How often do you encounter issues like this? The appearance of a part is critically important—any carelessness can directly affect the car’s overall image. This means that even when ordering parts from an authorized Mercedes-Benz dealer, one must proceed with caution.
Perhaps there is no better example to discuss than taillights.
Some people liken headlights to the eyes of an automobile… but what about taillights? If they are not the eyes of the “second face,” then at the very least they are the eyes on the butt. For W140, the butt can sometimes have an even greater impact on its image than the face—because those “butt eyes” can be quite unpleasant when they age poorly.
Today, even the youngest W140 is approaching 30. It is no exaggeration to say that out of 1,000 W140s, not even one will have taillights in truly decent condition. You will see six-figure “AMG unicorns” wearing deteriorated taillights—let alone cars worth only a few thousand dollars.
Straight forward: unlike headlights, the W140 taillights have a kind of “elastic lifespan”—or perhaps more accurately, a fragile one. This is determined by their material and construction. Headlight lenses are made of glass, whereas taillights are made of ABS. Headlight lenses are not only durable, but also easy to clean and service. One can simply remove the lens to clean the interior, and as long as minor stone marks are acceptable, they can be restored to a like-new appearance. Alternatively, new lenses or even reflectors can be ordered and replaced.
Taillight lenses, on the other hand, are made of ABS, which is prone to discoloration and leakage in our case, and cannot be disassembled or properly repaired. The reflector and lens cannot be safely separated—taillights are essentially like eyeglasses: disposable items. More importantly, achieving a presentable appearance for taillights is far more costly and far riskier than for headlights.
Today’s discussion focuses on the facelift taillights. Compared to the earlier design, these integrate the reverse light into the taillight lens while retaining the amber turn signal section. They were used from the facelift in March 1994 until the end of production in August 1998. Applicable SA codes include 856, 805, 867, 806, 807, 808, and 809.
In total, approx 240,000 sedans were equipped with this type of taillight. From MY1997 to the end of production, the only change was that the turn signal section became clear; therefore, everything discussed here also applies to those cars. In short, this concerns more than half of total W140 production.
Never perfect
1) Leakage
Even when considering only genuine parts, the first notorious flaw of the taillight lens is leakage. Depending on luck, a genuine taillight may remain perfectly sealed for 30 years—or begin to leak shortly after installation, or even immediately.
Anything that can find its way in will enter through weaknesses in the seal and become trapped between the reflector and the lens: water, dust, even insects. This is partly due to the lens design itself, which is relatively open—there is considerable space between the bulb base and the lens, effectively inviting foreign matter in.
Moreover, the sealing itself is far from ideal. The lens is made of two halves—front and rear—bonded together with sealant. Yet there are always weak points, or even areas that were never properly sealed in the first place.
Once foreign matter—especially water—enters the assembly, it tends to remain trapped inside. If not addressed promptly, the reflector’ s chrome plating will oxidize, leaving the taillight with a rather tragic appearance.
2) Discoloration
The second notorious issue is discoloration—more precisely, a shift toward pink or red. From the facelift onward, the taillight lens gained a clear section for the reverse light; from MY1997, the turn signal section also became clear. These transparent areas are quite large, and after prolonged exposure to the environment—especially sunlight—they tend to develop a pinkish tint. The red sections are also affected, of course, but the change there is far less noticeable. The pinking effect is usually more noticeable to the naked eye than it appears in photographs. As degradation continues, the color deepens progressively until it turns fully red.
Some studies have proposed explanations for this phenomenon: under UV exposure, antioxidants within the plastic can generate phenolic compounds—and phenols are reddish in color. These additives are intended to prevent fading, yet in this case they become the very elements that discolor first. I have experimented with several theoretical remedies, including reducing agents and controlled light exposure, but none proved effective. Since similarly transparent facelift W140 front turn signal lenses do not exhibit this issue, it is reasonable to conclude that this is a matter of manufacturing differences.
