The transfer box can (rarely) get out of sequence.
• Little is currently known about this issue, and some details on resetting it are also desired to be written by someone!

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.

Further reading

If you are having issues with the 4WD transmission system in your Jimny 3, the articles "Vacuum hubs - checking and testing" and "Manual or fixed front wheel hub heads" might be of great assistance.

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Page last edited on 4/03/2021 by user Bosanek