C4 Electrical System
The electrical system on the C4 generation Corvette® consists of a battery, an alternator, the ignition system, various lamps, several electric motors, an entertainment system, numerous fuses and wiring, various control and monitor modules and a large number of connectors which serve to connect the systems and devices together.
Although there are literally hundreds of individual items involved with the Electrical System, there are some portions of the system that can be checked or serviced rather easily.
The battery is located in the rear of the engine compartment on the driver's side. The positive and negative power leads attach to the side of the battery.
If it becomes necessary to replace the battery, a portion of the body must be removed. Fortunately, this sounds worse than it really is.
First, disconnect the two battery cables from the battery then, looking at the rear of the left front wheel well, you will find four bolts. Using a #30 Torx bit, remove the top three which hold the fender panel in place.
Next, measure the gap between the fender panel and the driver's side door (while door is closed) so you can reestablish the clearance when you reinstall the fender panel.
Next, remove the 10mm bolt at the top rear of the fender panel which attaches the panel to a support in the engine compartment.
Next, sliding a thin 10mm spanner under the bottom rear of the fender panel, you will be able to loosen the bolt hidden there enough to slide the panel towards you and off of the automobile.
You can now remove the 13mm bolt securing the battery hold down and remove the battery.
The alternator comes in several varieties depending on which year C4 you have. Corvettes have a history of problems with the alternator and electrical system faults can frequently be traced to this device.
In 1984, the alternator output amperage was 97 amps maximum load. In 1985, this was increased to 120 amps and there were three different models of alternator with GM P/N 1049868 being the number for all of them. If you replace the alternator with a GM unit, you will get the final version.
In 1986, the amperage was dropped back to 105 amps where it remained.
To determine if alternator is working, start the engine and turn on all accessories...radio, all lights, air conditioner...everything. Select volts on the function select switch panel. Observe engine RPMs and with your foot securely on the brake, place the automobile in gear and out of gear several times. (For a manual transmission, you will have to lightly load the engine but not to the point where is begins to stall, depress the clutch then load it again several times).
Observe the voltage display and if the indicator is less than 11.5 volts, either the alternator or battery (or possibly both) may be defective.
The ignition system on the 1984-1991 C4 L98 engine Corvette has the ignition coil housed in the distributor cap on the rear of the engine which is connected via a brush arrangement to the rotor. (Later models use the so called Opti-Spark system which has an optical isolator driven distributor and is prone to ingestion of water. This water contamination will prevent operation unless a special kit is installed to eliminate the moisture or unless your C4 is a later model which had the kit installed by the factory. Repair of the Opti-Spark system is difficult and best left to the pros. If you wash the engine of a later model (LT1 or later) C4 and it will then not start or starts but will hardly run, you have probably caused the Opti-Spark to ingest water and are looking at a large repair bill).
If the engine turns over but will not start, you should check for a spark before checking for sufficient fuel flow and pressure. (You should of course first verify that there is fuel in the tank via reference to the fuel gauge). Note that this troubleshooting order is not because the ignition system is more likely to fail than the fuel system but rather because it is somewhat easier to check and should be eliminated for that reason before moving on to the fuel system.
You must be certain that there is no fire hazard present. An odor of gasoline or any flammable mixture means you must totally clear that condition before you perform this step or else the possibility of severe injury of even death is present.
DO NOT PERFORM THIS STEP IF THERE IS ANY EVIDENCE AT ALL OF FUEL LEAKAGE OR IF THERE ARE UNCAPPED BATTERY FILLER OPENINGS PRESENT!
(Note that the following checks that there is at least some high voltage present. To determine that the voltage level is sufficient to completely ignite the fuel/air mixture, you will need to purchase a spark plug tester at your local auto part house. They cost between $15 and $20 dollars on average).
To check for a spark, gently remove one of the plug wires by grasping it at the connector boot (the part that goes over the spark plug) and pull it off with a twisting motion. Do not grasp the wire itself.
Carefully wedge a stiff wire (such as a straightened out paper clip) into the plug wire connector, hold onto the wire with an insulated tool (such as a large pair of needle nose pliers with rubber handles) and have an assistant crank the engine while you hold the wire approximately 1/2 inch from the engine block.
You should see a spark jump from the wire to the engine block if the ignition is working.
If there is no spark, check a second wire to make certain of your findings and if missing, suspect the distributor, ignition module or, on later modelsC4s, the Opti-Spark system which is located on the front of the engine and subject to moisture contamination problems.
If there is a spark present, you may have fouled spark plugs or the fuel system may be a fault.
It is also possible that you have a voltage present at the plugs but not a high enough voltage to reliably ignite the mixture. A tester will prove or disprove that theory.
To prevent ignition problems, it is important that the spark plugs be changed every 30,000 miles and at that time, the distributor and plug wires be closely examined.
On pre-LT1 Corvettes, the distributor cap interior and rotor device should be inspected for any signs of arcing or carbon tracking. If any signs of these problems are noted, do not attempt to clean the deposits. Replace the assembly.
Similarly, if any problems are noted with the plug wires (such as cracked insulation or broken connector ends), replace the entire plug wire set.
Although it is possible for the timing chain to slip and throw the ignition timing off this is not a common occurrence. If a strong spark is present and the plugs are not fouled but the car makes no attempt to start, this normally indicates a fuel problem.
In a C4 Corvette, the fuel pump is one of those crossover items that is part of both the electrical system and the fuel system. The pump is a submerged variety, electrically operated, contained in the fuel tank and somewhat difficult to get to.
Although the pump is difficult to physically check, you can make certain that it at least runs by turning the ignition switch to on (but don't start the engine).
If the pump motor is working, you will hear a whirring noise coming from from the rear of the vehicle that will last for 2 seconds. Note that this does not prove that the pump is actually pumping fuel in sufficient quantity and/or pressure but it does prove if the pump motor itself is operating.
If you do not hear any sound, check the fuse labeled "Crank" on the fuse panel.
If the fuse is not blown but there is no sound from the tank area, there is a problem with either the motor or the wiring feeding the fuel pump motor.
If the pump appears to be working, you will need to measure the fuel pressure at the Shraeder valve located on the fuel rail on the engine. Anything below 35 PSI is cause for alarm and can be the fuel pump (pumps---2 each---on a ZR-1), the fuel pressure regulator, the check valves in the fuel return line, the fuel filter or a obstructed fuel line.