Both of these problems are irreversible, and they often occur together, giving the taillights a rather forlorn appearance. Of course, defining this as a “fault” is ultimately subjective. People claim their cars are perfect and free of defects, while their taillights have clearly drifted from the factory color. Judging by the average condition of surviving cars, a considerable number of owners can tolerate this. This discussion, therefore, is intended for those who consider it a problem.
Because when one attempts—out of good intention—to replace aged taillights, even more complications arise. Over the roughly 30-year lifecycle of this product, countless subtle changes and variations have occurred. In short, it is extremely difficult to find perfectly matching parts. That is precisely what the following discussion will address.
Get Worse
Acting out of good intentions can sometimes make things worse, leading more people to become numb or indifferent to the problem. Given the current average residual value of a W140, replacing a set of three genuine taillights represents a significant expense—installation costs are negligible, but the parts alone amount to at least over €1,000. The issue is that these well-meaning efforts, driven by sympathy for the car, may end up making its problems appear more numerous rather than fewer.
Take this donor as an example. This 1997 S 420 had been run by a generous owner before being abandoned in an indoor parking lot. Although the rear had never been involved in a collision, two of the taillights had been replaced. The left appears the pinkest—it is the original part produced in 1997. The center reflector band is a replacement genuine part; it shows a greyish tone, but the right corner has developed leakage. The right was replaced together with center; it remains in good condition, though under certain lighting it appears slightly pinker than the center band. See below.
The previous owner likely replaced the taillights due to leakage rather than collision. Let us assume that before the replacement, there were two leaking taillights and one non-leaking unit—all of them showing uniform pinkish discoloration. If that pink tint were considered acceptable, then effectively there was only one issue. After the well-intentioned replacement, however, the three taillights now display three different shades, and the center piece has begun leaking again—turning what was once a single concern into at least three separate problems.
The taillight manufacturer for the W140 is, from beginning to end, a single supplier: ULO. Founded in 1947, ULO is a German manufacturer that has been supplying parts to Mercedes-Benz since from 1957. For the W140, in addition to the taillights, the exterior mirror assemblies were also supplied by ULO. Perhaps it is not as widely known as Hella, yet it produced even more Mercedes-Benz taillights throughout the 1990s.
For indicators, there were variations among suppliers such as Bosch, Automotive Lighting, and Magneti Marelli. But when it comes to taillights, it was always ULO. As a result, the company’s products exhibit a high degree of consistency. So the question remains—how different can these ULO units really be from one another?
Object of Study
Based on the potential issues that may arise after installation—such as leakage and color mismatch—several key points of attention have been established when examining the variations in these parts. They are as follows:
1) Sealing
On W140 facelift taillights, the reflector and the lens form a single assembly, joined by heat welding. This heat-welded joint is what provides the seal between the two components. The quality of sealing can be assessed visually by inspecting the seam along the joint. Poor sealing will fail to prevent water or foreign matter from entering the taillight.
2) Color
The primary source of color variation in W140 facelift taillights lies in the transparent sections, which can exhibit different color tones. These differences can be assessed visually. The causes of such variation can be divided into two categories: production and aging. Some variation is already present when the taillights leave the ULO factory, while further changes may or may not occur over time in use.
3) Overall Quality
Parts produced in different periods have been confirmed to exhibit variations in quality. There are multiple points of observation, such as the smoothness of the molded components, the finish and luster of the chrome surfaces, and the materials used for the fasteners. Beyond the most obvious difference—the color of the lens—these variations can also be observed after installation, potentially affecting the overall appearance of the vehicle.
What follows is a presentation of the differences observed among W140 facelift taillights produced between 1994 and 2016—from those fitted to the very first facelift production cars to the latest known production batch. The production date of the part is determined by the marking on the back of the reflector; this “clock” indicates the month with precision. We assume that the lens, which is fused to the reflector, was produced during the same period.
In theory, this clock should rotate continuously, but in later observations deviations were identified—these will be addressed separately. No units with a more recent production date have been identified. According to a representative from ULO, this part has been out of their product line for several years, and all W140-related components have already been removed from their online catalog.
