4WD transmission failure - emergency measures

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Main overview

If Jimny's 4WD system fails when 4WD traction is most needed, you might still have some options and emergency procedures at your availability.

If you are not sufficiently acquainted with Jimny's 4WD transmission system, then you might first need to read the articles "4WD transmission system operation" and "4WD transmission failure diagnostics".

1. The most common cause of a 4WD system failure is the failure of the vacuum system for the front wheel hub heads.
2. The next common cause is broken wiring between the 4WD controller and the transfer box, because the wires are relatively exposed under the vehicle.
3. The mechanisms in the transfer box can break.
4. Тhe least common cause is the failure of the 4WD controlling computer.

Warning about the ABS system

If your vehicle has ABS (anti-lock braking system), you have to be aware of the following hazards!

• If you manage to shift the entire transmission into 4WD mode while the 4WD controller still thinks that the vacuum system is inoperable (green 4WD light is blinking on the instrument panel), the 4WD controller will not inform the ABS controller that the vehicle is in 4WD mode (because it thinks that the hub heads have not engaged).
• Also, if you manage to shift the entire transmission in 4WD mode while the 4WD controller is inoperable (dead), the ABS controller will not be aware that the vehicle is in 4WD mode.

• Whatever the case, if the entire transmission system really is in 4WD mode and the ABS controller is not aware of that, this can lead to improper operation of the braking system while the vehicle is in 4WD mode.
• Therefore, drive slowly and cautiously while in 4WD mode like this!

Situation if the vacuum system fails

Overview

• The failure of the vacuum system will cause that the vacuum hub heads on the front wheel do not engage, leaving the front wheels disconnected from the rest of the drive train.
• However, there are several emergency options at your disposal, depending on your 4WD transmission system.

Options with lever operated 4WD transmission

Immediate options

• You can still shift the transfer box into 4WD-H and then into 4WD-L by using the lever, regardless if the vacuum system (or the 4WD controller itself) is operating or not.
• In this case, 4WD-H mode will bring no benefit over the ordinary 2WD-H mode.
• However, 4WD-L mode will effectively work as 2WD-L ("RWD-L") - even that's better than nothing!
• Even "2WD-L" mode can be a life saver compared to ordinary 2WD-H mode.
• The only visual nuisance will be having a blinking 4WD light on the instrument panel while the transfer box is in any of 4WD modes.

Further options

• You can also restore traction to the front wheels ("restore 4WD") by performing the vacuum hub head locking intervention as described below.
• After performing that intervention, 4WD traction capability will be effectively completely restored (except the blinking green 4WD light).

Options with button operated 4WD transmission

Overview

• You will have to fool the 4WD controller that the vacuum system is working fine in order to achieve any remedy of the situation.
• By blocking the vacuum system, or by shorting the pair of wires which go into the vacuum monitoring switch, you can fool the 4WD controller into entering 4WD-H transmission mode.

Option 1 - blocking the vacuum system

• Blocking the vacuum system can be achieved by disconnecting the vacuum pipe from the vacuum monitoring switch in the engine bay, and then by blocking that pipe connector on the switch.
• You can even use your finger to temporarily block the pipe connector on the switch, as it only needs to be blocked while the controller is performing the 2WD<->4WD (dis)engagement procedure.
• This way, when the switch starts to suck, the vacuum will be instantly created in it (as the air passage is blocked), and then the switch will inform the 4WD controller that proper vacuum is achieved.
• This fooling procedure with a finger is best done with one person holding the finger on the switch while the other person operates the 2WD / 4WD buttons, but even a single person can do this if he is very quick.

Option 2 - simulating response from the vacuum system

• On the other hand, if the vacuum system has failed in a way that it can't even generate vacuum at the vacuum control switch (some switch failed, vacuum source failed, electrical failures, part of 4WD controller failed, etc.), the blocking solution will not be applicable.
• In this case, you can disconnect the pair of wires from the vacuum monitoring switch, and short-circuit the wires.
• This will effectively tell the 4WD controller that sufficient vacuum is present in the system (as long as the wires are short-circuited).

Results

• If you fool the 4WD controller to enter 4WD(-H) mode, it will engage the front drive line (the rear is always engaged), and even the green 4WD light will be constantly lit as normal.
• The front wheels will of course still receive no torque, since there is no vacuum to engage the front wheel hub heads.
• However, once the controller is fooled and enters 4WD(-H) mode, it will then be possible to normally change it further into 4WD-L mode by pressing the 4WD-L button, without the 4WD controller performing any further checks if the vacuum system is working (until the vehicle is restarted).
• In this case, 4WD-L mode will effectively work as 2WD-L ("RWD-L") mode, but even that's better than nothing!
• Even "2WD-L" mode can be a life saver compared to ordinary 2WD-H mode.

Further options

• After fooling the 4WD controller as described above, you can also restore traction to the front wheels ("restore 4WD") by performing the vacuum hub head locking intervention as described below.
• After performing that intervention, 4WD traction capability will be effectively completely restored.

• Beware that the 4WD controller will check if the vacuum system is working once after each vehicle start-up.
  • Therefore, try not to turn off or stall the vehicle until you exit the "danger zone" where 4WD traction is required.
  • Otherwise, you will have to fool the 4WD controller about the operation of the vacuum system immediately after each vehicle start-up.

