Integrated HV Service Practical: Full Vehicle Workflow

Objective. Students will independently complete a full HV service workflow—PPE donning, system disable and LOTO, scan diagnostic, specified inspection tasks, re-enable, and professional repair order—on an unfamiliar vehicle variant, demonstrating integrated competence.

This lab is your capstone demonstration for EV and hybrid high-voltage (HV) service. You will work on an unfamiliar vehicle platform, applying every skill and procedure you have built across this course—without step-by-step instructor guidance. That independence is the point. Industry employers expect a certified technician to pick up an unfamiliar vehicle, locate the correct service information, and execute a safe, professional workflow from first contact to final documentation.

Why an Unfamiliar Vehicle Matters

HV architecture varies significantly across manufacturers and model generations. Battery pack voltage, service disconnect location, interlock wiring strategy, CAN bus topology, and shutdown wait times all differ by platform. A technician who has only practiced on one vehicle may miss a critical step on another. By completing this lab on a vehicle you have not serviced before, you prove that your competence is transferable—exactly what ASE credentials are designed to confirm.

Phase 1: Pre-Task Planning Before You Touch the Vehicle

Professional HV service always begins at the desk, not the vehicle. You will receive a work order containing the customer concern and the vehicle VIN. Your first task is to decode the VIN and locate the correct OEM service information for that exact model year and powertrain variant. Generic or third-party information is not acceptable for HV work; OEM data carries the manufacturer's verified shutdown times, torque specifications, and interlock locations.

From that information, write a step-by-step service plan that includes:

Required PPE for each phase of the job
The exact HV disable sequence with component names
All LOTO (lockout/tagout) steps, including the device type and location
The voltage verification method and acceptable threshold that confirms the system is de-energized
Inspection and diagnostic tasks in sequence
The HV re-enable procedure with any required wait times or system-readiness checks

Submit this plan to your instructor before approaching the vehicle. Instructor approval is a prerequisite gate—no plan approval means no hands-on work. This mirrors professional practice: planned work is safer work.

Phase 2: PPE Donning and Inspection

Approved HV electrical work requires class 00 or higher rubber insulating gloves rated for the system voltage, with leather protectors worn over them. Before donning, every glove must pass an air inflation test: roll the cuff, trap air, and inspect for leaks along the entire surface. Damaged gloves are removed from service immediately. You will also wear safety glasses or a face shield, HV-rated arc-resistant clothing appropriate to the task, and insulated footwear. All PPE must be inspected before use, not after.

Phase 3: HV System Disable and LOTO

Follow the OEM sequence precisely. Most procedures require:

1. Switching the vehicle to OFF and removing the start key or key fob from the vehicle

2. Waiting for capacitors in the power electronics to discharge (OEM specifies the duration)

3. Removing or disconnecting the manual service disconnect (MSD) or service plug, if present

4. Applying your personal lockout device to prevent inadvertent re-connection

5. Verifying zero voltage across HV conductors using a CAT III or CAT IV DMM set to DC voltage

Voltage verification is non-negotiable and is a safety-critical step in your rubric. Measuring less than the OEM's de-energized threshold—often under 60 V DC—is the only proof the system is safe. Never assume the system is de-energized because an indicator lamp is off.

Phase 4: Scan-Tool Diagnostics and Inspection Tasks

Connect an OEM-compatible scan tool and retrieve all stored and pending diagnostic trouble codes (DTCs). Record every code, its freeze-frame data, and any relevant live data PIDs such as pack voltage, state of charge, individual module voltages if accessible, coolant temperature, and insulation resistance readings if the vehicle provides them. Prioritize DTCs to determine which are the most likely root cause of the customer concern before beginning physical inspection.

Complete all assigned inspection tasks—high-voltage cable condition, connector seating, coolant hose integrity, interlock circuit continuity, and any tasks specified on your work order—using insulated tools and your DMM.

Phase 5: HV Re-Enable and System Readiness Verification

Re-enable the system in the exact reverse of the disable sequence per OEM instructions. Remove your LOTO device, reinstall the MSD, and follow any required initialization steps. Verify system readiness: the scan tool should show no new DTCs, and relevant live data should return to expected values. Some platforms require a specific key-on sequence or a calibration routine before the vehicle is returned to the customer.

