A Complete Guide to Wire Harness Testing: Methods, Scopes, and FAQs

Rigorous testing is performed on wire harnesses to screen out defects and validate long-term reliability in real-world conditions. Wire harness testing gives you peace of mind that all products are defect-free and of high quality. This guide will explain how to test wire harnesses, including the methods, scopes, and FAQs.

Test Methods

Wire harness testing involves electrical, mechanical, and environmental aspects, as well as visual inspection. With the help of testing, we can deliver electrically reliable, mechanically robust, and durable wire harnesses to you.

Electrical testing

Electrical testing is performed to validate key electrical parameters such as continuity, short circuits, resistance, and voltage, ensuring accurate and secure electrical connections.

Continuity test

The continuity test validates the continuity of each electrical path. It is intended to verify that all required connections are present.

The multimeter is the most commonly used tool for continuity testing in R&D, maintenance, sampling test, and small-volume production cases. Generally, a small voltage is applied between two pins that should be connected.

If the multimeter buzzer beeps or the resistance is below the preset threshold, current flows between these two pins, indicating a PASS result for the test.
If the buzzer does not beep or the resistance falls outside the preset threshold, there is no current flow, suggesting an open circuit. Then the test fails.

In the case of mass production, the continuity tester is preferred because it can automatically assess the continuity of wire harnesses in batches.

Short-circuit test

The short-circuit test measures the resistance between two suspected pins to check for short circuits. Usually, a multimeter is employed to apply a small voltage between two points that should not be connected.

If the multimeter buzzer beeps or the resistance falls within the preset threshold, a short circuit (current flow) is present between these two suspected points, and the test fails.
If the buzzer does not beep or the resistance falls outside the preset threshold, no short circuit exists and the wire harness passes the test.

A dedicated short-circuit tester is required for mass production cases.

Insulation resistance test

The insulation resistance test serves as a mandatory step to assess the quality of the insulation layer. The insulation resistance is measured using an insulation tester or Megohmmeter.

Specifically, a relatively higher DC voltage (e.g., 500 V or 1000 V) is applied between a pair of wires or between wires and the ground to measure the leakage current and calculate the insulation resistance. The calculated resistance is then compared with the preset threshold.

If it is greater than or equal to the preset threshold, the test is judged as PASS.
If it is below the preset threshold, the wire harness fails the test.

Hi-pot (high-potential) test

To avoid dielectric breakdown, a hi-pot test is required to verify that insulation can withstand high voltages. Instead of manual testing, the automated hi-pot tester is the mainstream trend in modern wire harness manufacturing.

The leakage limit, test duration, and target voltage are set at the beginning of the test. When the test starts, the hi-pot tester ramps the wire-to-wire or wire-to-ground voltage gradually from zero to the target value at a controlled rate and continuously monitors the leakage current throughout the ramp.

If the leakage current is constantly lower than the preset limit, the test is considered PASS.
If not, the test failsand a breakdown may even occur.

Once the test is complete, the tester automatically reduces the voltage to zero and discharges any residual charge from the tested harness.

Mechanical testing

Mechanical testing is to verify the physical strength and structural durability of a wire harness under mechanical stress. It evaluates multiple aspects, such as pull force, crimp cross-section, and intermittent disconnection.

Pull test

As the most fundamental mechanical test, the pull test measures the crimp strength, usually between the terminal and wire, so as to validate good crimping. The test can be performed in a non-destructive or destructive manner. Among all pull tests, pull & break (non-destructive) and pull & hold (non-destructive) are most commonly used.

Pull & break: A crimped sample is mounted onto the motorized pull tester to determine its maximum pulling force. With one clamp fixed to the insulation and the other to the terminal, the two clamps move away from each other along a common axis. Once a physical failure (e.g., wire break, terminal pull-out, or crimp barrel crack) occurs, related parameters (e.g., peak force and failure mode) must be recorded for traceability. This failed sample needs to be discarded properly.
Pull & hold: The tester applies aforce lower than the peak force to a sample at a specified rate and holds the force for a couple of seconds. If the sample remains intact within the specified duration, it passes the test. If any physical failure occurs, this sample needs to be isolated for troubleshooting.

Crimp cross-section analysis

Crimp cross-section analysis is to observe the structure of a crimped terminal under a microscope. This process is destructive because the crimped terminal is cut and polished before its cross-section is examined.

If all parameters fall within the standards and there is no critical defect in the microscopic structure, the test is considered PASS.
If any parameter exceeds the tolerance or any critical structural defect is observed, the test fails, and related parameters need to be adjusted.

Intermittent disconnection test

This test is designed to verify that no intermittent disconnection occurs under dynamic conditions such as vibration and shock.

A finished wire harness is connected to an intermittent monitor and then mounted onto the vibration table. The test must be conducted under a simulated real-world operating environment.

During the vibration, the electrical parameters of the circuit are constantly monitored and compared with the preset thresholds in the normal state. Once any parameter falls outside the threshold, an intermittent disconnection event is recorded.

