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transfer box - diff not unlocking

goodoldboy

Well-Known Member
Joined
Apr 30, 2010
Messages
1,630
the last few days have seen my transfer box becoming harder to unlock the diff - the trick has always been to "unload" it before shifting i.e. rolling backwards in neutral. i finally managed to change this morning by rolling forward with both the g/box & t/box in neutral.also i noticed that whilst driving with the diff locked the transfer lever would move itself from h/unlocked to h/locked under braking??? .im off to check the oil level after lunch - it was changed 4 years ago.The t/box gets plenty of use (at least once a day) so im hoping its nothing too bad.
p.s. certanly not an issue with the light as the first hairpin bend of the journey demonstrated.
 
The Centre Diff Lock on the '90' is essentially a simple thing - the Transfer Lever moves a shaft on which the Selector Fork slides. The fork is sandwiched between two springs so moving the lever tensions one of the springs and the fork will move in or out of mesh when the lock pinion is unloaded.

It sounds as though the mechanism is drying-up so treat it to an oil change and see what happens.

There is an oil pump in the box that circulates the oil (there is a large suction filter and a magnetic particle trap as well). There is very little to go wrong in there, certainly no electrickery as there is in the 100-Series box (apart from two detector switches on the manual gearbox version and three on the Autobox version).

Let us know how you get on :thumbup:.

Bob.
 
I checked the level & its full with clean oil so im a little mystified.Ive left it in high for the last two days as i had a long trip planed - got back late last night so im going to try it in low & see what happened.
 
also i noticed that whilst driving with the diff locked the transfer lever would move itself from h/unlocked to h/locked under braking???

You say that when driving with the diff locked, the lever would move from 'Unlocked' to 'Locked' . . Was the centre diff actually 'unlocked' and then 'locked' after you braked??

If it wasn't then the Transfer Lever isn't attached to the selector forks properly (which can happen if the lever assembly is removed and then not located properly when replaced).

Has the Transfer Lever been removed recently??

If the Transfer Box is doing what the Transfer Lever indicates and the lever is slopping about from Unlocked to Locked when braking then it sounds as though one of the 'Detent' balls and springs are not doing their job.

The two Detent balls & springs are in behind screw-in plugs on the upper rear of the box, in behind the speedometer sensor. The Right Hand (O/S) one is for the centre diff lock. The steel plunger in the picture is the 'Interlock' that prevents Low Ratio being Selected when the Diff Lock is 'open'.



DSCN7496.jpg




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Crawl underneath an check that the plug hasn't vibrated loose and got lost (I'd be surprised as they are in with thread locker). I suppose the spring could have broken.

A couple of questions there . . .

Bob.
 
ok bob thanks for that i will take a look but everything seems ok at the mo.The box hasnt been opened & the only change recently has been some A/T tyres fitted instead of M/Ts .My "driveway" has a rock slab section & its possible that the new tyres caused some wind up in the transmission.
 
Since you mentioned a change of tires. When I bought my Prado, it had a spare tire that was a slightly different size which I didn't realize at the time. I had to use the spare when I got a flat and converted the damaged tire into the spare since it couldn't be satisfactorily repaired (I had a tube put in it so I could use it in an emergency. This is Costa Rica.) After changing the tire it became more difficult to lock, and very difficult to unlock, the center diff. I replaced all the tires -- they needed it -- with new and the difflock has worked perfectly ever since. I have also heard of others having difficulty with the difflock when they had tires from different manufacturers of the same nominal size mounted on their vehicles. Apparently there can be enough difference in the actual diameters to cause the diff to be working even when running in a straight line.

During the time I was having trouble with difflock, I noticed that when I tried to disengage it, the lever would move by itself from the H to the HL position after the diff had failed to unlock for a period of time. I would then have to move it again to H to try to get it to disengage.

I don't know if that helps.
 
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During the time I was having trouble with difflock, I noticed that when I tried to disengage it, the lever would move by itself from the H to the HL position after the diff had failed to unlock for a period of time. I would then have to move it again to H to try to get it to disengage.

I don't know if that helps.

That makes sense :thumbup:.

When you move the Transfer Lever, you are moving the selector shaft - which then 'clicks' into position using the detent sprung ball at the rear of the shaft.

You are not actually moving the selector fork, only compressing a spring against it to make it move when its ready (i.e. when the sliding lock pinion is 'unloaded').

If the system is wound-up, the sliding lock pinion won't slide and therefore stays where it is - with the selector spring staying compressed.

Vibration will probably cause the 'detent' to let go - allowing the selector shaft (and Transfer Lever) to spring back into the 'locked' position as the selector spring relaxes.

There isn't a 'gate' on the Transfer Lever to prevent this.

Happy now - I understand what's happening :lol:.

Bob.
 
I always replace the tyres in pairs/sets but it looks like wind up has caused the problem.Bob didnt you do a thread about rebuilding the t/box?
 
I always replace the tyres in pairs/sets but it looks like wind up has caused the problem.Bob didnt you do a thread about rebuilding the t/box?

The problem with different sized tires can occur when tires on one axel are replaced but not on the other. For example if the tires are the rear are a different diameter than those on the front (even though they are the same nominal size), the central diff will be working to equalize the speed even when traveling in a straight line. This extra 'tension' can make locking and unlocking the diff more difficult. My mechanic diagnosed this easily by lifting all four wheel and testing the transfer. If it works fine with the tires in the air, look at the tires themselves.
 
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