If you have an engine computer that controls the alternator, you will have to check the wiring from the alt. to the PCM. Also check any fusible links and grounds are often overlooked.
Do you have dual alternators?
SECTION 414-00: Charging System - General Information
2002 E-Series Workshop Manual
DESCRIPTION AND OPERATION
The charging system is a negative ground system consisting of the following:
- integral/internal voltage regulator
- charging system warning indicator
- storage battery
- necessary wiring and cables
The generator is belt-driven by the engine accessory drive system. Refer to wiring diagrams for schematics and locations of the components and wiring.
The battery is a 12 volt direct current source connected in a negative ground system. The battery case is sealed and includes two vent holes to release gases. The battery has three major functions:
- engine cranking power source
- voltage stabilizer for the electrical system
- temporary power when electrical loads exceed the generator output current
The optional dual generator package (7.3L diesel engine only) consists of an upper/lower dual generator system. The upper generator is a 6G 50/110 generator with a 2.49:1 pulley ratio. The lower generator is a 6G 50/110 generator with a 2.32:1 pulley ratio. Thus the upper and lower generators are not interchangeable.
This system is also monitored and controlled by the powertrain control module (PCM). The PCM monitors both the upper and lower generator I circuits to determine both generators' output and set possible fault codes (diagnostic trouble codes [DTCs]). The PCM only controls the lower generator by turning it on or off by opening or closing the lower generator off only when the glow plugs are commanded on by the PCM to avoid possible damage (excessive voltage) to the glow plugs.
Both generators are independent of one another and only control themselves (through their own internal voltage regulator). If one generator fails, the other generator can maintain system performance under light load conditions. Under a heavy load, the PCM should detect a problem, set a DTC and illuminate the warning indicator.
The PCM controls the charging system warning indicator and will command the light on if the PCM detects a problem on the monitored circuits.