Vehicle Mechanical Diagnostic Techniques
Mechanical Diagnostic Techniques
Start all hands-on diagnostic routines with Hand and Eye Checks. In other words, look over the vehicle for obvious faults.
• If the engine is blowing blue smoke out of the exhaust – consider the worth of tracing the cause of a tapping noise in the engine.
• When an engine will not start – check that there is fuel in the tank
Note: All diagnostic routines should include the Hand and Eye Checks.
Noise, Vibration, and Harshness (Road Test)
Noise, vibration and harshness (NVH) concerns have become more important as drivers have become more sensitive to these issues. Drivers have higher expectations of comfort levels. NVH issues are more noticeable due to reduced engine noise and better insulation in general. The main areas of the vehicle that produce NVH are:
• Engine accessories
It is necessary to isolate the NVH into its specific area(s) to allow more detailed diagnosis. A road test, as outlined later, is often the best method. The five most common sources of non-axle noise are exhaust, tires, roof racks, trim and moldings, and transmission. Ensure that none of the following conditions are the cause of the noise before proceeding with a driveline strip down and diagnosis.
1. In certain conditions, the pitch of the exhaust may sound like gear noise or under other conditions like a wheel bearing rumble.
2. Tires can produce a high-pitched tread whine or roar, similar to gear noise. This is particularly the case for nonstandard tires.
3. Trim and moldings can cause whistling or whining noises.
4. Clunk may occur when the throttle is applied or released due to backlash somewhere in the driveline.
5. Bearing rumble sounds like marbles being tumbled.
Noise is very difficult to describe. However, the following are useful terms and are accompanied by suggestions as to when they are most likely to occur.
• Gear noise is typically a howling or whining due to gear damage or incorrect bearing preload. It can occur at various speeds and driving conditions or it can be continuous.
• Chuckle is a rattling noise that sounds like a stick held against the spokes of a spinning bicycle wheel. It usually occurs while decelerating
• Knock is very similar to chuckle though it may be louder and occurs on acceleration or deceleration
Check and rule out tires, exhaust and trim items before any disassembly to diagnose and correct gear noise.
Clicking, popping or grinding noises may be noticeable at low speeds and be caused by the following:
• Inner or outer CV joints worn (often due to lack of lubrication, so check for split gaiters)
• Loose drive shaft
• Another component contacting a drive shaft
• Damaged or incorrectly installed wheel bearing, brake or suspension component
The following may cause vibration at normal road speeds:
• Out of balance wheels
• Out of round tires
The following may cause shudder or vibration during acceleration:
• Damaged powertrain/drivetrain mounts
• Excessively worn or damaged outboard or inboard CV joints
The cause of noise can often be traced by first looking for leaks. A dry bearing or joint will produce significant noise.
- Inspect the CV joint gaiters (boots) for cracks, tears or splits.
- Inspect the underbody for any indication of grease splatter near the front wheel half shaft joint boots.
- Inspect the inboard CV joint stub shaft bearing housing seal for leakage at the bearing housing.
- Check the torque on the front axle wheel hub retainer.
Noise Descriptions with Possible Sources
- Tap - Valve clearances out of adjustment, cam followers or cam lobes worn
- Rattle - A loose component, broken piston ring or component
- Light Knock - Small-end bearings worn, earn or cam follower
- Deep Knock or Thud - Big-end bearings worn
- Rumble - Main bearings worn
- Slap - Worn piston or bores
- Vibration - Loose or out-of-balance components
- Clatter - Broken rocker shaft or broken piston rings
- Hiss - Leak from inlet or exhaust manifolds or connections
- Roar - Air intake noise, air filter missing, exhaust blowing or a seized viscous fan drive
- Clunk - Loose flywheel, worn thrust bearings or a loose front pulley/damper
- Whine - Power steering pump or alternator bearing
- Shriek - Dry bearing in an ancillary component
- Squeal - Slipping drive belt
A vehicle will produce a certain amount of noise. Some noise is acceptable and may be audible at certain speeds or under various driving conditions such as on a new road. Carry out a thorough visual inspection of the vehicle before carrying out the road test. Keep in mind anything that is unusual. A key point is to not repair or adjust anything until the road test is carried out. Of course, this doesn't apply if the condition could be dangerous or the vehicle will not start. Establish a route that will be used for all diagnostic road tests. This allows you to get to know what is normal and what is not. The roads selected should have sections that are reasonably smooth, level and free of undulations as well as lesser quality sections needed to diagnose faults that only occur under particular conditions. A road that allows driving over a range of speeds is best. Gravel, dirt or bumpy roads are unsuitable because of the additional noise they produce.
