Automotive Wire Harness Maximum Tensile Force Testing: Standards, Procedures, and Acceptance Criteri

Automotive wire harness maximum tensile force testing evaluates the mechanical integrity of crimped terminals, splices, and complete harness assemblies under axial loading. This test verifies that wire-to-terminal joints and harness structures can withstand the pull forces defined by OEM drawings and international standards such as QC/T 29106, USCAR-2, and IEC 60352-2. This article explains the sample types, test items, test methods, acceptance criteria, and practical applications of maximum pull force testing for automotive wiring harnesses.

Technical Overview

The maximum tensile force test, also referred to as the pull force or pull-out test, is one of the most fundamental mechanical verification methods for automotive wire harnesses. It determines the greatest axial load that a crimped connection, splice, seal, or complete harness assembly can sustain before failure occurs. Because a wire harness experiences continuous mechanical stress during vehicle assembly and service life, including vibration, thermal expansion, road shock, and manual handling during repairs, the tensile strength of every joint in the harness directly affects the electrical reliability and safety of the entire vehicle.

A weak crimp or an under-strength splice can separate under load, causing intermittent circuits, open circuits, or, in severe cases, short circuits that may lead to electrical faults or fire hazards. The maximum tensile force test therefore serves as a primary gate in both design validation and routine production quality control. Several standards govern this test, including QC/T 29106 for automotive wire harness technical requirements, USCAR-2 for cable-to-terminal crimp performance, IEC 60352-2 for solderless crimped connections, and ISO 6722 together with GB/T 25085 for 60 V single-core cables used in road vehicles.

In practice, the test is performed on a universal tensile testing machine using dedicated fixtures that grip the terminal on one side and the wire on the other. The machine applies an increasing axial force at a controlled speed until the specimen fails, and the peak force together with the failure mode is recorded and compared against the minimum values specified in the applicable standard or customer drawing.

Testing Samples

Crimped Terminal-to-Wire Joints

The most common specimen is a single wire of the specified cross section, typically ranging from 0.35 mm² to 16 mm² for standard automotive applications, terminated with a crimped contact or ring terminal. Specimens are produced on production crimping equipment or crimp presses under controlled crimp height and crimp width, and the free wire length is normally kept at a minimum of 100 mm to 150 mm beyond the crimp to allow proper clamping in the testing machine.

Splices and Solder Joints

Multi-wire splices, whether crimped, ultrasonically welded, or soldered, are tested to confirm that the junction can transfer the required load without wire pull-out or joint fracture. Specimens are prepared exactly as they appear in the production harness, including any insulation and covering applied over the splice.

Sealed Connection Assemblies

For connectors used in wet areas or underbody environments, the tensile test is often performed on the complete assembly, including the seal and the back plug, to confirm that wire-to-seal retention and seal positioning remain intact under the specified axial load.

Complete Harness Assemblies

Full harnesses are tested at defined pull points, such as grommets, retainers, branches, and connector interfaces, using the forces and directions specified on the harness drawing. This verifies the overall mechanical strength of the finished product rather than a single joint.

Wire and Insulation Material Samples

Bare conductor and insulation samples cut from the cable itself are tested in tension to determine conductor break strength, insulation tensile strength, and elongation at break, in accordance with ISO 6722 and GB/T 25085. These material-level values establish the upper reference strength against which joint performance is judged.

Testing Items

Depending on the validation plan or control plan, the maximum tensile force test program may include the following items:

• Crimp pull-out force between terminal and conductor, the core test item for crimp quality
• Conductor tensile strength and elongation for the base wire material
• Insulation tensile strength and elongation at break
• Whole-harness axial pull force at grommets, branches, and retainers
• Terminal retention force between the contact and the connector housing, often tested on the same equipment
• Seal and back-shell retention force for sealed systems
• Post-environmental pull force, retested after heat aging, thermal shock, humidity cycling, or salt spray exposure, to confirm long-term joint stability

Testing Methods

Specimen Preparation and Conditioning

Specimens must be produced with the same materials, tooling, and process parameters as series production. Crimp height and crimp width are measured and recorded for every specimen, and cross-section microsection analysis is frequently performed as a complementary check. Before testing, specimens are conditioned in a standard laboratory atmosphere, typically 23 °C ± 5 °C with a relative humidity of 45 % to 75 %, for a minimum of several hours in accordance with ISO 291, unless the governing specification states otherwise.

Equipment and Fixturing

A universal tensile testing machine with a calibrated load cell, accuracy class 1 or better, is used. The terminal is held in a purpose-made fixture or grip that applies load on the terminal barrel without stressing the conductor, while the wire is clamped in pneumatic or mechanical wire grips with smooth faces to avoid premature wire damage. The alignment of the load axis with the specimen axis is critical; angular misalignment can reduce the measured force and produce misleading failure modes.

