4WD system diagnostics

Got some issues with your 4WD system? Here’s a few things that might help you diagnose it.

Sections

  • Understanding how the 4WD system operates
  • Wiring and 4WD module
    • 4WD system logic
  • Vacuum system
  • ‘Tricking’ the 4WD module
  • Common issues
    • 4wd light doesn’t come on in high range, hubs don’t engage in high range, and 4WD light flashes in low range

Understanding how the 4wd system operates

The 4WD system in the Jimny is relatively simple, does not have many areas to cause issues and is relatively reliable. People do come across issues, however, and it’s important to understand how everything operates to help diagnose issues.

Wiring for the 4WD in particular is relatively simple in the Jimny. Note that this diagram (presently) does not show the full routing of the wires. There’s some power supply for both turning on the module and providing power to the vacuum solenoids, vacuum detection switch, 4WD and 4L detection switches from the transfer case and CAN wiring.

With regards to CAN signals, the 4WD module transmits relevant information across the CAN such as 4WD state. In turn, the 4WD system receives:

  • Vehicle speed signal and engine running status from the ECU
  • Transmission gear position and transmission output shaft rotational speed from the transmission control module (A/T only)

The system works via a physically shifted transfer case, and vacuum operated automatic hubs. The 4wd module controls the vacuum request to either engage or disengage the front hubs, and it some checks are involved to ensure that sufficient vacuum is provided for long enough to engage the hubs.

Note that this means that what is shown on the dashboard for 4wd engagement is demonstrating what the car is thinking is happening about the hubs, rather than the actual physical position of the transfer case. If the 4wd signals are disrupted then the car can be in 4wd but with the hubs not engaged

This is in contrast to later model generation 3 Jimnys which used an electronically shifted (‘push-button’) transfer case. This is important to understand if you are looking to modify the 4wd system: there are considerations you have to make with a generation 3 Jimny that don’t apply to a generation 4 Jimny, and vice-versa.

Wiring and 4WD module

The wiring for the 4WD system really starts at the transfer case. There are two switches on the transfer case: one goes from open to connected when 4wd is engaged, and one goes from open to connected when low range is engaged. The 4WD switch will stay connected in either high or low range, and the 4WD-L switch will only be connected in low range.

Note that when I say ‘signal’ for 4WD or 4WD-L then that signal is a ground signal. If the signal wire is ungrounded then the car will recognise that as no input on that signal wire. You can physically shift the transfer case but if the switches are not working or the wiring is broken then that shift will not be detected.

The 4WD switch is the one low and forwards on the transfer case, slightly on the left hand side of the transfer case. The 4WD-L switch for low range is nearer to the back on the right hand side of the transfer case.

4WD system logic

There’s a number of states of the 4wd system, so I’ll break it down by the various states that it could find itself in

  • 2H engaged
    Nothing really to report here. With 2H engaged the car will see neither a ground signal on 4WD or 4WD-L switches.
  • Shifting 2H to 4H
    The car will detect this as the 4WD switch now providing a ground signal.
    Physically the transfer case will have engaged the drive to the front output of the transfer case, which is synchronised so this shift can be undertaken when driving.
    Electrically the car will start trying to engage the vacuum hubs and it will flash the 4wd symbol while it pulls a vacuum from the reservoir to the hubs via the engagement solenoid. If -5.03 psi vacuum is not reached within 15 seconds of requesting hub engagement, then the 4wd light will flash to indicate that the hubs are (likely) not engaged.
    The car will also disable the A/C compressor for the time in which the vacuum solenoid is trying to engage the hubs.
    Also of note the car will also not engage the hubs at > 100 km/h speed signal
  • 4H engaged
    With the vacuum system able to reach required vacuum within 15 seconds of shifting into high range, the 4wd light will be displayed solidly and the hubs will be locked on. They will remain on until they are pulled ‘off’.
  • Shifting 4H to 4L
    When undertaking this shift there is no synchronisation and hence it must be done when the car is stopped. The car will detect this shift through a ground signal on the 4WD-L wire.
  • 4L engaged
    With 4L engaged, the car will switch off a number of the safety features and the cross-axle braking system (‘ALLGRIP’) will be enhanced to intervene in the case of a spinning unloaded wheel on one axle.
  • Shifting 4L to 4H
    When shifting 4L to 4H, which must be done with the car stationary, the car will re-enable certain safety systems like the autonomous braking system and will decrease the sensitivity to single-wheel spinning on an axle
  • Shifting 4H to 2H
    Disengage vacuum solenoid is engaged for 10 seconds to disengage the hubs. There is no active vacuum monitoring of this circuit, it simply will apply vacuum for a sufficient time for the hubs to disengage.

