Tech: 88-89 GMT400 Quad Sealed Beam upgrades (Bosch T84 E-code headlights)

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88GMCtruck

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I’ll start this thread off by simply stating that the output of the 88-89 sealed beam headlights are a joke. This thread will outline the OEM lighting in the quad headlight trucks, and available upgrade options. Many people simply say switch to composite headlights; while this is a valid option, I personally have an attachment with the quad headlights as they are the original style of the GMT400 truck and were only used for 2 years. They are much less common and have a more distinct look, IMO. If someone was to want to "upgrade" to quad headlights, swapping a newer truck (90-98) to the quad headlights is simple. All you need are the light adjusters and the proper electrical plugs. All the grille shells setup for composite headlights have the appropriate step in them for the quad headlights, and they mount in the exact same location on the core support.

Stock Sealed Beam H4703/H4701
The H4703 lowbeam and H4701 highbeam are DOT approved sealed beams that GM used in 88-89 trucks, 90-91 blazers and suburbans, 98-03 firebirds, and a few other, less popular car applications. The lights are a 55w lowbeam and 65w highbeam. Up until a few years ago the aftermarket for them was marginal and a few people were modifying the Sylvania brand replacement units (as they had a bulb that was easily removed) by gluing their own bulb in place of them. This still did not overcome the shortfalls in the light itself. The small size (92mmx150mm) and marginal optics of the DOT spec are what makes these lights very poor. I originally overcame the marginal output by adding factory fog lights to my truck. Also, another great upgrade is to run larger gauge wire and relays for the headlights. The factory wiring is ran through the headlight switch and column dimmer, and smaller 16ga wire. My first lighting upgrade was to power the lights with relays and 10ga wire. This increased the output of the stock lights greatly, and is a must if you upgrade to any of the following lights as the higher wattage will run too great of resistance.

Stock H4703 lowbeams (the blue is paint on the non-reflective portion to add a different look)
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Eurolight conversions
About 6 years ago, a company called Eurolights came out with a non-sealed beam version of the headlights, which utilized a replaceable H7 bulb. I quickly picked up a set of these. While they suffer from a very similar (although not DOT approved) cut lenses as the sealed counterparts, they are an ok upgrade from stock. They allow you to use a higher wattage bulb, which overcomes the cruddy optical quality. The glass lenses on them are considerably thinner than the stock sealed beams, and within a year or so both been broken due to rocks. I eventually replaced the lenses with those from a set of sealed beams, keeping the h7 bucket and brighter bulbs.

Eurolight H7 conversions compared to stock h4703 sealed lowbeam
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LMC “euro” conversions
About two years ago, LMC truck also came out with a replaceable “euro” style clear lense/diamond reflector style light. They are DOT approved however I have never used them, as they do not fit the style of my truck. I’m assuming them to be a decent upgrade over stock, probably similar to the eurolight conversions. EDIT: A friend of mine bought an 89 blazer with a set of the lowbeams LMC lights. I can actually say I'm impressed with them. They use 9006 bulbs, are of quality construction and actually have a very proper light output. I would highly recommend them if you cannot find the Bosch lights.

LMC conversion lights
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Bosch H4 headlights
And finally, the ultimate upgrade to quad headlights; Bosch H4 E-code headlights for GM export option T84.

I first learned of the Bosch E-code version of the headlights from another forum member, DaBurb. He has an Australian spec Holden Suburban. They came with quad headlights as well, but used a bosch capsule and H4 light. He's actually replaced them with the clear lensed conversions available from LMC. I've done a literal ton of research after these lights, but alas, for years I could only find the RPO T85 (Export-Left Rule of Road) via the Holden Suburbans. Howard gave me the part numbers off them and they are still in the GM parts system. They are, however, designed for the incorrect side of the road over here.

Eventually (about 4 months ago) I found that Bosch also made these in right hand rule of the road (GM RPO T84). They were used in German export Firebirds (of all things, they use the same headlights as 88-89 trucks here in the states). I found GM and Bosch part numbers. The GM part number no longer works, nor are there any in the system. I contacted Bosch and the lights have been discontinued for over 8 years now, and they also don't have any. Turns out there is a little place in PA called Susquehanna MotorSports that bought about 200 of them that were in a GM warehouse in Ontario. They have about 32 left, and apparently when they are gone, they are gone.

Part numbers:

Lowbeams:
T84 lowbeam (RHD) – 16506324
T85 lowbeam (LHD) – 16507483

Highbeams (T84 or T85, it has no rule of road cut since it's a highbeam.)
Highbeam (either) – 16506325

So after ordering the lowbeams from Susquehanna MotorSports, I decide it’s time to get a set of highbeams. The highbeam part number was still good in the GM system, so about a month ago I dug into my part numbers to order the highbeam light. Very funny phone conversation with my normal parts guy, List is $68/ea but he needs a vin for them and questions wtf they were off of. Anyway he tells me there are two left in the US, in a small dealership in Kansas. Called them and talked to their parts guy. He said they have obviously been there a LONG time, offers them to me for $51/ea just to get rid of them. So, I just may have bought the last set available. And I was worried about a rock hitting the lowbeams and breaking them, these ones are really going to make me sweat. So, the high-beams are no longer available from GM; I ordered the last 2 in the system.

