2WD versus 4WD

[pauld]

The default position on startup for the front diff is 4H. When you leave 2H selected it uses the mechanism to move it into 2H every time you start the car. I drove by default in 2H in my NP & then moved to 4H when wet, on gravel or dirt or when towing. I wore my mechanism in the front diff out by doing this. Note that the NP had different traction control programming allowing more wheel slip before clamping the power so u-turns or fast take offs in traffic never left you stranded with a cut engine.

I now have an NX and leave it in 4H except for the occasional move to 2H to ensure the mechanism still works. The NX traction control / ASC is over intrusive in certain situations so 2H is best to be avoided. If I put 2 wheels on the side of a highway in the wet & then take off in 4H I can give it full noise, try that in 2H.

I have never seen a difference in fuel economy between them, but IF it was 1L / 100 then it’s $150 per 10K or $1500 per 100K. The correction of the mechanism in the front diff was going to cost more than that.

Some have reported noise in 4H, I can’t tell the difference.

[GHendo]

I always though the ASC would transfer load to other wheels if one began to slip. This would happen regardless of being in 2wd. Correct me if I’m wrong.

I tend to always drive in 2wd and only flick over to 4wd when I need it. Might consider changing this habit after reading this thread. I’d like to understand the mechanics behind operation first.

The thing about leaving the vehicle in 4H is that drive is available to ALL FOUR WHEELS at any time. If you have it set in 2H, even if ASC cuts in, there is no way you’ll get drive to the front wheels – you are still just a two-wheel drive vehicle.

This business about … ‘if someone does something stupid, you’ll only use the brakes’ is an over simplification. You never know what can happen in a situation like that and the brakes might be the last thing you’ll use. If you happened to see an out of control vehicle heading for you, your response might be to accelerate hard (and maybe swerve) out of the way, and AWD could possibly be a big plus in that situation.

Incidentally, cycling the transfer case into 2H is a good idea. Every time I drive out onto the bitumen I flip mine into 2H for a minute or so before returning it to 4H. By doing this I hope to keep things lubricated and working.

[nj swb]

No. At low speeds the ASC will cut power. The changeover to not cutting the power is something like 30kph but I’m guessing on the exact figure.

I've had ASC cut power on me well above 70km/h. On dirt roads in 4H I often prefer to accelerate through the bend - make sure the front wheels are pulling the car around the bend, rather than the rear pushing and the fronts understeering. ASC will cut power to the point that the car is decelerating while I'm trying to accelerate. Returning from our recent Simpson trip, driving dirt roads from Birdsville to Innamincka to Arkaroola, after every stop we'd remind each other over the radio to turn headlights on - and ASC off (primarily a Pajero thing - OJ's Challenger is much better behaved in that respect).

I've also had ASC interfere on bitumen - open road curves that my MT51s can handle without any drama make the lights flash and power disappear.

[stumagoo]

I just tried it in my NL... 4h for 150kms.... I found it was definately down on acceleration (no ASC etc on mine) driving all 4 wheels definatley made the steering heavier and while the car felt more stable for sure the slight loss of power had me using more right foot for sure..... pretty much all semi busy freeway/Highway driving

[flyboy]

The default position on startup for the front diff is 4H. When you leave 2H selected it uses the mechanism to move it into 2H every time you start the car.

This might change everything for me. Can anyone else confirm if this is how the system operates? My natural instinct makes me skeptical. I certainly don’t hear any diffs disengaging or rengaging every time I turn my car on and off?

[nj swb]

The default position on startup for the front diff is 4H. When you leave 2H selected it uses the mechanism to move it into 2H every time you start the car.

This might change everything for me. Can anyone else confirm if this is how the system operates? My natural instinct makes me skeptical. I certainly don’t hear any diffs disengaging or rengaging every time I turn my car on and off?

Yes, it's true. The system is operated by vacuum, and defaults to the engaged (4wd) position - if vacuum is lost, the front disconnector will connect.

You don't hear it change when you turn your car off, because the vacuum takes some time to bleed off, and the diaphragm mechanism isn't exactly noisy. When you start your car, it won't switch again until enough vacuum has been generated, and again, it's not noisy.

[stumagoo]

Yes, it's true. The system is operated by vacuum, and defaults to the engaged (4wd) position - if vacuum is lost, the front disconnector will connect.

You don't hear it change when you turn your car off, because the vacuum takes some time to bleed off, and the diaphragm mechanism isn't exactly noisy. When you start your car, it won't switch again until enough vacuum has been generated, and again, it's not noisy.

This said both mine actually have been able to hold vacuum through the night (its used daily) and does not move out of 2wd.... this is not uncommon as I have heard of many Pajeros that seize into 2wd.

[flyboy]

As I suspected.

I can quite clearly hear mine go in and out of 4H, even if it’s just started up (but noisier) and I select it.

Just because if the vacuum source is lost it defaults to 4wd when the car is being driven, does not mean that every time it gets turned on and off the vacuum is being dropped and 4H is engaging. The actuator is staying exactly where it is, and is not “wearing out”.

I’ll put it on ramps later and listen to the front diff as someone turns the car on and off a few times in 2wd. I’m guessing the front diff will make no sound whatsoever.

[4ndy]

Hi @flyboy,

Did you manage to get the car on the lift and have a listen to the actuator/solenoid?

Would be interested to hear your opinion, as I'm currently "discussing" an issue with Mitsi concerning the 4WD system.

