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Automotive Wiring Harness Deterioration

Cracking - Breaking - Rodent Damage

Automotive wiring insulation and environmental concerns collided in the 1990s and the automotive industry lost. Or they didn’t do their research well. Or whatever. You be the judge after reading the following.

The search for biodegradable components in the manufacture of cars began at least as far back as the 1970s. In the late 1980s European manufacturing regulators mandated biodegradable wiring harness insulation for cars. Previously, wire insulation was made of PVC or other petroleum products, with a mixture of chromium, lead or other additives for specific applications. After a few years of research, car manufacturers turned to soy-based insulation and eliminated many if not all of the toxic substances. German manufacturers, particularly Mercedes-Benz, started to try it in the early 1990s, and eventually just about all manufacturers followed suit.

So, what is the point of making wiring insulation out of soy products? you ask. And the answer is: Over time it degrades and turns to dust. That is one definition of biodegradable. It is certainly possible that a concern for the environment, or a need to follow government environmental regulations, drove manufacturers to adopt this materials technology. It’s also possible that the inherent short life-span of such a product appealed to the built-in-obsolescence engineering philosophy of the automotive industry.


The actual outcome of the decision to use an edible product in the manufacture of automobiles was that rodents began eating wiring harnesses!

Rodent infestation of cars has long been known as a sporadic nuisance. A few years ago, after my wife and I visited friends in Montana and left our van parked on their lawn for several days, when we left a strange rhythmic thwap-thwap-thwap came out of our ventilation system when the fan was turned on. After about half a day, a nasty smell began to be emitted as well.
I knew what I had to do. I performed field surgery on the HVAC system. After removing the interior air filter behind the glove compartment, I reached in and pulled out a flattened white-footed mouse from the ventilation fan. Unfortunately, instead of saving the carcass for scientific study (white-footed mouse in Montana is a different species than the one we have in Massachusetts), I tossed it on the ground and drove on. Equally unfortunately the smell did not cease. I then had to dig deeper into the bowels of the HVAC system in order to find a second mouse carcass, this one smaller. A baby mouse had gone wandering into the bowels of my van followed by a concerned parent trying to save its wayward offspring – and they both met a gruesome death. Ah well…

So I have direct first hand experience with rodent  infestation of my vehicle. But aside from the bad smell and the annoying fan noise, nothing drastic happened to my van.

But varmint damage to wiring insulation is obviously not a minor issue. As automotive wiring harnesses have become more and more complex, with connections involving sensors, actuators and control modules, any breakdown in the insulation and isolation of the wires can lead to catastrophic failures.  And so it has happened. There are now multiple law suits against manufacturers such as Toyota, Honda and Mercedes alleging that their products attract destructive rodents which cause immense electrical damage and cause significant drivability problems and even complete breakdowns.

In Federal Court, in June 2018, Toyota won a rodent-damage class-action suit alleging that the wiring insulation it used was “defective” because it attracted rodent to chew on it. In its defense presentation, Toyota asserted that varmints can and do attack many parts of vehicles; tasty soy insulation is not the only thing that attracts rodents -- as I can testify from my own experience. In any case, the Court rejected the argument that Toyota’s cars were defective as sold in the following words:

"Plaintiffs are, in effect, asking the Court to stretch the implied warranty of merchantability to include some promise that no external actor will later harm Plaintiffs’ vehicles. The Court declines to extend the doctrine so far."

Nevertheless, Toyota and other car manufacturers have a public relations problem on their hands. As knowledge of potential rodent damage to wiring insulation becomes widespread, customers will be searching for cars that do not fall victim to varmints. Not to mention that car customers whose electrical wires are not chewed but still deteriorate due to age will be dissatisfied and looking for better quality in their next purchase.

A number of solutions to the rodent problem have been suggested and tried. Honda offers a variety of electrical tape which is impregnated with capsaicin, the hot ingredient in peppers. That could keep the mice from chewing. Another, more earthy solution is to create a smell “wall” of coyote urine around your vehicle. That should keep the varmints away.

But keep in mind that if you have biodegradable insulation, it will deteriorate anyway, regardless of attack by varmints.

Once the damage is done, either due to animal damage or deterioration, there is almost no other solution: Replace the affected harness or, if you are very handy and persistent, repair the damaged wires one by one. It seems to me simplest (and most reliable) to run complete replacement harnesses for all affected systems. You have to research carefully to make sure that you are purchasing non-biodegradable harnesses. But this way, once the job is done, you’ll know that at least the particular wiring system you repaired will not deteriorate again