Phase 6: Professional Repair Order and Debrief

A complete repair order includes: the customer concern as stated, the cause identified through diagnosis, the correction performed or recommended, and any parts and labor. Use precise, professional language—avoid vague terms like "checked" or "looks fine." Your five-minute verbal debrief with the instructor should walk through each test result, explain your diagnostic logic, and state your recommended repair. Clear communication of technical findings to a customer or service advisor is itself a professional competency.

Scoring and Safety-Critical Steps

You must score 80% or above overall on the integrated skills checklist and repair order rubric. However, every safety-critical step—PPE inspection and donning, LOTO application, and voltage verification—requires 100%. A single missed safety-critical step is an automatic fail. Remediation and a full re-attempt are required before credit is awarded. This standard exists because in the field, a single missed step can be fatal.

Key terms10

Manual Service Disconnect (MSD)A removable plug or switch, specified by the OEM, that physically breaks the high-voltage battery circuit to allow safe service; also called a service plug or service disconnect.

Lockout/Tagout (LOTO)A formal procedure—governed by OSHA standards—that uses a physical lock and informational tag to prevent accidental re-energization of a system while a technician is working on it.

CAT III / CAT IV DMMA digital multimeter with an IEC 61010 measurement category rating suitable for high-energy distribution circuits; CAT III covers fixed installation equipment and CAT IV covers the utility service entrance level—both are appropriate for HV vehicle work, providing protection against transient voltage spikes.

Insulation ResistanceA measurement, typically in megohms, of how well the insulation separating HV conductors from the vehicle chassis is maintaining isolation; a low reading indicates a potential fault to ground.

Interlock CircuitA low-voltage safety circuit built into connectors, covers, or the MSD that signals the power electronics controller to disable high voltage if a cover is removed or a connector is disconnected without authorization.

State of Charge (SOC)An estimate, expressed as a percentage, of the remaining usable energy in the traction battery pack relative to its fully charged capacity.

Freeze-Frame DataA snapshot of selected powertrain and battery sensor values recorded by the control module at the moment a diagnostic trouble code was set; used to reconstruct the operating conditions that triggered the fault.

Air Inflation TestA pre-use inspection method for rubber insulating gloves in which the glove is rolled at the cuff to trap air and then inspected along its entire surface for any leaks that would compromise dielectric protection.

De-energized ThresholdThe maximum voltage level, stated in OEM service information, below which the technician may consider a previously energized circuit safe to contact; commonly below 60 V DC for HV systems, but the OEM value always governs.

OEM Service InformationRepair and diagnostic procedures published by the original equipment manufacturer for a specific vehicle model year and powertrain; the authoritative source for shutdown times, torque values, and interlock locations in HV service.

In class: Pre-task planning: student is given a work order describing customer concern and vehicle VIN; student independently locates correct OEM service information, writes a step-by-step service plan with LOTO steps and PPE requirements before touching the vehicle, and submits the plan for instructor review before proceeding. · Independent execution: student performs the complete workflow without instructor assistance—donning and inspecting PPE, disabling HV and applying LOTO, performing scan-tool diagnostics, completing all assigned inspection tasks, re-enabling the HV system per OEM procedure, and verifying system readiness—while instructor observes and scores using the skills checklist. · Professional repair order and debrief: student completes a full professional repair order and presents findings in a five-minute verbal debrief to the instructor, explaining each test result, the diagnostic conclusion, and the recommended repair; instructor provides structured feedback against the rubric.
Materials: Unfamiliar HV/BEV training vehicle (different platform from prior labs), OEM service information access, OEM-compatible scan tool, CAT III/IV DMM, full HV PPE set, LOTO equipment, insulated hand tools, torque wrench, repair order form, integrated skills checklist rubric
Check: Instructor-scored integrated skills checklist plus repair order rubric; student must score 80% or above on the rubric overall and achieve 100% on every safety-critical step (PPE, LOTO, voltage verification). Any missed safety-critical step is an automatic fail requiring remediation and re-attempt.

Practice check8

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