Getting 4WD out of a jam

I have a 4WD vehicle that runs normally in 2WD and sometimes I struggle to engage the 4WD and transfer box gear lever. What is the likely cause of that?

  Short answer: The problem can be maladjusted or worn gear linkages…or poor technique. On the most sophisticated 4WD systems, 4WD and low range and diff locks can be engaged at the press of a button. On more basic designs a manual transfer gear-shift lever is used and the ease of engaging it depends on whether the transfer box has synchromesh or old-fashioned fixed gears.

If your 4WD is a basic model or quite old its transfer box will probably be non-synchro with square cut gears. That can often be troublesome and drivers need to know fixed gear techniques like double-declutching, pausing in neutral, straight steering, possible brief reversing, low revs) to overcome and minimise the problems.

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Longer answer: When fixed and square-cut gears were more common (nowadays synchromesh and helical gears are the norm) drivers had to get the revs just right or double-declutch to get them to engage smoothly.

Without getting too techy, when a gear is engaged the teeth of two cogs mounted on parallel (and spinning) rods are meshed with each other. When you change gear, one of those cogs has to move and engage another cog of a different size. Synchromesh allows them to automatically “synchronise” their speed and alignment so their teeth mesh together smoothly. Helical gears make that even smoother.

With fixed gears, that is the driver’s job. As you disengage one gear and go through “neutral” to engage with another, you need to change your engine revs (with the accelerator pedal) to suit the level of the gear you are going into. Pausing briefly in neutral or double-declutching helps that. Depress the clutch to move from a gear to neutral. Fully release the clutch while in neutral and depress it again to shift to another gear. With practice, it takes about two seconds.

 This “feel” is even more important when engaging a transfer box, because you are not just moving from one ratio to another in the same box connected to the same drive-train; you are also connecting to a whole other part of that drive-train – a second propshaft is engaged, and it is connected to another differential and two more wheels (that up to that moment have just been coming along for the ride).

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Even older designs usually allow a shift from 2WD to 4WD while moving, but not when turning. For shifting into low-range, first come to a halt. If the lever won’t budge, roll the car forwards or backwards a bit to help the gears lock in.

 If a transfer box is especially stubborn, reversing the car 20 metres or so can help unjam it. So does ensuring the front and rear wheels are parallel (steering straight); using low revs; and pausing the transfer shift in neutral for a moment before moving on to 4WD.