Check the main power distribution panel inside the cab first, then inspect the engine compartment relay center to isolate electrical faults quickly. Match each circuit slot with its labeled function before replacing any blown link.
The cab-mounted panel is typically positioned beneath the dashboard on the driver side. Each slot controls specific systems such as lighting, gauges, and auxiliary equipment. A broken metal strip inside a blade insert indicates a failed component, while repeated failure points to wiring damage or overload conditions.
In the engine area, the primary distribution unit sits near the battery and contains relays along with high-capacity protection links for systems like starter circuits and cooling components. Disconnect the battery before handling these parts to prevent short circuits and electrical damage.
Use a multimeter to verify continuity across each connection rather than relying only on visual checks. This method helps detect hidden faults where the insert appears intact but does not pass current, reducing the risk of misdiagnosis.
1990 Peterbilt 379 Fuse Box Diagram with Panel Locations and Circuit Details
Locate the cab power distribution panel under the driver side dashboard and use the printed layout on the cover to match each slot with its assigned circuit before making any replacements.
This interior unit controls systems such as dash lighting, gauges, heater controls, and auxiliary switches. Each slot has a fixed amperage rating, typically marked in numbers like 10A, 15A, or 20A, and must be matched exactly during replacement to avoid overheating or wiring damage.
Check the engine compartment distribution center positioned near the battery; it contains relays and high-capacity protection links for starter circuits, charging system, and cooling components.
Open the protective cover carefully and read the labeling on the underside, which identifies each relay position and high-load circuit without removing additional parts.
Common circuit assignments include:
Headlights and marker lights in the cab panel, ignition and starter relay in the engine compartment, blower motor and HVAC controls in the interior unit, battery feed and alternator circuits in the engine distribution center.
Use a multimeter to test continuity across each connection point instead of relying only on visual inspection, since some damaged inserts may appear intact while blocking current flow.
If repeated failures occur in the same slot, inspect wiring harnesses for abrasion, loose grounds, or corrosion near connectors.
Disconnect the battery before handling any electrical components to prevent accidental short circuits and protect sensitive systems.
Where to locate dashboard and engine bay fuse panels in Peterbilt 379
Check beneath the driver side dashboard first; the main interior power panel is mounted low behind a removable cover near the steering column.
Reach under the dash and locate a rectangular plastic lid secured by clips. Remove it to access rows of blade-style inserts aligned in a grid. Use a flashlight to read slot numbers and circuit labels printed on the cover or surrounding trim, which identify systems such as gauges, lighting, and auxiliary switches.
Open the hood and find the engine compartment distribution center near the battery; this unit is enclosed in a protective housing and contains relays and high-capacity protection links arranged in a spaced layout:
- Mounted close to battery for direct power routing
- Protected by a snap-on or clip-secured cover
- Handles starter, charging system, and cooling components
- Cover underside shows labeled positions for each circuit