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928-H4-Cap Porsche 928 8″ H4 Headlight LED H4 Bulb Sealing Cap: A Solution to Sealing the H4 Bulb after Upgrading from Halogen H4 Bulb to LED H4 Bulb

Background: The Problem

If you are in the USA, you may have already upgraded your Porsche 928's 8″ H5 headlights to the widely-believed-to-be-superior 8″ H4 headlights. Or you may — especially if you are outside of the USA — have a Porsche 928 that was originally equipped with 8″ H4 headlights. Either way, the next logical step in headlight upgrades is likely to be replacing the original halogen H4 bulbs with LED-based H4 bulbs. This LED H4 upgrade is becoming increasingly popular for a number of reasons, among them:

Despite the advantages of LEDs over halogen bulbs in this application, though, there is one problem that's not so easy to overcome: you will discover that the cap that seals the original halogen H4 bulb is not at all suitable for use with the H4 LED bulb. When you remove the cover on your halogen-H4-equipped headlight, you will see something like this:

Removing the cap reveals the halogen H4 bulb.

If you remove that bulb and replace it with an LED H4 bulb like this (unplugged, with the heat sink temporarily removed) …

… then you will find that you cannot reseal the back of the bulb with the cap; it's just not at all compatible.

It's not uncommon at this point to think, "Well, I just won't seal it." Put on the heat sink, plug the LED H4 bulb into the LED H4 driver module, plug the LED H4 driver module into the H4 socket on the car-side wire harness, and finally, try to fasten the LED H4 driver module to something with something crude like nylon ties and let it go. Not sealed, and rather messy looking. After a few years, it's likely that the headlight glass and reflector will start turning cloudy, due to the contaminants in the air. Compare the clean headlight on the left to the cloudy one on the right; the one on the right is the result of running it unsealed for few years. It looks even worse when illuminated at night, and certainly has a detrimental effect on projected beam brightness as well.

Our Solution: The 928-H4-Cap Family

The 928-H4-Cap family was designed from the ground up to solve exactly the problem described in the previous section. Due to the variety of different LED H4 bulbs, we expect over time to introduce variants of the cap that are carefully designed to seal the bulbs, each of which might have different gemoetries requiring different hole sizes, and might also have other clearance issues.

The first such variant of the 928-H4-Cap cap is the 928-H4-Cap-22. The fronts and backs of an original factory cap (Porsche part number 928 631 923 00) and a 928-H4-Cap-22 are shown below.

The cap seals the LED H4 bulb on the smooth part of the bulb's barrel; this is visible in the photo below, where the LED H4 bulb is positioned in the cap at the same depth it would be after installation into the headlight assembly.

928-H4-Cap Family Features

928-H4-Cap-22 Specifications

Assembly and Installation

The 928-H4-Cap-22-Kit-A provides a pair of caps and the hardware you need to mount the LUMILEDS H4 LED driver shown earlier on this page.

You first attach your H4 LED driver module to the cap using the brackets and other hardware supplied with the kit.

Then you:

  1. install the cap over the LED H4 bulb (unplugged with its heatsink removed):
    1. tilt cap so bottom of cap is closest to headlight assembly as shown and feed LED H4 bulb's wire through the center hole;
    2. carefully center cap's center hole over LED H4 bulb's barrel and rotate cap slightly about its hole while applying gentle pressure toward headlight assembly, taking care not to dislodge O-ring;
  2. clamp it down with the headlight assembly's cap clamp wires;
  3. install the LED H4 bulb's wave washer and heatsink;
  4. plug the LED H4 bulb's driver in the H4 socket on the car-side wiring harness and plug the LED H4 bulb into the LED H4 driver.

Your completed installation will look like this:

To purchase the 928-H4-Cap, visit our on-line store.
Last updated by EJS,