Volvo FL Workshop Manual. Wiring Diagram

A detailed repair and electrical diagram manual for Volvo FL series trucks, covering all major electrical systems: power supply, lighting, engine start, ABS, CAN bus, charging system and control unit communication. The manual is intended for technical personnel, auto mechanics and electricians involved in diagnostics and repair of electrical equipment.

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Password for PDF: truckfixdiagn.com

1. 🔌 Grounding problems

Symptoms: Headlights flickering, instrument panels going out, random errors on the display

Reason: Oxidation or loosening of ground bolts (especially under the cabin and near batteries)

Solution: Cleaning, lubricating and tightening all "ground" connections

2. 🔧 Control unit errors (MID 128, 144, 140)

Symptoms: "Engine fault", "Gearbox communication lost", engine does not start

Reason: Loss of communication on the CAN bus, broken wires in the harnesses

Solution: Checking the CAN resistance (should be ~60 Ohm), diagnostics via Volvo Tech Tool

3. 💡 Failure of light circuits

Symptoms: Headlights, turn signals, brake lights do not work

Reason: Burnout relay, oxidation of connectors or contacts in the rear of the chassis

Solution: Replace the relay, clean the contacts, especially in the lights and in the rear harness

4. 🧠 Central comfort unit (in the cabin)

Symptoms: The heater fan, wipers, cigarette lighter do not work

Reason: Failure in the "cabin control module" (CCU), overheating or glitch

Solution: Check the power supply of the unit, reset the power (disconnect the battery for 10 minutes), replace if damaged

5. ⚙️ Sensor errors (temperature, pressure, level)

Symptoms: Incorrect readings, false errors

Reason: Damaged insulation, moisture ingress, poor contacts in the harnesses

Solution: Test with a multimeter, replace the sensor or harness

6. 🔋 Rapid battery discharge

Symptoms: Does not start in the morning, requires "jump starting"

Reason: Constant leakage current through faulty components (e.g. digital radio, security systems)

Solution: Measure the current leakage in idle mode (<0.05A), alternately disconnect consumers

7. 🧵 Hidden wiring breaks

Symptoms: Sporadic errors, some functions do not work

Reason: Harness breakage at bending points - at the doors, under the cabin, at the fuse box

Solution: Visual and continuity inspection, restoration or replacement of sections

📌 Helpful tips:

Check the harnesses at the rear axle - they suffer from dirt and salt

Make sure that the connectors at the fuse box are not oxidized

Check the fuses in the cabin and under the hood - sometimes they oxidize even without burning out

Carry out annual cleaning and treatment of contacts (especially at the trailer socket and rear electrics)

🛠️ Recommended software for diagnostics:

Volvo Tech Tool (TT)

Jaltest, Texa (not original, but works)

MID/SID/FMI error code reader