Test Procedure

The test is conducted in the following sequence:

• Mount the specimen vertically, securing the terminal in the upper or lower fixture and the wire in the opposite grip, with the load axis straight through the crimp
• Set the crosshead speed to the value required by the specification, commonly between 25 mm/min and 250 mm/min, with 100 mm/min widely applied for crimp pull-out tests
• Apply the tensile load continuously until failure, defined as wire break, crimp pull-out, or another documented separation mode
• Record the maximum force reached, the failure location, and the failure mode, supported by photographs where required
• Repeat the test on the specified number of specimens, typically a minimum of three to five per condition for validation and per-lot sampling for production control

Acceptance Criteria and Typical Minimum Values

The measured peak force must meet or exceed the minimum pull force specified for the conductor cross section. Representative minimum crimp pull-out force values commonly referenced in industry specifications such as QC/T 29106 are listed below; the applicable customer drawing or standard always governs the final acceptance limit, since OEM requirements may differ.

• 0.35 mm² — minimum 60 N
• 0.50 mm² — minimum 60 N
• 0.75 mm² — minimum 100 N
• 1.00 mm² — minimum 130 N
• 1.50 mm² — minimum 200 N
• 2.50 mm² — minimum 300 N
• 4.00 mm² — minimum 350 N
• 6.00 mm² — minimum 400 N
• 10.0 mm² and above — increasing values up to approximately 1200 N for very large cross sections

For complete harness assemblies, a static force defined on the harness drawing, often between 100 N and 250 N depending on the branch and attachment point, is applied and held for a defined duration, typically 10 to 60 seconds, without conductor break, insulation cracking, connector unseating, or seal displacement.

Failure Mode Classification

The recorded failure mode is as important as the force value itself:

• Conductor break outside the crimp zone with force above the minimum limit is the preferred and accepted result, as it demonstrates that the joint is stronger than the wire
• Crimp pull-out, where the conductor slides out of the barrel, is rejected regardless of the force achieved
• Conductor fracture exactly at the crimp mouth is evaluated against the governing specification and is frequently cause for rejection when found repeatedly
• Insulation slippage, seal displacement, or housing damage on assembled parts is assessed against the requirements of the applicable connector standard such as USCAR-21

Applications

Maximum tensile force testing is applied across the entire wire harness lifecycle:

• Design validation and product validation programs, where full test sequences including environmental aging followed by pull force retest demonstrate that the design meets OEM performance requirements
• Production process control, where first-piece inspection, periodic sampling, and crimp press setup verification rely on pull-out force results to confirm stable crimping parameters
• Supplier qualification and incoming inspection of terminals, contacts, splices, and cables
• Failure analysis of field returns and manufacturing defects, where residual joint strength helps identify root causes such as tooling wear, wrong wire size, or contaminated conductor surfaces
• Regulatory and customer compliance, where test reports from accredited third-party laboratories or from in-house laboratories operating under ISO/IEC 17025 are submitted as objective evidence during PPAP and audit processes

FAQ

What is the purpose of the maximum tensile force test for automotive wire harnesses?

The test verifies that every mechanical joint in the harness, including crimps, splices, seals, and attachment points, can withstand the axial loads encountered during vehicle assembly and service. It is the primary indicator of crimp quality and mechanical joint reliability.

Which standards apply to the maximum pull force test?

Commonly referenced standards include QC/T 29106 for harness technical requirements, USCAR-2 for cable-to-terminal crimps, IEC 60352-2 for crimped connections, and ISO 6722 or GB/T 25085 for the underlying cable materials. Connector-level retention is additionally covered by USCAR-21.

What test speed and how many specimens are required?

Test speeds typically range from 25 mm/min to 250 mm/min depending on the specification, with 100 mm/min in widespread use for crimp pull-out testing. Validation programs normally require at least three to five specimens per condition, while production control follows the sampling plan defined in the control plan.

Does the failure mode matter if the force value passes?

Yes. A crimp pull-out failure is rejected even if the measured force exceeds the minimum, because it indicates an unstable joint mechanism. The accepted result is a conductor break away from the crimp at a force above the specified minimum.

Can the test be performed after environmental exposure?

Yes, and it is standard practice in validation programs. Pull force is retested after heat aging, thermal shock, humidity, and other environmental sequences to confirm that joint strength does not degrade below the specified limit over the service life of the harness.

Where should the test be carried out?

Testing can be performed by accredited third-party laboratories for certification and customer submissions, or by manufacturers in their own laboratories provided the equipment is calibrated and the laboratory operates a documented quality system consistent with ISO/IEC 17025.