Vibrations: Rakesh K Anand -Technical rep: GWOA Ltd

Vibrations have been one of frequent complaints cropping up every now and then, on the G-Wagen forum. Therefore, an opportune time to write on the topic comprehensively. This complaint can be as basic as, an out of balance wheel or as difficult to diagnose vibration, emanating from the transfer box. So, what can be done to diagnose vibrations from its onset? Unless, one has gained some experience on diagnosing vibrations’ it would be looking for a “pin in a hay stack”. The initial action would be, to start with the most obvious and progress on diagnoses, by a process of elimination. One needs to consider what occurs to the vehicle when, vibrations emanate. As the vehicle travels, “the wheels on a truck go round and round….” “Attached, to the wheel is the wheel bearing, and attached to the wheel bearing is…” which could be a source of vibration. From the above paragraph, what I am trying to drive home is, Identify a possibly area fault, expand on what that unit is attached to and investigate that as well. In our case, as I mentioned earlier, the wheels could cause vibration, should these be out of balance. Our starting point would be initially checked on the tyre condition and wheels. Do basic checks on both, in case of the tyres, look at the wear, is there a pattern developing to the wear on threads? If so, suspect the shock absorbers or consider wheel alignment issues. As tyres wear down, the front tyres in particular, will unevenly wear, especially, at the outer edges. Hence, the reason, tyres should be rotated every 6,000 miles; front to rear (in keeping with the same direction of rotation), initiating even wear and prolonging tyre life. Correct tyre pressures are paramount, easily overseen. The wheels need to be checked for any caked on mud, this could be as a result of some off roading, in which case, vibrations will occur almost immediately or after a few days later. Wash the mud off, thoroughly; check the inside of the wheel, not just the outer wheel face. These steps are necessary before any wheel balancing is commissioned. Checking the tyre pressures, wheels and having checked and balanced the tyres as necessary, given that vibration is persistent, check on the wheel bearings next. To do this, jack each of the wheels, safely propping each wheel at the axle with appropriate axle stands or collectively working the front and rear axles alternatively, in all cases making sure you work to health and safety rules. Rock the wheel from top and bottom (12 o’clock and 6 o’clock positions), pushing at the top, whilst pulling from the bottom and inversely, pushing at the bottom and pulling from the top; noting the wheel bearing play. Note also, if play is at the hub, indicating wear at the King pin bearings. Mentally gauging the movement or play, this should be approximately, less than a millimeter or so. Any more than this; you could try readjusting, but experience has shown that the readjustment works temporarily. Play reoccurs because, adjusting the bearing simply puts more pressure on the bearing cage, and it consequently collapses more, extenuating play. Ideally, the wheel bearing needs replacing. Professional advice must be sort, or get a second opinion of this. Rear wheel bearings are not adjustable; likewise, if there is play found, these will need replacing. Attention should now turn to the propeller shafts (p/shafts). There are three p/shafts in the drive train. The first is in between the gearbox (g/box) and transfer box (t/box). For future reference we will dub this, as the g/box, p/shaft. The second, in between the t/box, running rearwardly, to the rear axle (rear, p/shaft). Lastly, the third p/shaft is in between the t/box and front axle (front p/shaft). The p/shafts consist of a universal joint (u/joint or U J) attached to either end, (correctly known, as cardasian joint). The middle, consisting of a tubed shaft, incorporating a slide joint to enable the shaft to extend or contract as the suspension falls and rises accordingly, to the road terrain. The servicing of the p/shaft consists of grease nipples, being “politically correct, that should be grease points”. These grease points (g/points) are on each U J and on the slide joint. (That is three g/points, per p/shaft). The greasing of these points should be every 6,000 miles intervals and more frequently, when off roading. In cases of Fording, immediately, or shortly after, at your convenience. The next logical step in checking for vibrations, would be, is checking the p/shaft are secure at the flange. Check the tightness of all nuts and bolts of the p/shaft flange. Check wear at the U J’s. hold the tube side of the joint and firmly shake to and fro, on the axis of the bearing joints. Any excessive wear will show up. One may even twist the shaft, to test the same, allow for the black lash play of the differential. To test the theory further, grease the joints and slide joint, road test the vehicle and note any difference in vibration