SORTING DRIVETRAIN GREMLINS...
A car's drivetrain not only has to transmit the power from the engine to the road, it also has to be resilient enough to cope with the torque loads produced by the engine under hard acceleration. Obviously the torque produced by different engines in various states of tune varies considerably, so making sure that the drivetrain of a car is up to the job when a tuned or larger, more powerful engine is fitted is particularly important.
Most drivetrain problems can be easily diagnosed by the symptoms they produce and only in extreme cases might something break, a driveshaft for example, without some form of prior warning.
THE CLUTCH
The clutch is the component that bears the brunt of the engine's torque and is the first link in the chain of components to transmit the drive to the wheels. A modern clutch in a well driven car should easily be able to cope with in excess of 100,000 miles but is a component that will inevitably wear over a period of time. Slipping is the most common clutch fault and generally occurs as a result of a worn friction disc. Clutch slip can also be caused by oil contamination of the friction disc and if this is the case the disc will still need to be replaced and the source of contamination found and remedied. If you think that a clutch is slipping, put the car in second or third gear with the handbrake applied and then slowly let out the clutch while gently revving the engine. When the clutch bites the car should stall, if it doesn't then the friction disc is past its useful life.
Clutch judder can also be due to a worn friction disc but chances are something else will be amiss. The juddering might be down to worn splines on the gearbox input shaft or possibly just a case of worn engine mountings. Some cars are particularly prone to clutch judder (ask an early Morris Marina owner) and there may actually be nothing wrong with any of the components at all. Sometimes even a sticky clutch cable can cause a juddering feeling due to the clutch not engaging smoothly. In such cases a little grease may work wonders.
Difficulty engaging gear may be due to a gearbox fault, but if the problem presents itself irrespective of which gear you are trying to engage then this indicates a fault in the clutch. If you are really lucky then you might find that maladjustment is the problem, but the clutch release lever may be broken or bent in such a way that the travel of the clutch is restricted. At worst the pressure plate may need replacing due to worn or broken springs, though when this is the case the clutch is also likely to be noisy in operation.
A rumbling or rattling sound when the clutch pedal is depressed generally indicates a worn release bearing.
THE GEARBOX
Most modern gearboxes are complicated beasts and stripping them down for inspection is an involved job. If a gearbox is just a little noisy then it may be possible to live with it in the short term but if the 'box has a habit of jumping out of gear then this is not only a pain but can be downright dangerous. Driving along with one hand against the gear lever holding the car in gear means that you have one less hand to control the vehicle, and imagine the possible consequences of the 'box suddenly jumping out of gear during an overtaking manoeuvre. A refusal to maintain a selected gear can be due to a number of internal problems such as worn synchromesh hubs, a worn or broken detent spring or locator, or a damaged selector fork.
Noises from a gearbox, or which appear to come from the gearbox, can often be identified depending upon when exactly they are present. A constant rumbling, for example, often indicates that the bearings which support the countershaft(s) are worn. Should the rumbling decrease when the box is in neutral but the car is still moving then point the finger of suspicion at the differential/final drive bearings. However, if the noise persists at the same level in neutral then suspect the mainshaft bearings. If the noise is present in only a single gear, then chances are the gear wheels or roller bearings for that particular gear are probably severely worn.
Ineffective synchronisation when changing gear could just be a result of worn bushes in the gear lever / change rod mechanism rather than worn synchro hubs or baulk rings but don't ignore the fact that difficulty in engaging gear might be due to one of the clutch problems mentioned earlier.
If you are unfortunate enough to suffer from major gearbox problems and don't fancy stripping the unit yourself then an exchange unit is probably the way to go. If you do decide to exchange your 'box then check exactly what are getting for your money, i.e. confirm that the quoted price includes fitting and lubricant , and be sure that your existing 'box is acceptable as an exchange unit - in extreme cases it may be considered to be beyond repair and a surcharge may be imposed.
AUTOMATIC TRANSMISSIONS
If modern manual 'boxes are complicated for DIY overhaul then automatic gearboxes most certainly are. Unless you know what you are doing and have the appropriate equipment, they are best left to the experts when major problems arise. However, many seemingly major problems can be caused by a lack of fluid, or fluid which just needs replacing. Good, clean fluid is generally opaque and red in colour. If the fluid is thick and brown / black then it desperately needs to be drained from the 'box and replaced.
Problems caused by a low level of good, clean fluid can include incorrectly timed up and down shifting and a refusal to move when the car is placed in 'Drive'! A stubbornness to move with fluid at the correct level may be nothing more serious than an incorrectly adjusted selector cable or rod, or perhaps wear in the rod bushes. Vibrations and unwelcome noises can be caused by faulty fluid pumps or blocked fluid strainers. If the car attempts to move when neutral is selected then the internal clutch in the automatic 'box is asking for attention.
PROPSHAFTS AND DRIVESHAFTS
With the vast majority of cars now being front wheel drive, propshafts are becoming a thing of the past. However, if you drive a classic, modified or standard, then there's a high chance that your motor is rear wheel drive. If so and you experience a drivetrain vibration at certain speeds, then inspect the propshaft joints for wear. Check also that the bolts that the secure the propshaft to the differential flange are secure. Look for signs of any balance weights which have gone absent without leave. If the prop' does need to be rebalanced then there are specialists to cater for this.
Differential / final drive units are usually capable of high mileages before demanding attention, but subject one to more power than it is capable of handling and it will soon give up the ghost. Excessive noise might just indicate a lack of lubricant rather than impending doom. Lubricant normally escapes through a worn pinion seal so replace it and refill quickly or that impending doom may soon become a reality. If lack of lubricant isn't the problem then worn bearings, worn crownwheel and pinion or possibly a loose mounting probably are.
Vibrating or knocking from driveshafts, especially on full lock, indicates that the universal joints are worn. In some cases just the joints can be replaced but if the joints are of the sealed variety then a replacement shaft will be required. Frustratingly, some driveshafts, those fitted to a Triumph Acclaim for example, have a sealed joint at one end and a replaceable joint the other. Inspect the rubber boots which cover the joints for any sign of perishing or splitting. Grip the shaft and try to turn it back and forth and try to slide the shaft back and forth along its length. Rotational movement should be minimal and there should be a fair degree of resistance against lateral movement.
OTHER CONSIDERATIONS
Other causes of noise or vibration that may appear to be coming from the drivetrain may actually originate from elsewhere. A pronounced rumbling may be worn wheel bearings. Any undue play can usually be felt by jacking up the car and vigorously shaking the road wheel. In most cases wheel bearing replacement is an easy DIY option though you may need the use of a slide hammer or puller to remove a bearing from its shaft.
Vibration can also originate from unbalanced wheels. Switching the wheels round may confirm this by altering the characteristics of the vibration.