Note that the car only ever indirectly detects aspects of where the 4wd system is. It does not directly detect if you have engaged a gear, just a ground signal is sent for being in 4WD (i.e. in either 4H or 4L) and another ground signal is sent for being in 4L. The same for the hub engagement: it is not actively monitoring hub engagement, it is simply displaying the status of pulling a vacuum to engage the hub, and then pulling a vacuum to disengage the hub (and assuming it’s worked).

Vacuum system

Vacuum solenoid 1 (the forward of the two in the engine bay, grey connector) disengages the hubs. It does this through a vacuum line connected to the outer wheel spindle (unmarked vacuum line on factory vacuum lines)

Vacuum solenoid 2 (the rear of the two in the engine bay, red connector) engages the hubs. It does this through a vacuum line which connects to the centre of the wheel spindle (white marked vacuum line on factory vacuum lines). There is a vacuum switch which is connected to the output of this solenoid and the two hub lines to verify if there is sufficient vacuum to engage the hubs.

The hoses are also fairly easy to follow: the engagement hose is the ‘front’ hose on the axle housing and the disengagement hose is the rearwards hose on the axle housing. You can also confirm this by following the hose that is connected to the vacuum switch by the two solenoids: only the engagement hose connects to the solenoid connected to the vacuum switch; the vacuum switch is not used for disengagement purposes.

The reservoir for vacuum is a tank in the drivers side wheel arch, which receives vacuum from the intake system via a one-way valve to ensure there is a reserve of vacuum available to engage the hubs. This means that you can engage the hubs even under significant acceleration where the engine would be producing no appreciable vacuum.

‘Tricking’ the 4WD module

Since the 4WD module can only passively sense aspects around the function of the 4WD system, it is possible to trick elements of the system in multiple ways.

If you interrupt the 4WD and 4WD-L signal then you can engage 2L as the hubs will not be engaged. A caveat to this would be for automatic cars and if you drove it fast enough then the mismatch between gear position and transmission output speed vs. road speed has the potential to introduce an error state in the 4WD system. Manual cars do not know what gear they are in nor the output speed of the gearbox so this would not be a problem in a manual.

If you ground the 4WD-L signal wire when in high range, then you will have the safety features disabled in high range and the higher intervention of the ALLGRIP system while being in high range to maintain momentum. From testing when these cars first came out this will produce an error in the 4WD module (and some others) which you will need to clear afterwards. No problem in manual cars though and is another solution to traction control off in high range: just make the car think you’re in low range!

When replacing the vacuum hubs with manual hubs, if you want the 4WD indicator light to work as normal then you need to make sure the engagement vacuum line is plugged after the vacuum switch. So long as that vacuum switch sees a bit lower than -5 psi vacuum then the indicator will not flash and the car will think it has engaged the vacuum hubs.

This article isn’t necessarily about how to replace the vacuum hubs with manual hubs. People do this because they find it easier than troubleshooting the vacuum system, however, given that the vacuum system is relatively straightforwards then I always advise people to look at troubleshooting it first. If one does change the hubs for manual hubs then obviously plug vacuum lines and make sure you remember to engage the hubs when 4WDing. Many people have done a manual conversion and forgotten to engage the hubs and wondered why they were not making progress offroad…

If you are interested in replacing your hubs with manual ones, then these hubs (noting they also need a 10 mm spacer) will apparently work.

Common issues

4wd light doesn’t show in high range, hubs don’t engage in high range and 4wd light flashes in low range

For some reason or another, the car is not seeing a signal from the 4wd switch. When it sees a signal from the low range switch, it is seeing that signal alone (rather than seeing both 4wd and 4wd-low signals).

There can be a number of reasons for this:

  • The wiring for the 4WD switch has been cut (this can happen offroad)
  • The 4WD switch itself has failed
  • The 4WD switch is not properly engaging on the internal shift shaft, possible if the transfer case has just had a rebuild

First step with this one is the simplest option first: a broken wire underneath the car. Should be fairly obvious.

One can verify the rest of the system’s operation by grounding out the 4WD line with the key on. It should noticeably clunk when engaging the hubs. If this doesn’t happen then the break in the wiring could be inside the car (or where the wiring runs across the top of the engine bay and to the drivers side where the 4WD module is located).

I engage low range, but the safety features are not disabled

This is another common one, and again the most likely cause is a break in the wiring of the 4WD-L switch. Same kind of diagnosis as a broken wire for the 4WD system, but different wires to follow.

