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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Preventive Maintenance for Diesel & Heavy Trucks › Lesson 11
TRN-DIES20-06 · Lesson 11 · 16 clock hours · lab

Electrical and Lighting PM Checks

Objective. Students will test all required vehicle lighting circuits, check battery state of charge and case condition, inspect wiring harnesses at known chafing points, and record all defects on the PM form, referencing FMCSA lighting requirements for interstate operation.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Electrical and Lighting PM Matters

A Class 8 tractor operates under Federal Motor Carrier Safety Administration (FMCSA) regulations that require specific lighting to be functional any time the vehicle is operated on public roads. An out-of-service inspector can place a truck out of service on the spot for a lighting violation, costing the carrier time and money. Beyond compliance, working lights and sound wiring prevent collisions and fires. This lab trains you to find every defect before the truck leaves the yard.

Safety Before You Start

Park the truck on level ground, set the parking brake, and chock the wheels. Before working near or under the vehicle, follow your shop's lockout/tagout (LOTO) procedure for any high-voltage or powered system you need to de-energize. Wear safety glasses at all times; battery acid and arcing connectors are eye hazards. When testing batteries, remove rings and metal watchbands — a short across a fully charged battery bank will produce enough current to cause severe burns in an instant. Keep a Class C fire extinguisher within reach whenever you are near batteries. Wear nitrile gloves when handling battery terminals or applying anti-corrosion spray.

Walk-Around Lighting Inspection

FMCSA regulations specify which lamps must be present and functional on a commercial motor vehicle. The major categories you will verify are:

Work with a helper. One person operates switches and pedals from the cab; the other walks around with the PM form and records each lamp position, the lamp type (incandescent or LED), and whether it passes or fails. Do not rely on memory — write it down immediately. Note the exact location using standard truck position language (e.g., "driver-side front marker, amber, inoperative"). This description must be specific enough for a repair technician who was not present to find and fix the defect without guessing.

Battery Testing

Heavy trucks typically run a battery bank of two to four 12-volt batteries wired in series-parallel to supply 12 volts at high cranking amperage. Before testing, visually inspect each battery: look for a swollen or cracked case, evidence of electrolyte leakage, and terminal corrosion. A white or blue-green crust on terminals increases resistance and can prevent starting.

Open-Circuit Voltage (OCV): With the engine off and no loads active for at least two minutes, measure voltage across each battery with a digital multimeter (DMM) set to DC volts. A fully charged 12-volt lead-acid battery reads approximately 12.6 V. A reading of 12.4 V indicates about 75% charge; below 12.2 V the battery is significantly discharged and may not pass a load test.

Load Test: A dedicated battery load tester applies a load equal to half the battery's cold-cranking amp (CCA) rating for 15 seconds. If voltage drops below 9.6 V under load at room temperature, the battery is considered failed and must be replaced. Always follow the tester manufacturer's instructions for the specific tool in your shop.

After testing, clean corroded terminals with a terminal brush, rinse, dry, and apply anti-corrosion spray. Verify that hold-down clamps are tight — a loose battery vibrates, shortening its life and risking a short circuit.

Wiring Harness Inspection

Vibration, heat, and sharp edges are the enemies of wiring harnesses. Inspect harnesses at these high-risk locations:

When you find a defect, write its location precisely on the PM form: frame rail side (driver or passenger), distance from a fixed reference point (e.g., "approximately 18 inches forward of the rear axle"), and the nature of the defect. A repair technician must be able to go directly to the spot.

Recording Defects on the PM Form

Every defect gets its own line. Record the system, the location, the observed condition, and the corrective action required. Do not combine multiple defects into one line. Leave nothing to verbal communication — the PM form is the legal and operational record of the vehicle's condition at the time of inspection.

Key terms8

Open-circuit voltage (OCV)The battery terminal voltage measured with no load applied and the surface charge allowed to dissipate; used to estimate state of charge.
Cold-cranking amps (CCA)A battery rating describing how many amperes the battery can deliver for 30 seconds at 0 °F while maintaining at least 7.2 volts; used to size load tests.
Load testA battery test that applies a controlled load (typically half the CCA rating) for 15 seconds while monitoring voltage to determine if the battery can deliver usable power under starting conditions.
Clearance lightsLights mounted at or near the widest point of the vehicle — amber at the front, red at the rear — required by FMCSA to mark vehicle width.
Identification lightsThree closely spaced lights mounted at the center top of the front and rear of vehicles wider than 80 inches; amber front, red rear.
ChafingWear damage to wire insulation caused by repeated rubbing against a sharp or abrasive surface, which can lead to a short circuit or open circuit.
GrommetA rubber or plastic ring inserted in a hole through which a wire or harness passes, protecting the insulation from the sharp edge of the hole.
Out-of-service (OOS) criteriaSpecific defect conditions defined by FMCSA and the Commercial Vehicle Safety Alliance under which an inspector may prohibit a vehicle from operating until repairs are made.

