Why is the PUR outer skin the golden shield for in-vehicle wiring harnesses?

Ordinary PVC Cables Can Become "Hard, Brittle, and Cracked" in the Engine Compartment

If you have ever opened the hood of an older vehicle and taken a close look at the exposed wiring harnesses, you may have noticed something concerning: the cable insulation feels hard and brittle, and in some areas it has already cracked, exposing the copper conductors inside.

This is not an isolated phenomenon. It is a common aging issue associated with PVC (polyvinyl chloride) cables operating in high-temperature engine compartments.

The problem lies in the material characteristics of PVC. To make PVC soft and flexible, manufacturers add plasticizers - chemical additives that make otherwise rigid PVC easier to bend. However, prolonged exposure to high temperatures in an engine compartment (80°C to 150°C, with localized temperatures near the exhaust system sometimes exceeding 200°C) can cause these plasticizers to gradually evaporate or migrate out of the material.

Once the plasticizers are depleted, PVC gradually returns toward its inherently rigid and brittle state.

What happens next?

The hardened PVC insulation loses its flexibility and begins to develop cracks under continuous vehicle vibration. As the cracks expand, sections of the insulation may flake or break away, exposing the copper conductors to oxidation and corrosion.

More seriously, exposed conductors may come into contact with one another and cause a short circuit, or contact the vehicle's metal body and cause electrical leakage or a ground fault. The consequences can range from distorted sensor signals to, in severe cases, an electrical fire.

Some vehicle owners have reported that when replacing a radiator or performing other maintenance, the protective PVC tubing underneath the hood had become so brittle that it crumbled like cookie crumbs when touched.

This is not simply an isolated failure.

In the high-temperature environment of an engine compartment, PVC cable aging is not a question of "whether" it will happen, but "how long" it will take to happen.

PUR's Chemical Resistance: Protection Against Engine Oil, Coolant, and Fuel

PUR (Polyurethane) has a fundamentally different material structure from PVC, resulting in very different long-term performance.

PVC has limited resistance to oils and certain chemicals. PUR insulation, by contrast, offers strong resistance to many substances commonly encountered in industrial and automotive environments, including engine oil, coolant, hydraulic oil, gasoline, dilute acids, and alkaline solutions.

Its chemical resistance can be summarized as follows:

Resistance to engine and lubricating oils: Compliant with DIN EN 50363-10-2 requirements, with good resistance to long-term exposure to mineral-oil-based lubricants without significant swelling or decomposition.

Coolant resistance: Maintains stable performance when exposed to coolant or coolant leakage.

Gasoline and fuel resistance: Resists degradation when exposed to automotive fuels.

Acid and alkali resistance: Provides resistance to a range of acidic and alkaline chemicals.

Hydrolysis and microbial resistance: Performs well in humid environments and is resistant to degradation caused by hydrolysis and microbial growth.

PVC may gradually swell, soften, and eventually crack when exposed to engine oil. Under comparable conditions, PUR can maintain its physical properties for years.

This is one reason PUR cables are widely used in demanding environments such as machinery manufacturing and automotive production, where exposure to oil and other contaminants is common.

PUR is also a halogen-free material, meaning it does not contain halogen elements such as chlorine, fluorine, or bromine. Under extreme conditions such as fire, PUR does not produce the same type of hydrogen halide gases associated with the combustion of PVC, which can generate hydrogen chloride and other toxic fumes.

Flexing Test Data: Millions of Bending Cycles in Drag-Chain Applications

Engine-compartment and door-hinge wiring harnesses face more than just high temperatures and chemical exposure. They must also withstand repeated bending and mechanical friction.

Every time a vehicle door opens and closes, the wiring harness in the door flexes. Every vibration experienced while driving also places mechanical stress on wiring inside the engine compartment.

Over time, this can add up to millions of repeated bending cycles.

So how does PVC perform under these dynamic conditions?

In drag-chain testing, PVC cables may typically withstand around 200,000 to 500,000 bending cycles at room temperature. And that is under room-temperature conditions. At the elevated temperatures found inside an engine compartment, service life can be reduced further.

What About PUR?