Thanks, Andy.

[Vicbitter]

. If anything I’d say there is more strain on the driveline in 2H given all drive has to go through two rather than four wheels.

This is my thought also.
But then again, I like the lighter steering in 2h.

[miniyazz]

I would suggest that there is more strain on the drivetrain as a whole in 4WD. Front and rear being driven = more parts with stress on them in particular CV joints. Naturally focussing on the rear only there will be somewhat more stress in 2WD mode but on bitumen I doubt this is significant and the majority of the time would be noticed by slightly increased tyre wear compared to 4WD - an effect which will be countered by the decreased tyre wear at the front in 2WD mode.

An interesting point might be that 4WD non-locked (AWD) means if e.g. your front right tyre hits some black ice and loses traction you would lose acceleration/engine braking on all four wheels - at least until traction control kicks in. In 2WD mode, you would lose engine power/braking only if that happens to one of the rear wheels.

[erad]

"An interesting point might be that 4WD non-locked (AWD) means if e.g. your front right tyre hits some black ice and loses traction you would lose acceleration/engine braking on all four wheels - at least until traction control kicks in. In 2WD mode, you would lose engine power/braking only if that happens to one of the rear wheels."


If you are driving a 4WD car in conditions where you may meet black ice, you should be in 4WD anyway, simply as an extra measure to keep your car on the road. In the NSW Kosciuszko National Park, chains are not required to be carried/used by 4WD's. I think this is crazy, because a 4WD coming down a hill with the driver having his big foot on the brakes is as dangerous than a 2WD car in the same situation. Actually a 4WD is actually more dangerous than a 2WD because normally they weight about 500 kg more than a regular car.

[marcthelegend]

Hi @flyboy,

Did you manage to get the car on the lift and have a listen to the actuator/solenoid?

Would be interested to hear your opinion, as I'm currently "discussing" an issue with Mitsi concerning the 4WD system.

Thanks, Andy.

Hi Andy,

Can indeed confirm that the system defaults to 4wd when you turn off the vehicle.

The front diff actuator (dubbed the "freewheeling clutch" by mitsi) has a spring in it that, when vaccum is lost, forces the front diff to be engaged with the driveshafts. This is a failsafe so that if you're offroad and the solenoids die for whatever reason you still have 4wd to get yourself out and to a dealer.

I found this out by smashing mine to bits on a rock and having to rebuild it.

Note though that the actuators within the transfer case remain in 2H when off and when driving in 2H.

I suspect you get the flashing centre diff light when driving for a certain period of time? If so, likely suspect is going to be the 2 vacuum solenoids that control the "freewheeling clutch", you can select 2H in the cab and the transfer case will cut drive to the front prop shaft and if the solenoids don't function you will still have the front diff engaged.
In this instance it will behave as if it were in 2H (there is NO drive to the front wheels in this instance) but the system will wait for a period of time for the front diff to disengage, if that doesnt happen it will start flashing the centre diff and put the transfer case into 4H to warn you of a system fault.

Hope this helps!

Cheers,

Marc

[miniyazz]

"An interesting point might be that 4WD non-locked (AWD) means if e.g. your front right tyre hits some black ice and loses traction you would lose acceleration/engine braking on all four wheels - at least until traction control kicks in. In 2WD mode, you would lose engine power/braking only if that happens to one of the rear wheels."


If you are driving a 4WD car in conditions where you may meet black ice, you should be in 4WD anyway, simply as an extra measure to keep your car on the road. In the NSW Kosciuszko National Park, chains are not required to be carried/used by 4WD's. I think this is crazy, because a 4WD coming down a hill with the driver having his big foot on the brakes is as dangerous than a 2WD car in the same situation. Actually a 4WD is actually more dangerous than a 2WD because normally they weight about 500 kg more than a regular car.

The counter argument to the risk of losing traction if a front wheel loses grip is that each driven wheel is less likely to lose grip due to acceleration in AWD because each driven wheel will only have about half as much force between the road and the tyre so can tolerate surfaces twice as slippery, in acceleration, as in 2WD. As you point out, braking and cornering is unaffected by AWD.

[4ndy]

Hi Andy,

Can indeed confirm that the system defaults to 4wd when you turn off the vehicle.

The front diff actuator (dubbed the "freewheeling clutch" by mitsi) has a spring in it that, when vaccum is lost, forces the front diff to be engaged with the driveshafts. This is a failsafe so that if you're offroad and the solenoids die for whatever reason you still have 4wd to get yourself out and to a dealer.

I found this out by smashing mine to bits on a rock and having to rebuild it.

Note though that the actuators within the transfer case remain in 2H when off and when driving in 2H.

I suspect you get the flashing centre diff light when driving for a certain period of time? If so, likely suspect is going to be the 2 vacuum solenoids that control the "freewheeling clutch", you can select 2H in the cab and the transfer case will cut drive to the front prop shaft and if the solenoids don't function you will still have the front diff engaged.
In this instance it will behave as if it were in 2H (there is NO drive to the front wheels in this instance) but the system will wait for a period of time for the front diff to disengage, if that doesnt happen it will start flashing the centre diff and put the transfer case into 4H to warn you of a system fault.

Hope this helps!

Cheers,

Marc

Thank you very much for the reply - this makes sense. Apparently Mitsi are stumped and a master-tech is going to look at the car. Glad I'm not paying for this merry-go-round!