Procedure to manually engage the vacuum hub heads

The procedure

1. Mark the current position of each hub head on the wheel hub assembly.
2. Demount both vacuum hub heads using an E10 "torx" bolt driver (which you should always carry in the vehicle).
3. Using a finger, slide the inner collar in each vacuum hub head forward to manually lock the hub head.
4. If you can, try and jam the slider forward by forcing something behind it (a piece of plastic or so?).
5. Carefully reinstall each hub head in the same position on the wheel hub assembly as it used to be, ensuring that the slider inside does not move.

Cautions when driving like this

1. As soon as you leave the slippery terrain, try to disengage the transfer box back into 2WD.
2. If you manage to disengage the transfer box back into 2WD, you are fine - it will not really matter if the hub heads are still locked in reality.
3. If you can't disengage the transfer box back into 2WD, then you must immediately demount the hub heads again and manually unlock them (essentially the same procedure as above, except sliding the inner collar back in).
4. Once the front hub heads are disengaged, you can drive around with the transfer box still in 4WD mode, without risks of developing a "transmission wind up".
5. Otherwise, if the entire transmission is still in 4WD while driving on non-slippery surfaces, you risk expensive transmission damage through a severe transmission wind up!

• You can visually check if the transfer box is in 2WD or in 4WD mode by lying on the ground near the vehicle and observing the front propeller shaft while the vehicle is moving.
• If the front propeller shaft is rotating, then the transfer box is in 4WD mode, and if it is not rotating, then the transfer box is in 2WD mode.

Situation if the wiring under the car is broken

Overview

• With a lever operated 4WD transmission, the only transmission-related wires under the car is a pair of feedback signal wires, going between the 4WD controller in the cabin and the transmission mode detection sensor in the transfer box.
• With a button operated 4WD transmission, there is also a pair of control wires, going between the 4WD controller in the cabin and the electromechanical transmission mode actuator in the transfer box.

Measures common to both transmission systems

• First of all, try to temporarily fix the issue with some insulating tape or other ingenious methods.
• If you don't have the tape (shame on you!) or if the connection won't hold on itself, at least try to hold the wires connected while someone else is shifting the transmission mode in the cabin.
• This will be sufficient to allow the 4WD controller to finish its job with the transfer box, so that it will then proceed to operate the vacuum system.
• If there isn't anybody else with you, try befriending a wolf or a fox to assist you. A bear is too clumsy. A monkey should be the best. Parrots just talk but don't do crap. A weasel will trick you.

• Now, after the shifting into 4WD has been successfully accomplished, before you release the wires and the connection is broken again, the other person should disconnect the vacuum pipe from the vacuum monitoring switch, and hold the finger on the pipe connection on that switch.
• Then you release the wire. After approx. 15 seconds, the other person can release the finger and put the vacuum pipe back into its position.
• Reason:
  • Depending on which wire is broken, the 4WD controller might interpret the disconnection of the wire as the return of the transfer box into 2WD mode, and will consequently attempt to operate the vacuum system to disengage the front wheel hub heads.
  • Removing this vacuum pipe will thwart its action and the entire drive train will remain in 4WD mode.

Additional measures with button operated 4WD transmission

• You might also have to disconnect the wires from the actuator in the transfer box immediately after the shift into 4WD mode is successful.
• Reason:
  • To prevent the 4WD controller from automatically using the actuator to shift the transfer box back into 2WD mode after it loses the connection to the transmission mode detection sensor in the transfer box.

Situation if the transfer box fails

• Failure of the transfer box will probably be rather obvious.
• Either the transmission lever (if equipped) will be jammed or very loose, or something will menacingly bang, grind or squeak in the transfer box during the shifting attempt or during vehicle motion.

• When a transfer box fails, you are unfortunately out of options before a proper servicing is done.
• This happens very rarely on its own, but it can easily happen if shifting in/out of low range gearing is performed when the vehicle is not completely stationery - "user error"!
• It can also happen when driving in 4WD modes on non-slippery surfaces (severe transmission wind-up) - "user error" again!

Situation if the 4WD controller fails

Overview

• If the 4WD controller has failed completely, that can be most easily detected by noticing that the green 4WD light on the instrument panel is staying OFF (not even trying to blink) when attempting to shift into 4WD mode.
• This is not a fool-proof detection method, as it can happen that actually the light itself is faulty.
• The vacuum system will also implicitly become inoperable if the 4WD controller itself has malfunctioned.

Options with lever operated 4WD transmission

• Your situation and options are essentially the same as in the case of a failure in the vacuum system (read above).
• The only additional difference is that the green 4WD light on the instrument panel will be inactive as well.

Options with button operated 4WD transmission

• You are out of options - ordinary 2WD-H is your only transmission mode (unless you know how to manually operate the actuator in the transfer box by "feeding" it with required electrical inputs directly from vehicle's battery!).
• Evaluate whether walking or staying put gives you better chances of a rescue. Some honest praying wouldn't hurt either.
• Failure of a 4WD controller is a rare occurrence, but the only proper insurance against it is having a spare identical 4WD controller with you ...

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