Initial Inspection and Diagnostic Overview

Road Tests

Internal diagnosis examination of the radio should be left to the authorized radio service center. However, the automotive technician should be able to analyze and isolate radio reception conditions to the proper area or component causing the condition. All radio conditions can be isolated to one of four general areas. The trouble will be found in the antenna system, radio chassis (receiver), speaker system, or radio noise suppression equipment.

Radio Reception

Tuning

Fine tuning is not required for the electronic radio.

FM multiplex has a range of about 32 km (20 miles) before reception noises are heard. This means that in problem areas with tall buildings or hills it is necessary to select the strongest possible station. The electronic radio automatically tunes to the center of any given station, eliminating the need for manual fine tuning.

Tone Control

Favoring the rear speakers in noisy areas will help to reduce noise.

Antennas and Mobility

Although an automobile radio will give outstanding mobile reception, it cannot provide the continuous reception of that enjoyed in the home radio. The home receiver is not limited by the vehicle operating characteristics and certain geographical effects as is the mobile unit. For example, for the best FM reception, the automobile antenna should be designed similar to a TV antenna and pointed in the direction of the station. The best AM antenna is a long piece of wire; the higher the wire the better the reception. However, because of design necessity, the automobile antenna is restricted in size, height, direction and must receive both AM and FM stations. This means that a limited amount of the station's signal reaches the vehicle radio. In addition, the vehicle and its radio are portable. This mobility and reduced signal pickup results in FM flutter. This is normal.

Interfering Noise

The vehicle ignition system is a source of radio interference. This high voltage switching system produces a radio frequency electromagnetic field that radiates at AM, FM and CB frequencies. Although components have been designed into the vehicle to maximize this problem, the noise is more noticeable if the radio is tuned slightly off channel when listening to FM programs. Vehicle electrical accessories and owner add-on accessories may also contribute to radio interference. Furthermore, there are many noise sources which are external to the vehicle. These include power lines, communication systems, ignition system or other vehicles.

Noise or static may result from many causes. Two of the most common sources of radio noise are listed below.

Ignition Noise

The most effective method of evaluating ignition noise is to compare the radio performance with engine on, versus engine off. If ignition noise is present with engine running:

- Ensure that the spark plug wires are the suppressor type and that the spark plugs are the correct resistor type.
- Ensure that the carbon center insert in distributor cap is secure.
- Check distributor cap and rotor electrodes for silicone grease as this may cause ignition noise on FM or CB. This noise is characterized by a "motor boat" type sound on weak to moderate strength stations. The noise can only be eliminated by replacing the distributor cap and rotor with a cap and rotor that does not have grease.

NOTE: Silicone grease must not be removed from vehicles equipped with electronic engine controls (EEC-IV).

Missing or Faulty Noise Suppression Component

- Noise suppression components may be malfunctioning or missing.
- Check bond strap grounding effectiveness by wedging a large file between metal parts to ensure proper ground, such as between the tailpipe and body, or between the fender and frame, while the radio is playing and the engine is running. Listen for a decrease in the objectionable radio noise. If a reduction in radio noise is noted, first try tightening body and exhaust system clamps and brackets. Then, if necessary, install a new bond strap between the two metal parts to ensure proper ground.

FM Flutter

FLUTTER can best be described as repeated pops and hissing bursts heard in the speaker, during an otherwise good broadcast. Usually this condition exists while traveling in the fringe area of the station. Flutter will become more severe beyond approximately 40 km (25 miles) of the station. The signal loss becomes greater away from the station, until finally noise takes over and you can no longer receive the station. Flutter may also be noticed near the station because of the line-of-sight characteristics of FM waves. This condition can happen when a building or large structure is between the radio receiver and the station being received. Some of the FM signal bends around the building, but certain spots have almost no signal. Some of these signal losses are only a few inches wide and if the vehicle is parked in one of these dead spots, only noise from the speaker will be heard. As you move out of the shadow of the structure, the station will return to normal. Flutter will not occur on AM because the AM radio waves are much longer than FM waves.
FM Multi-Path Cancellation


Another effect caused by the line-of-sight characteristic is called CANCELLATION. This condition exists when the radio waves are reflected from objects or structures. The noise produced by cancellation are similar to flutter, with the addition of distortion in the program. A more familiar description of cancellation is its similarity to the multiple ghosts and picture jumping that occur on television when a low flying plane passes. The same condition exists in the vehicle, except that the vehicle is moving and the reflecting structure is stationary. The reflected signal cancels the normal signal, causing the antenna to pick up noise and distortion. Cancellation effects are most prominent in metropolitan areas, but can also become quite severe in hilly terrain and depressed roadways.


FM Strong Signal Capture and AM Overloading

FM strong signal capture is an unusual condition that occurs when traveling in the vicinity of a broadcast tower. If listening to a weak FM station, when passing the broadcast tower, a stronger station occurs without changing the radio dial. When passing the tower, the station may switch back and forth a few times before returning to the station originally tuned. When several broadcast towers are present (common in metropolitan areas) several stations may OVERLOAD the receiver resulting in considerable station changing, mixing and distortion. Fortunately this condition is localized and will not harm the receiver. Some overloading may also be noticed on AM, but usually to a lesser degree.

Receiving FM Multiplex

Because more data is carried in the FM multiplex waves than in the monaural FM broadcasts, flutter, cancellation and capture are even more noticeable. The FM multiplex noise free broadcast range is approximately 8 km (5 miles) less than that received with the monaural FM radio.

The AM/FM, FM multiplex radio may never encounter any of these troublesome conditions as they are more prominent in metropolitan areas hilly terrain and depressed roadways. However, when diagnosing FM multiplex, it is recommended to accurately tune to the strongest FM multiplex station.

Stereo Indicator Lamp Inoperative or Flickering (AM and FM Reception Are OK)

- Verify that customer is listening to stereo stations.
- A weak or distant signal may cause stereo indicator lamp to flicker. Tune radio to nearby FM stereo station. If reception is good, but stereo indicator lamp is still intermittent, remove radio chassis for service.
- Intermittent FM stereo indicator lamp operation is sometimes caused by damaged AM/FM band switch. If normal operation can be restored by switching back and forth from AM to FM several times, radio should be removed for service, and this condition should be noted on service tag.

Tape Player Will Not Accept Cartridge, Eats Tape, Plays Too Fast or Too Slow, Etc. (AM and FM Operating Properly)

NOTE: Complaints about poor performance while using tapes usually indicates a dirty head in the tape deck. Poor fidelity of tape, low tape volume, garbled sound on tape, poor treble response or muffled sound are indications of a dirty head.
- Ensure tape cassette has not come to the end.
- Check operation of tape player by using a known good cassette.
- If condition is not corrected by substitution of known good cassette, radio chassis must be removed and sent to an authorized service facility for service.