The 90 Quattro came stock in the U.S. with fairly poor DOT aerodynamic headlamps. Due to the fairly compromised 9004 stock bulb, and perhaps Audi not caring as much about the lighting for the U.S. market, the beam is poorly shaped, rather diffuse, and hard to improve. The sandblasting suffered by 200,000 miles on the front of a New England car does not help much, either...
The solution is to obtain a set of 80 (or 90) European headlight assemblies, which use the vastly superior H4 (and H1) bulbs to put plenty of high quality light where you need it.
It is also quite likely (I have not done any measurements) that the wiring harness is partly to blame. To get to the bulbs, the current must start at the battery, run to the fusebox, to the headlight switch, high beam switch, back to the fusebox, through some fuses, and out to the front of the car. This twenty feet or so of wire and numerous connections will certainly cause some voltage drop, and thus a dimming of the lights.
Rewiring the headlight circuit to use relays switching good voltage through thick wire is so important that it should be implemented on any well kept old Audi to improve even the stock lighting. This always should always come before you start hanging expensive european or auxiliary lights on your car.
The headlight assemblies I bought are the ones used in Europe for the 90, with H4 high/low beams and an additional H1 high beam. Due to the space taken up by the two reflectors for these bulbs, there is no room for the amber turn/park light found stock on the US 90. (The european models have their turn and park lights mounted in the corner of the bumper.)
To address this I am installing a pair of type 85 bumper lights just inboard of my foglights. These will be driven directly by the turn signal flasher to prevent it from flashing at the higher rate that indicates a bulb out. By using the sockets that came with my car in them I will also drive a parking light filament in that fixture. For safety (letting drivers to my side know my intentions) I am installing a set of current production repeater lamps in the fenders, relayed so they provide a marker light and turn signal function.
The relays are mounted in an auxiliary relay bracket from an 84-87 coupe or 4000. It has locations for six relay sockets in a row, and notches to allow more relays or similarly based devices to be attached to its sides. The seventh relay, to run the front parking lights, is side mounted, as are a pair of dual circuit breaker sockets (used on all the old Audis for the power window and sunroof circuits). By removing all the wiring from about a dozen parts cars, I was able to select appropriate colors and gauges with the right connectors on their ends to build this monstrousity the way I wanted to - to match the car.
I got my 12 volts directly fromt he alternator output post, and used various center support assembly screws for grounding points.
The new headlight assemblies have a single four pin connector on their back. I did not have two matching sealed connectors to use so I bought new ones from the dealer.
My new bumper mounted lights forced the issue of AC deletion in my car - the right side unit required some of the space hogged by the AC compressor for its socket and plug. Considering there was zero pressure in the system and the cost of servicing AC systems these days, I was not giving much up.
The fender mounted repeaters were mounted in holes in the fenders with some of the ever-handy 3M strip-caulk i keep around to hold my cars together. I broke off their lock tabs and cut off the little alignment ribs so I could use a simple round hole. The sophisticated installer might like to duplicate the factory hole shape and indent the sheet metal around it for a truly slick look (or alternately patch in a small bit of sheet metal from a donor vehicle). The hole location was aided by access to a wreck I could examine for interference with other parts under the fenders. I carefully painted the hole to discourage rust.
The headlights I bought have an additionaI socket in the reflector for a small bulb, known as a city light. I will wire this to run with the parking lights, since their primary purpose is to allow the shutting off of the normal headlights to reduce unnecessary glare in well lit urban areas. They helpdrivers see the car and "locate" its shape properly without confusion since there will still be some light at the headlight lens.
I will also rewire my foglights (at the switch) so they will run with the parking lights. Then if I encounter very poor visibility conditions, using the parking light setting in conjunction with the fog lights will provide good illumination of the road without excess glare reflecting back from the oncoming snow, rain or fog, - and still provide other drivers with location cues from the dim bulb in the headlight lens.
The automatic (low beam) bulb check system on the 90 works by comparing the current flowing in each of the left and right low beam wires soon after the headlight switch. After installing relays to run your headlights, it will no longer function properly. You have two choices, to disable the autocheck or to install it in your new wiring. Personally I think I can trust my eyes to tell me if a headlight is out, but I will explain how to keep using it anyway.
Disabling the autocheck system:
Remove the drivers side underdash panel to expose the auxiliary relay bank. There is a small relay-like device a tthe right end of the main relay bracket, probably with the number "281" on it. Pull it out, and to verify you have the right unit, look for it to have markings that say 56bR, 56bR1, 56bL, and 56bl1 next to the terminals.
With it out, the autocheck will no longer be activated, but you have broken the signal path to the headlight relays you installed - those terminals I described are the first place the headlight current path goes. it continues on to the lights (or your relays) via the stock harness.
To re-energize the stock wiring that triggers your relays, you will need to jump one or both of the tracks previously running through this control unit. This is actually very simple. Why one or both? Because you only have to get the voltage to your low beam relay(s). I used one relay, switched by the right side (yellow) wire only, so I only had to jump the "R" terminal sockets to each other. the yellow/black wire is served by the "L" sockets. If you used both stock wires to trigger two relays, you need to jump both.
The "R" terminals are the top and left larger tabs, the "L" terminals are the right and bottom ones. To jump them use a short length of wire with male terminals on each end and insert them into the appropriate sockets. Test the results before closing up the dashboard.
Another way to disable the system is to bend over or cut off the "K" terminal on the control unit and reinstall it, or to identify the wire leading from that socket and cut it, or remove it from the socket and tape it off. Anyone for cat-skinning?
Re-implementing the autocheck system:
In order to keep using this function, you will need to move the control unit into a position such that its "R" and "L" terminals are in the current path from your relays to your low beam filaments. I do not provide a schematic for this because I do not love you enough.
Having doen this, the "K" terminal will have to be connected to the socket in the auxiliary relay panel (top right smaller tab), or the wire going to that socket will have to be extended to the contro units new location. You will probably also have to ground the "31" terminal, if it has one.
Insertion of this device will cause a small voltage drop at your bulbs - one reason not to engage in the foolishness of trying to keep using it. Also, if you are using higher wattage bulbs, I wouod be a little worried about meltdowns, sinc ethe control units does say "55w" on it.
The graphic documentation of this project is divided up among these files:
You can easily navigate between them or back to sections of this file.