First off, the glass lenses on the Bosch lights are thick, thicker feeling than the sealed beams and WAY thicker than the eurolight conversions. They include a very large rubber boot that cover the light and keeps moisture out. Under it is a very nice locking setup for the bulbs. The lowbeams also include a “citylight” which is a separate 194 blub that is effectively the European equivalent of a DRL. I choose not to wire this up in my truck as I hate DRLs.

Lowbeams (notice the Bosch and GM part number tags)
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Lowbeam compared to Eurolight H7 light (with highbeam glass)
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Highbeams
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Highbeam compared to Sealed Highbeam H4701
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Side by Side (lowbeam on left, highbeam on right)
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Bulbs, boots and city-light
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The lowbeam light output is simply amazing. Being they are E-code, they have a much cleaner and better focused light pattern that puts the light where you need it. They also have a very sharp (almost projector like) cutoff line. I have noticed much less eye fatigue on long drives or in inclement conditions. Now, being the lowbeams are H4 bulbs (dual filament) I wired them as a true low and high beam. So when they change between filaments the optics change, and illuminate farther up.

The highbeam lights are of course the same quality. They are a similar cut to the US highbeam; however the cuts are much more pronounced and create a much better light spread on the road. Combined with the high beam filament of the lowbeam (and my other lights) the output is nuts. It is also worth noting that the lowbeams light pattern and output is far better than any composite headlight I have seen, OEM or not, in a GMT400 truck.

So I have a full set of T84 lowbeams & highbeams in the truck now, and have a second set of lowbeams sitting in a box just in case something happens. I also covered my lights with a layer of 3M stoneshield just to help protect them. Nothing like losing $300 in headlights. I’ll try to get some pictures of them in the truck, as well as the lighting pattern.
 
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88GMC

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Great write up. The lack of light was one of the main reasons, on top of my buckets being rotted past repair, I switched to composites and I deeply regret it. You mentioned that you bought the last set of light housings. Any leads on anyplace else to look? Also by any chance do you have pics of the entire light assemblies (mounting brackets, adjusters, etc.)? I'd love to switch back to the dual sealed beams. Composites on the 88' grille just arnt the same.
 

88GMCtruck

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Thanks man. It's surprising how horrible the sealed beams and composite lights are from the factory... They just don't cut it honestly.

Well, as far the lights go, the lowbeams are still available through Susquehanna MotorSports. I just checked their inventory, they have 24 left. Like I said before, once they are gone, they are gone. The highbeams however, I have no idea where you'd be able to find a set, as I bought the last two in the GM system. The highbeams aren't a whole lot different cut than the DOT spec highbeam, but they use H4 bulbs. Honestly you'll be more than happy with the lowbeams by themselves, and since they are H4 bulbs you can wire them as a low/high light all in one... on top of your current highbeams.

The light adjusters aren't anything special, they come across ebay for $40 or $50 from time to time. Here are a couple of shots from when I installed my HIDs
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And an output shot of the lowbeams with H4 HIDs. They have an extremely nice pattern.
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If you hadn't caught it yet, after getting these housings I also did an HID conversion in them, if your interested in that info as well http://www.gmt400.com/forum/showthread.php?948-88GMCtruck-s-non-projector-HID-conversion
 

88GMC

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Thanks again man for the pics and write up. As always, the truck looks awesome. Love the way these things look with the stock grilles and lights. I have a second frame, D60, FF14, NP205 awaiting once the body is finally buttoned up. I can only hope it looks as clean as yours lol.
 

88GMCtruck

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Thanks again. You'll be happy with a SAS, honestly it was one of my best decisions.

When you do your SAS, the NP205 is going to give you problems as it doesn't have an electric speedo.... unless it's the uber rare one from a 90-91 R/V Series. You'll be fine running a pass drop NP241 they are rated for 5,000 something ft/lbs, are lighter and have lower gearing.
 

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Yeah, quad seald beams makes the best light. But it's a real hemorrhoids if you need to change the bulb. I've made some upgrades with the parking light with w5w miniature bulbs
And when one of these tiny bulbs has fused, i had to remove the grille and take off the hanger to change it.
Btw the quad headlights was avaliable until 1999 (i've got 99 Tahoe and it always was with that quad headlights). BUT, it wasn't an option. In late 97 Chevrolet decided to bring Tahoe and Suburban on European market. But Europe have it's own standarts and Chevy had to make some changes. First of all they reaplaced stock headlight with Bosch headlights with european standart beam. If someone don't know the difference between american and european headlight is the light beam, so called "jackdaw". And just for information other changes were: yellow side markers on fenders, yellow turn signal in the tail lights (reverse light was replased with the yellow one, changed the wiring and reverse light removed under the rear bumper), rear fog light, mud flaps and more.
 
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88GMCtruck

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Just letting you all know, I have a contact that has a large supply of the the High Beam H4 bosch lights. They include the OSRAM H4 bulb in them. You can use them for both low and high beams, however they wont' have as good of a pattern as the lowbeams would so you can point them farther down.

Asking $100 for highbeams, let me know if anyone is interested.
 

Darkrider

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Just letting you all know, I have a contact that has a large supply of the the High Beam H4 bosch lights. They include the OSRAM H4 bulb in them. You can use them for both low and high beams, however they wont' have as good of a pattern as the lowbeams would so you can point them farther down.

Asking $100 for highbeams, let me know if anyone is interested.

I know this is a hell of a long shot but are there any of these lights still left?
 
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