Environmental testing

Environmental testing is to verify that wire harnesses can withstand various climatic and chemical environments, such as extreme temperature and humidity, chemical corrosion, salt fog, and dust. Not all harnesses require full environmental testing. What tests are required depends on the intended application scenarios and customer demands.

IP test (water/dust)

Wire harnesses typically adhere to IEC 60529, where ingress protection (IP) ratings are defined for water and dust protection. The IP test aims to validate the reliability and stability of wire harnesses in water and dust ingress cases.

In the two-digit IP code, the first digit represents the dustproof capability, while the second reflects the waterproof capability. Higher digits indicate better protection. In IP67, for example, 6 indicates complete protection against dust; 7 means temporary immersion, showing that wire harnesses can survive in water up to 1 meter deep for 30 minutes.

Thermal cycling test

The thermal cycling test is designed to validate the reliability and durability of wire harnesses under high/low temperature cycling during their service life.

In the test, an assembled harness is placed in a thermal chamber through temperature cycles (e.g., between –40°C and +125°C) to observe its electrical parameters.

If the parameters remain within the specifications and the harness is visually acceptable, the test is judged as PASS.
If any parameter falls outside the specifications or any visible damage is observed, the test is judged as FAIL.

The electrical parameters after the test are also compared with the pre-test baseline values. The optimal outcome is that those parameters exhibit no significant change between pre-test and post-test values while remaining within the specifications.

Visual inspection

Visual inspection is a foundational check spanning the whole testing process. It allows technicians to identify obviously defective products at a glance, thereby saving test time and cost.

Defects that can be identified by visual inspection

Conductor/insulation defects: wrong wire color, insulation scratch, burned insulation, etc.
Terminal crimp defects: terminal crack, terminal deformation, excessive stripping, etc.
Connector assembly errors: terminal backout, missing seal, connector not fully mated, etc.
Taping/fixing defects: wrong tape color, loose tie, wrong tie location, etc.
Labeling mistakes: missing label, wrong label location, wrong information, etc.
Contamination: foreign matter in connector, oil stain, water stain, etc.

Test Scopes

The scope of wire harness testing varies with the test objectives and cost considerations. The test scope is typically divided into three levels: routine, sampling, and type tests.

Routine test

Routine test, also known as 100% inspection, examines key electrical parameters (e.g., continuity, dielectric strength, and insulation resistance) of each wire harness after being manufactured. It is expected to quickly identify and eliminate obvious manufacturing defects.

Sampling test

Representative samples are selected by time, batch, or quantity for more detailed testing, aiming to monitor the stability of the manufacturing process.

Type test

Type test is performed on representative samples at critical time nodes, such as product R&D, major engineering changes, or re-qualification. As the most comprehensive level of testing, it aims to prove the long-term reliability, robustness, and durability of wire harnesses. However, it is noteworthy that the type test is destructive.

FAQs

Can wire harnesses passing an IP67 test be used underwater?

No, IP67 means that wire harnesses can be submerged in water up to 1 meter deep for a maximum of 30 minutes under laboratory conditions. Those passing an IP67 test can resist rain, splashing, and quick submersion. It is suggested to look for a higher IP rating for underwater applications.

Why perform destructive testing?

Destructive testing is necessary. Only through destructive testing can ultimate strength be determined. Moreover, the internal structure of a crimp can be observed only when we break it. We perform destructive testing on a few samples to ensure the reliability and robustness of all products to be delivered.

Can visual inspection be skipped?

No, visual inspection matters. It is the quickest and most fundamental way to identify and eliminate obvious defects, with almost zero cost. It also serves as a complementary step for electrical, mechanical, and environmental tests, avoiding a waste of testing time.

Can the hi-pot test be skipped after the insulation resistance test passes?

No. Although both of these tests are intended for insulation, they serve different purposes and use different methods. A wire harness passing an insulation resistance test may fail the hi-pot test. The insulation resistance test is to examine whether the quality of insulation meets the standard, whereas the hi-pot test is to check whether the insulation is strong enough to withstand high-voltage stress. To ensure absolute safety under high voltages, both tests are required.

What industry standards does wire harness testing comply with?

Major global industry standards for wire harnesses fall into four types:

Manufacturing and acceptance standard: IPC/WHMA-A-620
Performance standard: ISO 16750, USCAR, IEC 60529
Quality control standard: IATF 16949, ISO 9001
Environmental standard: RoHS, REACH

What ECOCABLES Can Offer Through Testing

Wire harness testing keeps all products under quality control, which guarantees defect-free delivery. Test schemes can be tailored to your requirements depending on the product type and application scenarios. ECOCABLES ensures that every wire harness we deliver is of high quality, reliable, and durable, meeting the state-of-the-art standards. Choosing us will offer you reliable quality assurance and a long-term, stable supply. Feel free to contact us.