If the concern is a noise or vibration on a particular road and only on a particular road, the source of the concern may be the road surface. Test the vehicle on the same type of road. Make a visual inspection as part of the preliminary diagnosis routine prior to the road test. Take note of anything that doesn't look right.
1. Tire pressures, but do not adjust them yet
2. Leaking fluids
3. Loose nuts and bolts
4. Bright spots where components may be rubbing against each other
5. Check the luggage compartment for unusual loads
Road test the vehicle and define the condition by reproducing it several times during the road test. During the road test recreate the following conditions:
1. Normal driving speeds of 20–80 km/h (15–50 mph) with light acceleration – a moaning noise may be heard and possibly a vibration is felt in the front floor pan. It may get worse at a certain engine speed or load.
2. Acceleration/deceleration with slow acceleration and deceleration – a shake is sometimes noticed through the steering wheel seats, front floor pan, front door trim panels, etc.
3. High speed – a vibration may be felt in the front floor pan or seats with no visible shake, but with an accompanying sound or rumble, buzz, hum, drone or booming noise. Coast with the clutch pedal down or gear lever in neutral and engine idling. If vibration is still evident, it may be related to wheels, tyres, front brake discs, wheel hubs or wheel bearings.
4. Engine rpm sensitive – a vibration may be felt whenever the engine reaches a particular speed. It may disappear in neutral coasts. Operating the engine at the problem speed while the vehicle is stationary can duplicate the vibration. It can be caused by any component, from the accessory drive belt to the clutch or torque converter, which turns at engine speed when the vehicle is stopped.
5. Noise and vibration while turning – clicking, popping or grinding noises may be due to the following: damaged CV joint; loose front wheel half shaft joint boot clamps; another component contacting the half shaft; worn, damaged or incorrectly installed wheel bearing; damaged powertrain/drivetrain mounts.
After a road test, it is often useful to do a similar test on a hoist or lift. When carrying out a ‘Shake and Vibration’ diagnosis or ‘Engine Accessory Vibration’ diagnosis on a lift, observe the following precautions:
• If only one drive wheel is allowed to rotate, speed must be limited to 55 km/h (35 mph) indicated on the speedometer. This is because the actual wheel speed will be twice that indicated on the speedometer.
• The suspension should not be allowed to hang free. If a CV joint were run at a high angle, extra vibration as well as damage to the seals and joints could occur. Support the front suspension lower arm as far outboard as possible. This will ensure that the vehicle is at its correct ride height. The procedure is outlined by the following steps:
Support the front suspension lower arm as far outboard as possible. This will ensure that the vehicle is at its correct ride height. The procedure is outlined by the following steps:
- Raise and support the vehicle.
- Explore the speed range of interest using the road test checks as previously discussed.
- Carry out a coast down (overrun) in neutral. If the vehicle is free of vibration when operating at a steady indicated speed and behaves very differently in drive and coast, a transmission concern is likely.
A test on the lift may produce different vibrations and noises than a road test because of the effect of the lift. It's not unusual to find a vibration on the lift that was not noticed during the road test. If the condition found on the road can be duplicated on the lift, carrying out experiments on the lift may save a great deal of time.