Please note that, due to the limited sample size, the following summary combines left and right units, as well as the 1994–96 and 1996–98 ranges. In terms of production methods and product quality, we assume that left and right units produced within the same period are identical in all respects, and likewise that the 1994–96 and 1996–98 versions are equivalent.
Production Timeline
1) For the assembly line
The earliest facelift taillights began production at ULO in late 1993. After undergoing Mercedes-Benz testing and approval, they were first installed on vehicles for internal company use, and then entered full-scale production in March 1994. These fresh taillights met Mercedes-Benz’s latest quality standards and were produced exactly according to their original design specifications. Most taillights were supplied directly to the assembly line, while a small portion was allocated to the spare parts inventory.
04.1994
This taillight was removed from Furi, an early facelift car produced in April 1994. When it was discovered, the car had covered only 9,966 km and had always been stored indoors, leaving the taillights in near-mint condition. This one therefore provide an excellent reference for early facelift taillights.
Based on the markings, this was produced one month after the facelift was introduced, which aligns with the car’s production date. This makes it one of the closest examples to the original version. As such, we will treat it as a reference standard and highlight the areas where later versions exhibit changes.
The joint between the lens and the reflector is uniform, and the sealing appears to be good.
However, some gaps that should have been sealed were left unsealed.
The reflector is chrome-plated on both sides—plated internally for function, and externally with a decorative chrome finish as well.
All the screws are zinc-plated.
Marking on the lens.
A mosquito lies inside the reverse bulb reflector.
This fortunate example has not developed the aging it normally would have, simply due to lack of use. It should not be taken as an indication of superior quality in early production—any ULO taillight will discolor or leak over time, as proven across more than 200,000 vehicles.
After use
For comparison, here is a well-used taillight produced in September 1994, which shows badly discoloration. But fortunately, it has not leaked.
Note that on this slightly later version, the rear of the reflector is no longer chrome-plated.
The clock indicates September 1994; however, this taillight comes from a car produced in January 1995.
The updated taillights with a clear turn signal section began production for MY1997 around April 1996. Compared to the previous version, only the color of part of the lens was changed, so in theory there is no fundamental difference. Whether subtle variations occurred during production, however, requires examination of actual examples.
12.1996
This is a NOS genuine part marked as produced in December 1996. As an example of something not used in the packaging, there was no discoloration, leakage, or any form of aging. Making it a suitable reference example.
The chrome plating on the rear of the reflector was discontinued shortly after production began in 1994 and was never reinstated.
The date of production falls shortly after the start of MY1997. Compared to the April 1994 version, it has already undergone some changes.
The joint between the lens and the reflector remains uniform, and the sealing appears to be good.
Even a standard, flawless taillight has at least two weak points: one at the upper bend, and another at the upper corner near the license plate bracket. Both areas can easily admit a wire—and just as easily allow dirt and debris to accumulate.
These two gaps are not manufacturing defects, as the sealant is actually located further inside. However, once water enters these areas, it cannot easily drain out, and over time it may find its way into the interior. It is therefore difficult to argue that this is not a design flaw.
After use
For comparison, here is a used—but not extreme—example. This taillight was removed from a car with 55,000 km. This can perhaps be seen from the lens, which shows almost no fading.
The back also looks to be in very good condition.
The clock indicates February 1997. The car itself was produced close to 1998, so the later the car, the earlier the production date of the taillights it carries. This suggests that a certain degree of over-ordering of taillights existed.
Unfortunately, despite the low mileage, this taillight has already developed a leak. Water entered the reflector through the second weak point mentioned earlier and became trapped inside, dissolving the chrome plating and exposing the underlying grey plastic.
This gap has also allowed a significant amount of dirt to enter, which helps retain moisture.
Between 1994 and 1998, there were no significant changes in the quality of the taillights. Although parts supplied to the assembly line are regarded as the highest quality, they are not flawless in design—and unfortunately, these shortcomings were never corrected before the end of production.