if any, depending upon wear, this will dampen down a little and re-develop to a stronger vibration as before, confirming wear of the bearings or slide joint as the grease works out of the bearing. When greasing the U J’s, there are two schools of thought. One advocates greasing until the grease just lifts the seal(s) of the bearing and the second suggests greasing until the grease oozes out of the seal(s). In the latter, the excess grease will shoot off as the p/shaft spins upon drive and the natural elasticity of the bearing seal will reseal the bearing, so there is no risk of contamination. For the slide joint, Mercedes-Benz workshop manuals suggest five strokes of the grease gun. In my opinion, this is fine, if you are familiar on the greasing history of the vehicle. My practice, was to grease the slide joint, until the slide just began to move apart (this could take some twenty strokes to achieve this stage), particularly on vehicle with no history, consequent greasing was then, just five strokes, as mentioned above. The slide joint is checked for, in a similar manor, twist both halves of the joint against each other, excess play will show up. Some play is normal, but the trick is to determine if the slide joint play is attributing towards any vibration, this is difficult to gauge. Out of probability, the front p/shaft and in particular, the front U J, is common, for excessive wear. Vibration from p/shafts tends to shake the body and t/box gear lever. Front wheel imbalance or wheel bearings will tend to shake the steering wheel and body, rear wheel imbalance or wheel bearing will shake the body and t/box lever. If, by this point the source of the vibration is not located, then, one needs to look at mounts. There is the engine, g/box and t/box mounting bushes to consider. The engine mount will need the wheels to chocked at the front of, and rear of, wheel(s). This is as safety against the vehicle moving either forward or rearward. Obviously, the hand brake needs to applied as well. The engine needs to be lightly loaded in a forward gear and rearward gear without moving the vehicle. This test can be only momentarily conducted. Additional safety requires for the foot brake to applied, whilst executing this test. The object of the test is to observe the play of the engine mounting. Is the rubber firm or detached from either its bonding or is there a crack in the rubber part of the mounting? Any excess play will show up in the lift of the mounting; this can cause vibrations. Examine both sides of the mounting bushes. The same principle applies checking for the g/box and t/box bushes. Unless, a sunken pit is used to inspect the g/box and t/box bushes; it is advised that these are tested by shaking the inspected units by hand or using a lever to lift the bushes to simulate the action of engine load. If the vibration is not solved checking the points discussed, above, and then probable culprit is the t/box. Confirming vibrations from this unit becomes difficult to verify. One can remove the front or rear p/shafts to eliminate possible source of vibration, but that does not answer if the vibration is from the t/box or the other end of the p/shaft is attached to. There are limitations on road testing, is that it is not possible to see the moving parts to witness play or vibration if any. Well equipped garages may use “chassis ears”, electronic listening devices to listening to noises caused by vibrations. These are an aid, but results from such equipment can be vague. Other substitution methods may be employed, such as safely mounting the front and rear axles on adequate axle stands and simulating drive conditions. With no weight on the wheels, proper simulation cannot be achieved unless; the front and rear axle lock differentials are engaged. In case of the older G-Wagens’, the t/box needs to be in four wheel drive mode. Without this, the action of the differentials, will not allow all wheels to turn, in order to simulate, as near to road driving conditions, as possible. Inspection under these conditions requires caution at all times; they’re going to be moving parts under the vehicle and the rotating tyres. Any more than this, it would be strongly advised that a professional is commissioned to diagnose the problem for you. On the whole, this advice is good for the W460 and W463 Chassis. There are a few differences between the two chassis models. The W463 are essentially, permanent four wheel drive; the W460 is selectable four wheel drive. After a short production of the earlier W463 chassis; the centre p/shaft between the g/box and t/box was changed in design. These were replaced with C V joints. Over time the grease in the CV joint hardened and contributed to vibration, the only to confirm this is to disconnect one end of the p/shaft and turn or articulate the p/shaft; if there is hardened grease in the CV the p/shaft will have limited articulation. The cure is simple, by dismantling the boot cleaning the old grease out and re-pack with fresh grease.