My 4wd light is just flashing at me when I engage 4H and doesn’t change from this when I engage 4L

Assuming you’re not doing > 100 km/h when this happens, the most likely scenario is a vacuum leak. Lots of spots to check, and often people can’t be bothered but if you work through methodically you can verify what’s happening.

You can disconnect the vacuum lines from the solenoids and test each one down to the axle housing and the two hubs by using a small vacuum pump (like a brake bleeder vacuum pump). You need to be able to sustain -5 psi for 15 seconds, if you can’t then you have a leak somewhere.

Another avenue that can cause this are seal issues in the spindle hub, or even the actual vacuum hub itself. If you don’t seal the hub gasket correctly when reassembling a hub after doing a job like a swivel hub rebuild then you can have vacuum leaks there.

Something super common are people who discover their 4wd system doesn’t work after having a fancy snorkel installed. A lot of the metal fabricated snorkels require relocation of the vacuum reservoir from the drivers side mudguard area, and some installers have been known to discard the vacuum reservoir not understanding what it is for. With a lack of vacuum to pull from then the hubs may never get to full vacuum and engage.

My hubs are engaging/disengaging seemingly at random

This is another likely wiring issue. If you have a break in the insulation or a totally broken wire in the 4WD switch wiring, then the car will think you are shifting into and then out of 4WD repeatedly. Every time it sees the engagement signal as the 4WD signal wire is being grounded then it will engage the hubs. Likewise, when the 4WD signal wire is not grounded after being grounded it will then try to disengage the hubs.

The diagnosis flow on this is relatively straightforwards: you need to follow the wiring from the transfer case up into the car, from under the centre console to the left hand side of the car, through a connector and along the top of the firewall, and then back into the car on the right hand side of the car and into the 4WD module connector. A break anywhere along there causing the wiring to be able to be grounded either permanently or intermittently will cause exactly this symptom of trying to engage or disengage the hubs without touching the transfer case lever in the car.

I cannot seem to get my car out of 4WD or low range

The first check for this is to ensure you are shifting it correctly. While the engagement of low range requires you to push down on the transfer lever, disengaging low range just requires you to push the lever forwards into high range; the same applies to shift from 4WD to 2WD. Just push the lever forwards.

If you still cannot do it then it is possible the 4WD system is somewhat ‘bound up’. The Jimny does not run a centre differential, so any turning or other wheel speed difference between the front and rear wheels cannot be accommodate other than pushing parts around in the 4WD system. This ‘bind up’, or ‘wind up’ as some people refer to it, makes the car much, much harder to shift out of 4WD. It can also make it harder to get the hubs to disengage as the vacuum system may lack sufficient force to be able to release the drive from the amount of force being pushed into the front axle by the bound up transfer case.

The best solution for this is to reverse for a short distance in a totally straight line. This essentially undoes the winding up of the transmission by pulling it in the opposite direction, and can help you then disengage 4WD.

My car won’t stay in 4WD or low range

This one is a bit harder to provide advice on, especially because when the Jimny is relatively new a lot of people report issues with it staying in 4WD.

One common cause are the almost ubiquitous cheap cupholders that sit on the transmission tunnel. They can interfere with the transfer case lever boot and can force it forwards a bit when in low range because of how they are designed. As the transfer case lever only has to be pushed forwards, not pushed down and also pushed forwards, disengagement can be relatively easy.

It is also very possible to insufficiently engage low range, especially as you have to push down quite firmly on the lever and pull back quite firmly simultaneously to shift from 4WD and into low range. This will make the car much more likely to pop out of low range if it is only, say, 80% engaged.

Staying engaged in 4WD is usually much more straightforwards and most people do not report an issue with this. Again it is possible to insufficiently engage and the drive forces could force a shift back to 2WD, but since you only have to pull the lever back to engage 4WD most people can do this with sufficient force to be fully engaged. If there is an issue here then it could be something internal to the transfer case and is the kind of thing for a dealership or a mechanic to investigate.

The car seems to skip when driving, or occasionally loses drive, often with a crunching noise

This, sadly, is the kind of symptom of a stretched transfer case chain or other issues with the transfer case chain and the gears that it engages with. Even in 2WD the transfer case chain provides drive for the car, and this chain will eventually wear out. Some reduction gears may also have insufficiently cut teeth in the gears that engage the chain and this can cause premature chain wear; if you have reduction gears fitted then this might be a symptom to be a bit more attuned to.