If you own a C4 equipped with a Bose sound system, you'll eventually need amplifier repair or replacement. The question is when. Every C4 Bose sound system uses an amplifier per speaker. The amplifiers are the weak link due to capacitor failure. The capacitors are filled with electrolytic fluid, which ultimately leaks onto the circuit board, causing failure. If the leak is slow, the amplifier's circuit board can be damaged beyond repair. The electrolytic fluid is acidic, causing corrosion to anything it touches.
It's common to have multiple amplifier failures at one time. In most situations, you suddenly lose sound from a speaker. Sometimes, all amplifiers fail. If one or two speakers aren't working, check the amplifiers. When the radio receiver fails, all sound output is usually lost.
There are some differences in the early and late C4 receivers. In the early cars ('84-'89), the radio control head and receiver are integrated, and late cars ('90-'96) have a separate receiver. The radio receiver is under the passenger-side dash in the '90-'93 cars, and behind the passenger seat in the rear compartment in '94-'96 Corvettes. We have found some failed '90-'96 receivers causing no sound output. The '90-'96 radio head is for controlling the receiver only and to play CDs or cassettes. The sound bites go to the receiver for output to speakers.
You can perform some simple tests to check the amplifier's integrity. If all of the amps (speakers) go out at once, first check the Bose relay. The Bose amp relay can cause a dead battery. Visually inspect the amplifiers for dark staining on the circuit board. Failed amplifiers usually have a burned wire or a plastic odor.
Water in the speaker/amplifier enclosure in the door is a common problem with '84-'89 cars. If the door drain isn't working, the enclosure can be submerged, allowing water to seep inside the enclosure. The enclosure is sealed, but if it's been opened and the gasket is damaged, a bad outside seal strip allows water in. It's possible to have radio-receiver water damage from a leaking rear-hatch window in '94-'96 Corvettes. In severe cases, the receiver is submerged in the compartment.
To rule out the speakers, test them with an AA 111/42-volt battery. To make sure the amplifier is receiving a signal, receiver output can be checked with a voltmeter for varying current output. If moving the radio control knobs to adjust the radio on the '84-'89 cars causes static or noise, the radio most likely has a problem, not the speakers or amps. In some cases, simply touching the control knobs causes static.
Removing the speaker/amplifier enclosure isn't too difficult, except on the '90-'96 cars. The front speaker enclosure is under the sill cover and requires sill removal for access. The sill cover sticks to the sill, so be careful during removal. Slowly lift one side of the cover to loosen the bond from the sound deadener. Try to avoid touching the sound deadener; it's difficult to remove from your clothes or skin. Once the speaker/amplifier enclosures are removed, the speakers and amplifiers can be tested for proper signals.