Headlamps Fogging/Yellowing

Page Revised: 10 May 2019


Article by
Eric Dess

1987-88 Cougar Yellowed Headlamp

If you own a 1987-88 Cougar, you've probably experienced some kind of funky deal with your headlamps getting all fogged up or yellowed in appearance. In 1987, most Ford vehicles joined the Lincoln Mark VII with new style aerodynamic style flush headlamps. Now headlamps were permanent parts of the car's front end styling, with only the bulbs needing replaced. The 9004 bulb (high beam/low beam combination) was favored so all headlamp assemblies were designed for a single bulb, or later on, two bulbs side by side. As time wore on, however, people began noticing that their headlamp assemblies started to exhibit yellowing, moisture condensation, or both.

The good news is that, in most cases, the lenses can be salvaged and even repaired. Shawn has sent this information from his experience:

"My headlights were full of water, yellow, hazy and overall nasty, I used a buffing wheel with some Turtle Wax rubbing compound. It's pretty gritty and it took most of the haze and yellow out. The compound stuck on the rougher spots so I used it as an indication as where I need to focus more on cleaning. From there I used Meguire's Cleaning Wax on a new, clean buffing wheel bonnet. I did that until the lights became clear and shiny. From there I used a polish just for temporary protection.

"Since my headlights leaked, I washed them out using Armor-All Car Wash. I put a small amount in the headlight bucks, added some water, then shook them like crazy. Then I stuck the nozzle of the garden hose in the light, turned the headlight upside down so the lens was facing upwards and went full pressure until the suds stopped coming out of the lights. I let them sit and dry in my house overnight and used some tub and tile sealant around the bead of the lights and rubbed it into the seam with a lot of pressure to ensure it filled in the seam. Once that cured I ran a thicker bead of the sealer on top of the previous one and made sure it was even, and let that cure. After that, I wrapped electrical tape around them and reinstalled them."

So why does this happen anyway? Jim has provided the information below:

Yellowing occurs with a combination of extended nighttime driving, acid rain, and possibly condensation. It mostly occurs on the inside of the lens but can also be found on the outside. The hard plastic used for the lens is susceptible to high heat, and yellowing is a side effect of that.

Since they introduced the plastic lens in 1984 on the Lincoln Mark VII, Ford has used polycarbonate—a high heat resistant, high impact strength plastic that also has low resistance to UV light and moisture. Since headlamp lens light sources generate a lot of heat and require impact resistance, polycarbonate has been the material of choice for all but a couple of GM programs (like the Chevy Berreta/Corsica program, which was an overlay lens—polycarbonate inner/acrylic outer).

As indicated, polycarbonate is very sensitive to UV and moisture. In addition, it is a very soft material when compared to acrylic and even more so when compared to glass. Hence, all polycarbonate headlamp lenses required the application of a UV blocking, silicone or acrylic hard-coat on the surface to provide for some acceptable length of performance. These coatings work much like a sponge and will only absorb so much UV before it starts to get to the base polycarbonate. With time, the surface of the polycarbonate starts to degrade and several things start to happen:

  • The substrate starts to micro-fracture/crack which leads to cracks in the protective coating.
  • The substrate starts to degrade and changing surface characteristics which leads to haze (fogging) and coating delamination.
  • Loss of topcoat leads to significant abrasion.

It should be noted that in spite of these factors, the impact resistance of the degraded lens is still much better than a glass lens. However, you can still get good light from an unbroken glass lens.

Yellowing on the inside may be from various materials outgassing from the inside of the lens. It is also possible that the coating being used on the reflector portion of the lens housing is yellowing. Another possibility is that the light source is emitting UV which is degrading the polycarbonate lens. However, if one can remove this yellowing using solvent, it is probably not the UV attacking the polycarbonate. It may be possible to remove the assembly and pour in some type of detergent to try and dissipate the yellowing. Since you cannot physically get your hand inside the assembly, or even pry it apart, that would probably be about the only thing you can do to try and lessen the situation.

If the yellowing is on the outside, it may be caused by one of two things:

  • A residual hard-coating that has maintained adhesion to the base polycarbonate. These areas may require concentrated sanding to remove the coating.
  • The polycarbonate has weathered so severely that it will require removing a significant amount of material to remove the weathered area. It could also be that because polycarbonate is so porous and/or crazed, the cleaner is working its way into the fine cracks/pores of the polycarbonate.

It's possible that a low-speed buffing (less than 1500 rpms) will bring the clarity and luster back to the lens. This is also a good idea if they've become dull. The trick is to use a substance that will not scratch the surface. This should only be attempted by someone who has some experience with polishing polycarbonate. Keep in mind the once the polycarbonate has started to degrade, it is self-propogating. And without the top coat, UV and moisture will readily attack the lens. However, for cars that will see limited exposure, it may provide some extended life.

Polycarbonate, in itself, is relatively sensitive to solvents. As a result, one needs to be careful when exposing a polycarbonate lens to any solvent. Attack may not be readily apparent until the solvent dries and the lens undergoes thermal expansion and contraction under heat and cold exposure.

Moisture condensation on the interior of the lens seems to be common as well. When these assemblies were made, the clear outer lenses were joined to the ABS plastic housing through a heat-bonding process. Over time and usage, and the constant heating and cooling of the materials from daytime and nighttime driving, the two unlike plastics tend to pull away from each other and let water inside the lens, forming a clouding inside the assembly. To clear up the inside of the lens, it must be removed and place inside a warm, dry environment for a minimum of 24 hours (inside the house is fine). Unless the problem of water leakage is resolved, though, this will happen again and again. We've not run across the perfect solution for this problem yet, but through some tips from other Cougar owners, we can pass along some solutions to you.

For starters, drilling a small hole (1/8") in the bottom corner of the clear part of the lens, underneath the assembly, provides a drain hole for any moisture and keeps it from clouding up the lens in the future. Also, you can cover the entire perimeter of the seal with a thick layer of clear silicone. That should last you at least a year of more if properly done.

If All Else Fails
Still can't solve your problem? Then you'll want to look into replacement headlight assemblies. There are several places on the Internet that sell them, and there's always eBay. Or you can take a trip to the local salvage yard for used lights. Here is the information you need to know:

  • 1987-88 Mercury Cougar headlamps and 1988-94 Lincoln Continental headlamps interchange.

We've not found anything to interchange with 1987-88 Cougar side markers or inner markers; those will have to be for a Cougar only. You'll have to scour salvage yards for those, or perhaps eBay. Be aware that most yards do not like to separate complete front ends. When in doubt, be sure to ask someone there. They may even have some already pulled and sitting on a back shelf.