Re:Gen 3 Jimny did transfer box engage at speed?

Hello I have a 2001 gen 3, 50k miles, no service history, so subject to recent axle oil change and transfer box oil change, also had to repair 4wd system (loose bolt in end of o/s driveshaft + small vacuum leak - replaced all vacuum pipes and removed locking hub caps both sides, cleaned and regreased.

Right now I've explained what I've done I'll get to the point! driving the car at 40mph on the road, out of nowhere a buzzing / gearing sound like trying to change gear without the clutch and crashing the gearbox - have I somehow managed to engage the 4wd by accident? Lever firmly in the 2wd position. Sound lasted for about 8 seconds before returning to normal. any thoughts please? Thanks

You can safely select 4wd while driving at up to 60mph, and apart from the click of the front hubs the process is essentially silent, so it's very unlikely to be linked to the transfer case or any other part of the 4wd system.

When you say you cleaned and regreased the auto hubs, you didn't pack them with grease did you? They just need the slightest touch. If over packed it is possible for the grease to expand due to heat and move things.

Hi Busta ok thanks for that reassurance. Can’t think what’s caused it, it’s happened twice not long after driving car from cold.

You’ve got me worried now about grease quantity! I don’t think I stuffed it full, it was lightly applied - and I’d only been driving for about 2 minutes when it occurred, so figuring excess grease if there was any, might not have had a chance to heat and expand...maybe it did. Will strip down again and take a look. I thought I’d been so clever sorting it! The loose screw I found I thought had been left in when someone had done some work previously, maybe trying to pull the half shaft out? It was stopping the collar sliding and locking the hub. Now I’m paranoid that it was left in deliberately because the collar has got a life of its own and tries to engage - but as Busta notes, I was doing 40mph and wheel was straight ahead and normal engagement should be possible. A mystery but thanks for your input!

A nut or bolt is often tucked into the hub for the opposite reason, to keep them locked all the time because the vacuum system will not lock them.

Did it with mine as a stop gap whilst looking for manual hubs

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That would make sense, thank you for the observation.

Transfer case engaging is different to the hubs engaging. In normal driving the hubs are spinning with the wheels and the driveshafts are stationary. If the collar is loose and comes into contact with the end of the stationary driveshaft then it would make a bit of a racket, so that explains the noise.
The collar is held into the part of the hub that moves back and forth with the vacuum by a wire ring. The lip this ring registers on wears and then the collar becomes loose. Time for a new hub! If you ask around here are plenty of used ones available for pennies because people love to fit manual hubs instead.