2) For the genuine replacement
After the last non-armored W140 left the assembly line in August 1998, the Sindelfingen plant no longer ordered taillights from ULO, the remaining stocks were transferred into spare parts inventory. ULO continued to produce W140 taillights, but from that point onward, they were no longer intended for new vehicles delivered to customers expecting Mercedes-Benz factory quality.
After the W140 was discontinued, the production volume of its taillights at ULO dropped sharply. Whereas the factory once required around 300 sets per day, production thereafter became demand-driven, based on aftermarket needs. This led to a “punctuated” pattern on the calendar rather than continuous output: a batch would be produced when inventory ran low, and the next batch only when stock was depleted again.
One notable production cluster that has been identified so far is around 2004. The taillights underwent noticeable changes during this period. Two new features have been observed in this batch of taillights:
Lower lens color temperature
Earlier versions of the taillights typically developed a pink tint relatively quickly in use—this could occur within five years. Around 2004, ULO appears to have revised the production process to address this issue, resulting in new lenses with a visibly lower color temperature compared to earlier versions.
No pink discoloration
Even after nearly or over 20 years of use, these newer taillights retain their original color. This is a positive outcome of the changes made in the production process. Therefore, these parts are worth reusing.
05.2007
This unused taillight produced in 2007 was selected because we also happen to have a used 2007 unit for comparison. Although the turn signal colors differ, parts produced in the same period should be of identical quality.
2007 is also the first year in which a change in the screws was observed. Note that only one screw on lower right remains zinc-plated, while the others are treated with Dacromet.
The clock indicates May 2007.
The sealing during this period still appears to be good.
One detail that has become rougher is the presence of excess sealant residue at the joint between the lens and the reflector.
After use
This used taillight produced in 2007 was selected because we have the unused 2007 unit above. Although the turn signal colors differ, parts produced in the same period should be of identical quality.
Interestingly, this unit also has only one zinc-plated screw—and in the same position—while the rest are Dacromet-coated.
Is it simply because these two were produced in the same month?
Compared to the unused 2007 unit, the used one shows no visible color difference or leakage. The absence of discoloration can be stated with confidence, while the lack of leakage may simply be a matter of luck.
Because…
The leakage issue was not improved. On the contrary, it became even more common in parts produced during this period. After Bianca was restored, a set of three taillights manufactured in 2007 was installed. In order to preserve originality, no additional sealing was applied prior to installation. Yet after the very first wash, moisture had already appeared inside the center band.
In summer, even before the newly installed taillights had been driven on the road, a mosquito had already found its way inside.
After promptly drying out the moisture and extracting the insects, the taillights were resealed. We use LOCTITE® SI 595, which is transparent and offers a relatively long curing time, allowing for adjustments and easy removal of excess sealant. It is also convenient to remove if needed later.
We have collected many taillights from 2004–2007. None of them show any discoloration; unfortunately, due to weak sealing, there are always areas where the internal chrome has oxidized.
Due to the color differences compared to parts produced before 2002, it is best not to mix them with earlier version. When properly sealed and used as a matched set from the same period, taillights from this era present a reasonably good appearance—though their tone tends to be slightly cooler than the factory look. However, today it is already difficult to find a complete set from this period.
Some argue that the quality of service parts is inferior to those installed at the factory. However, in the taillights produced in before 2007, we do not observe sufficient differences to qualify as meaningful deviations. It is reasonable to read that, they continued to adhere to the same quality standards.
3) For old relic
By 2013, facelift W140 were approaching 20 years of age. Most W140s were no longer used as daily drivers, and their residual values had reached a low point. As the cost of parts became increasingly disproportionate to the value of the cars, demand for taillights in the aftermarket declined significantly compared to the previous decade. During this period, both genuine parts and ULO’s own distributed versions experienced slow sales, with large amounts of inventory being discounted. Correspondingly, the intervals between new production batches continued to lengthen, and production occurrences became increasingly infrequent.