Industry test data provides some impressive examples:

L-com Cat5e PUR Industrial Ethernet Cable: Passed a 1 million-cycle flex test at a bending radius of 10× the cable diameter and a 10 million-cycle flex test at a bending radius of 20× the cable diameter.

Phoenix Contact PUR Sensor Cable: Dynamic bending testing can withstand up to 10 million bending cycles.

igus chainflex® PUR Data Cables: Designed and tested for millions of bending cycles in drag-chain applications while maintaining signal integrity.

HELUKABEL MULTIFLEX 512-PUR Drag-Chain Cable: Tested to approximately 10 million alternating bending cycles.

What Does 10 Million Bending Cycles Actually Mean?

Suppose a vehicle door is opened and closed 10 times per day. That would equal approximately 3,650 cycles per year.

At 10 million cycles, that would theoretically correspond to approximately 2,740 years of door-opening cycles.

Of course, real-world service life depends on many other variables, so this calculation should not be interpreted as an actual product lifespan. Nevertheless, it illustrates the exceptional fatigue resistance that PUR can provide under repeated flexing.

PUR achieves this performance because its molecular structure provides excellent elasticity and tear resistance.

During repeated bending, the PUR jacket distributes mechanical stress more evenly, much like a spring, rather than concentrating stress at a single point and allowing cracks to develop as PVC can.

Combined with finely stranded, highly flexible copper conductors, PUR cables can achieve minimum bending radii as low as 6× the cable diameter, depending on the specific cable construction.

High/Low Temperature Cycling: -40°C to +105°C

Automotive wiring harnesses must withstand a much more extreme temperature range than most people realize.

Winter

In regions such as North America, Northern Europe, and Russia, outdoor temperatures can fall to -40°C or even lower.

Summer

In desert regions or enclosed parking areas, temperatures inside a vehicle can exceed 60°C.

Engine Compartment

After prolonged driving, temperatures inside an engine compartment can easily exceed 100°C.

A cable that must operate year-round under these conditions places extremely demanding requirements on the insulation material.

PVC Temperature Limitations

A typical PVC operating range is approximately -20°C to +70°C.

Below -20°C, PVC can become stiff and brittle, increasing the risk of cracking or breakage when bent.

During prolonged operation above 70°C, plasticizer migration and thermal aging can accelerate significantly.

PUR Temperature Performance

Automotive-grade PUR cables meeting ISO 6722 requirements can provide an operating temperature range of approximately -40°C to +105°C.

What does this mean in practical terms?

At -40°C

In extremely cold conditions, PUR insulation retains its flexibility and is less likely to become brittle and break when bent.

Some specialized PUR cable constructions can maintain good flexing performance even at temperatures as low as -60°C.

At +105°C

At elevated temperatures, PUR insulation is designed to resist softening, deformation, and accelerated aging.

According to the referenced long-term high-temperature testing data, PUR cables can maintain stable performance during 3,000-hour high-temperature tests.

The More Important Challenge: Temperature Cycling

The real challenge is often not simply high or low temperature, but repeated temperature cycling.

For example, a vehicle may experience a sequence such as:

-30°C overnight → engine start → engine compartment temperature rises to +80°C during operation

This repeated expansion-and-contraction cycle places considerable stress on cable insulation.

Under these cycling conditions, PUR is designed to resist microcracking caused by repeated thermal expansion and contraction, while PVC can gradually become harder and more susceptible to cracking.

This is why high-end automotive diagnostic harnesses use PUR jackets:

They are designed to maintain reliable mechanical and electrical performance across a wide range of climates - from the Arctic Circle to the Sahara Desert, and from a cold engine start to full-load operation.

When PVC and PUR are compared side by side, the differences are clear:

PVC cables are inexpensive and easy to process, and they can perform adequately in indoor, dry environments.

However, in the engine compartment - a demanding environment combining high temperature, oil contamination, vibration, and repeated flexing - PVC insulation is ultimately more vulnerable to aging.

PUR is often regarded as a "protective shield" for automotive wiring harnesses because its chemical resistance helps withstand exposure to engine oil and coolant, its elastic structure withstands millions of flexing cycles, and its wide operating temperature range enables reliable performance across extreme climates.

For a high-quality diagnostic cable harness, the material used for the outer jacket can make the difference between a cable that lasts for a year and one designed to provide reliable service for many years.