Towing a 4X4
With the variety of four wheel drive transmissions - part-time 4WD, full-time 4WD, centre diff's, viscous couplings, ... - it is not at all obvious what preparations must be made before a 4WD can be towed any significant distance behind another vehicle. It is therefore essential to consult the manufacturer's handbook for your 4WD, or to contact the manufacturer directly.
This question hangs over a 4WD because it may have an extra gearbox, i.e. the transfer case, certainly has extra transmission components over a 4x2 and these are lubricated in various ways.
We can distinguish several types of towing:
Towing a four wheel drive on a trailer should have no consequences for its transmission. The primary concerns are that the trailer and towing vehicle are road-worthy and that the 4WD is securely tied to the trailer. Similarly, a wheeled "dolly" can be used to support the trailing wheels of the towed 4WD if its other end is raised clear of the ground by the towing vehicle. The 4WD's wheels do not rotate and the main concern is that the equipment used is adequate to the task.
Towed on Own 4 Wheels
Some four wheel drives should not be towed on their own 4 wheels, at least not for prolonged distances and not at high speed. The issue is that the transfer-case ouput shafts are driven by the road-wheels. The engine is turned off and the gearbox input shaft is therefore stationary. Hence there is high-speed relative movement between some components of the gearbox and/or the transfer-case which rotate together in normal driving. These components might not be adequately lubricated for prolonged towing in this way, depending on the transmission design. If such a 4WD absolutely must be towed in this way, it might even be necessary to remove the front and rear prop' shafts. (NB. This would render a transmission hand-brake inoperative.) Check with the manufacturer.
For example, Suzuki Australia advise that the current model Jimny 4x4 (aus' 1999) should not be towed on its own 4 wheels. (Presumably very short distances in offroad vehicle recovery at low speeds are acceptable.)
The Grand Vitara 4x4 (aus' 1999) may be towed from the front with all 4 wheels on the road, provided "that [the] towing speed does not exceed 90kph (55mph)". The transfer lever should be in neutral and the manual transmission in 2nd gear (an automatic in park `P'). But note, "Stop towing the vehicle every 300km (200 miles). Start the engine. With the manual transmission in 2nd gear (automatic in `D') and the transfer lever in neutral, rev the engine for about one minute with the clutch engaged to circulate oil in the transfer case." Sounds like a bit of a chore.
(If the 4WD is being towed on a tow-rope, with a driver in control, be aware that power assistance of the brakes and steering will not operate on the towed vehicle without the motor running.)
Front end Lifted
The front wheels and transfer-case front output shaft do not rotate with the front end lifted clear of the ground (by towing vehicle or on a dolly). However the rear wheels (in contact with the ground) and the rear ouput shaft do rotate. This causes relative movement between transfer-case components. If the vehicle is a full-time 4WD and has a centre diff' that diff' would be forced to act well outside its intended design conditions. Problems could also occur in a part-time 4WD if relative movement between the transfer-case input shaft and related gears, and the output-shaft(s) and related gears are not well lubricated in this situation.
As an example, the Suzuki Jimny 4x4 (aus' 1999) should not be towed in this way. The Grand Vitara 4x4 (aus' 1999) may be towed in this way but only up to 80km (50 miles) at no more than 50km/h (30mph), and then four wheel drive must be disengaged, with the transfer lever in neutral, and the manual transmission put in 2nd gear (automatic in park `P'). (Look, tow-truck operators are great guys but I think I'd want to be in the cab:-)
As another example, Land Rovers with the `LT95' full-time 4WD transmission must have the rear prop' shaft disconnected and removed if they are to be towed in this way - so that the transmission does not rotate.
If a part-time 4WD is predominately rear-wheel drive with front drive engaged on demand, and if it has manual free-wheel hubs (FWH), then depending on the vehicle, it may be acceptable to tow it (backwards) with the 4WD's rear wheels free of the road and the front wheels on the road if both FWH are disengaged.
e.g. The Suzuki Jimny 4x4 (aus' 1999) may be towed in this way at up to 90km/h (55mph) provided that the transmission is in `2H', 4WD is disengaged, the FWH are unlocked and the manual transmission is in neutral (or an automatic in park `P'). The Grand Vitara 4x4 (aus' 1999) can also be towed in this way at up to 90km/h (55mph), with the transfer lever in neutral,
Vehicle steering locks are not strong enough to hold the steering of a towed vehicle "straight ahead" if its front wheels are on the ground. Instead a purpose built device must be used to clamp the steering wheel in this position.
An indicator light should not necessarily be trusted to show if 4WD is engaged or not etc. when preparing a tow. At least make sure that the indicator light is not blown by giving it a positive test with the ignition turned on.
Get positive confirmation that your vehicle is suitable for a proposed tow by consulting the owner's manual for that model and/or contacting the manufacturer.
You must also satisfy all road regulations for your state with respect to showing adequate lighting, on-tow signs etc. as required.
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