10.2013
This is an unused example from 2013, in which a decline in quality was first observed. In all previous years, the taillights maintained a relatively consistent standard of construction, but by 2013 the product had clearly changed.
All of the screws are Dacromet-coated. Have you noticed any other differences in the back?
The clock indicates October 2013—by this point, the part had been in production for nearly 20 years.
There is some chrome overspray on the rear, and these areas show a cracked, pattern-like texture.
The previously grey plastic areas are now also chrome-plated, clearly indicating that the production process has changed. In fact, the luster of the chrome has also changed compared to earlier examples, and this difference is visible to the naked eye.
Compared to earlier samples, the assembly quality of the 2013 units has clearly declined, with noticeably larger gaps.
The edges of the sealant show multiple areas of peeling.
Residual sealant can be seen everywhere.
07.2016
This unused 2016 example, serving as a further subject of study, is the latest production version we have obtained. Although it feels quite recent, nearly ten years have already passed…
At first glance, the quality of this latest product is frankly disappointing. This is by no means an isolated case—there are multiple 2016 units in our inventory, and they all appear to be like this.
The clock indicates July 2016, and the surrounding pattern is clearly visible.
A preliminary analysis suggests that this phenomenon is stress whitening, or environmental stress cracking. The likely causes are either an excessive proportion of recycled material in the ABS, or damage introduced during the pre-treatment process prior to chrome plating. In short, it points toward two underlying factors: cost reduction and a decline in process control.
One intolerable defect is the presence of chrome runs—something never observed in earlier examples. This is caused by excessive application during the plating process. The direction of the runs even reveals how the part was positioned on the workbench during production.
The gap between the reflector and the lens has widened considerably compared to earlier examples.
At the sealing weak point mentioned above, this 2016 example allows a wire to be inserted directly into the interior. With a sufficiently long wire, it can even reach the opposite end of the taillight.
At the second weak point, a wire can now pass directly through the gap and reach the innermost corner—the very area where leakage most commonly occurs.
Residual, fibrous sealant is still widely present.
The 2016 product is, of course, unfortunate. Beyond the numerous visible defects, it also differs in appearance from earlier versions. First, the chrome is overly bright, resulting in a level of reflectivity that differs from previous iterations. Second, the transparent housing is also excessively glossy—its overly smooth surface feels more like that of a modern taillight rather than a plastic component from the 1990s.
We have not collected any used taillights from 2013–2016, but their water resistance may no longer be worth testing. For this reason, it is strongly recommended to seal all gaps before installing taillights from this period. If alternatives are available, it is best to avoid units produced during these years altogether.
Current availability
ULO is the sole supplier of genuine taillights for the W140 (sedan), covering all variants. Depending on the market, it supplied SAE versions for North American vehicles and ECE versions for the rest of the world. The difference between the two lies only in a single bulb; the lenses themselves are interchangeable. Since the bulb base is not manufactured by ULO, it is not included.The facelift taillight variants are shown as follows:
The W140 product line last appeared in ULO’s catalog in 2019. According to correspondence with a ULO sales manager in 2021, W140 taillights are “no longer within the product range.” The increasingly tight availability in the market today might confirms this. However, MB aftersales system still holds some inventory. It is possible that ULO now supplies new units only to Mercedes-Benz, rather than distributing them to other partners.
ULO, part of the Odelo Group, was acquired by the Turkish company Bayraktarlar in 2011. However, the 2016 products are still labeled as “Made in Germany.” In general, a decline in quality can result not only from cost control measures, but also from tool wear and the gradual loss of process know-how. Based on the observed results, the inability to faithfully reproduce the original manufacturing process appears to be a critical factor. In the final products, both the material selection and the processing show clear deficiencies.
At present, all ULO products are difficult to source and command high prices. There are also color differences between units from different production periods. As such, reusing surviving taillights from the 2004–2007 period can be a viable option. Purchasing full-price Mercedes-Benz genuine parts is not only more expensive, but may also introduce additional complications.