Worked examples4

1. You measure an open-circuit voltage of 12.38 V on a 12-volt battery that has been sitting with no load for five minutes. Estimate the approximate state of charge and state whether a load test is likely to pass.
  1. Recall the reference points: 12.60 V ≈ 100% charge, 12.40 V ≈ 75% charge, 12.20 V ≈ 50% charge, 12.00 V ≈ 25% charge.
  2. 12.38 V falls just below the 12.40 V mark, placing this battery at approximately 70–75% state of charge.
  3. A battery at 70–75% state of charge is below full but may still pass a load test if the plates are healthy. However, it is worth noting that a marginal battery at 70–75% SOC that barely passes in a warm shop may fail in cold weather when CCA requirement rises.
Answer: Approximately 70–75% state of charge. The battery is not at full charge and warrants a load test; a borderline result should be flagged for close monitoring or replacement before cold weather operation.
2. A battery is rated at 800 CCA. What load should a technician apply when performing a 15-second load test, and what minimum voltage must be maintained for the battery to pass?
  1. Load test load = CCA ÷ 2 = 800 ÷ 2 = 400 amperes.
  2. Apply 400 A for 15 seconds while watching the voltmeter.
  3. The battery must hold at least 9.6 V throughout the 15-second interval at approximately room temperature (about 70 °F) to be considered serviceable.
Answer: Apply a 400 A load for 15 seconds; the battery passes if voltage stays at or above 9.6 V.
3. During a walk-around you measure voltage at a marker light socket with a DMM and read 11.8 V with the circuit energized, but the lamp does not light. The ground circuit checks good. What does this tell you, and what is the next step?
  1. Voltage is present at the socket (11.8 V is close to system voltage with some drop, which is acceptable for initial diagnosis). The ground is confirmed good.
  2. Because power and ground are present but the lamp does not illuminate, the fault is most likely the bulb itself (open filament) or a severely corroded socket preventing contact.
  3. Remove the bulb, visually inspect the filament, and check socket contacts for corrosion. Replace the bulb; if it still does not light, clean the socket contacts and retest.
Answer: Voltage and ground are present; the bulb or socket contacts are the most likely cause. Replace the bulb and clean the socket contacts as the corrective action.
4. A truck has four 12-volt batteries wired in a 2S2P configuration (two strings of two batteries in series, then those strings in parallel). What is the nominal system voltage, and what OCV would you expect to read across the whole bank if all batteries are fully charged?
  1. Two batteries in series: 12 V + 12 V = 24 V per string.
  2. Two 24 V strings wired in parallel: voltage stays at 24 V (parallel wiring does not add voltage, it adds capacity).
  3. Fully charged 12-volt batteries read approximately 12.60 V each; two in series = 25.2 V across the bank when fully charged.
  4. Note: this is a 24-volt system, common on some heavy trucks and off-highway equipment. Confirm system voltage before testing.
Answer: Nominal system voltage is 24 V. Expected OCV across a fully charged bank is approximately 25.2 V.

The math in this lesson2

Learners set load at half of CCA and estimate state of charge from open-circuit voltage.
7.RP.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Walk around the truck with a helper activating each circuit: headlights (low and high beam), turn signals, brake lights, marker lights, clearance lights, and reverse lights; record any non-functioning lamp position, lamp type, and corrective action required on the PM form. · Using a digital multimeter or dedicated battery tester, measure battery voltage and perform a load test on each battery in the bank; inspect terminal clamps for corrosion, check hold-down security, and record state of charge. · Trace wiring harnesses along the frame rail and at the cab-to-chassis junction, checking for chafing against sharp edges or hot surfaces, missing grommets, and loose connectors; document any defect with location description sufficient for a repair technician to find it.
Materials: Class 8 tractor, digital multimeter, battery load tester, test light, wiring diagram (OEM), PM inspection form, FMCSA lighting requirements reference card, terminal cleaning brush, anti-corrosion spray
Check: Instructor spot-checks by introducing one hidden defect (disconnected lamp connector or low battery) before the student begins; student must find the seeded defect and all other actual defects; missing the seeded defect = remediation and repeat.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Preventive Maintenance / Lighting Regulations
1. Under FMCSA regulations, identification lights on a vehicle wider than 80 inches must be:
Electrical / Battery Testing
2. A battery rated at 600 CCA is being load-tested. What is the correct load to apply, and what minimum voltage must the battery maintain to pass?
Electrical / Wiring Inspection
3. You find that a wiring harness on the driver-side frame rail is missing a clip and resting directly on the rail flange. What is the primary risk of leaving this condition uncorrected?
Safety / PPE
4. Before removing a battery from a truck's battery bank, which personal protective equipment is MOST important to put on first?
Electrical / Battery Testing
5. An open-circuit voltage reading of 12.20 V on a 12-volt lead-acid battery indicates approximately what state of charge?
Preventive Maintenance / Documentation
6. During a lighting walk-around, which procedure best ensures accurate documentation of a failed lamp?
Electrical / Wiring Inspection
7. A technician pushes on a wiring harness connector at the cab-to-chassis junction and it separates with minimal resistance. What is the FIRST corrective action?
Electrical / Battery Testing
8. Which of the following conditions would most likely cause a battery to pass an open-circuit voltage check but fail a load test?
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