Rear Lighting Basics


Theres always more to swaps than initially meets the eye, and the addition of updated lighting is certainly no exception. Personal preference, amount of work, vehicle originality, the ability to reverse modifications all come into play. Due to the wide variety of ways to get this done I will touch on the basics of the systems. If you have a question that goes unanswered, feel free to e-mail me. (gmguy89@netscape.net)

As with any swap, some folks want to know why bother? If you drive your old iron on todays roadways, any device which wakes up Jerry or Suzie Sleepy-head behind you is worth the effort. Especially upgrading to dual brake lights. The volume of traffic, and the excessive amount of the above mentioned pair of typical drivers warrent this mod regardless of vehicle. No matter what happens, those of us piloting old iron will get the short end of the stick if an accident was to occur. Jerry and Suzzie are happy to go buy a spanking new electronic laden, looks like every other vehicle out there, with their insurance pay-off, we on the other hand will be holding that bent trim peice comtemplating the best way to revive our old companion while muttering various epithets about the people involved and various objects and animals that happen to stray near them.

Back to the what we came here for, as with any swap this will be a custom job, but you'll note that most, if not all of it, is an independant system, and won't butcher existing vehicle wiring for the most part. In a word, reversable. Planning is essential. Consider what sort of lighting upgrade you wish, and how you would want it incorporated into your vehicle.

Things to think about; Lighting fixtures utilized, how and where they will be mounted. Brake switch used (add another or use an existing switch). Best place to run the wiring to the rear of the vehicle. In what way do you wish to upgrade your lighting? Dual Brake? Signalling? Hazzards? A combination of these? Are you doing a column swap? If a column swap is the order of the day you can use the columns signalling/hazzard components. Do you want separate brake/signal lamps?

Regardless of method used, you can mock up the system on the work bench. Nothing like experimenting while sitting on a chair as opposed to tucked upside down under the dash.

Basic Circuits

Dual Tail/Brake Lamps

A dual tail lamp upgrade is probably the simplest of all the rear lighting options, as all you require is a second lamp which can be wired in parallel with the existing rear light.

It would be wise to check the wires running to the existing lamp for broken down insulation (ie. brittle, cracked) or inadequate wire guage. Replace as required.

The Signal Lamps

Add-On Signaling
This consists of adding a switch (ON-OFF-ON) in reach of the driver, a flasher, and additional bulbs/lighting fixtures to the rear of the vehicle. This signal device is not auto-cancelling. Use a switched (ignition on) 12 volt source--this prevents pets or left behind passengers from activating the signals while parked. Two wires will need to be run to the rear of the vehicle. Additionally, a set of indicator lights or LEDs can be added, along with signal lighting to the front of the vehicle. These would be wired into the terminals feeding the signal lights.

A textual walk through of the signalling circuit;
A 12volt source through a fuse, from the fuse (10A) into one terminal of the flasher, the other flasher terminal to the switch (3 posistion switch with center off)(1 pole triple throw), one throw of the switch (left for instance) gets wired to the left signal lamp. the other throw of the switch (right) gets wired to the right signal lamp. Each lamp being grounded through the frame.
Signal schematic.

Hazzard Flasher
Adding a hazzard function to the above system is basically a repeat of the above signaling circuit. However, the switch used to activate the hazzard funtion is a Double Pole, Single Throw (or Double Throw). As well, we should use an unswitched 12 volt source (So we don't need to leave the key in the ignition for the hazzards to operate).
[We could use a multipole switch in this situation to select hazzard and signal functions, along with feeding in the desired voltage source. Expansion on this will be a future addition.]

The need for a double pole switch stems from the fact that when the hazzard function is OFF, each lamp needs to be independant from the other for signalling purposes.
Hazzard schematic.

Signalling w/Dual Filament Tail/Brake Lighting

What looks deceptively easy to accomplish requires an elaborate switching mechanism when compared to the basic signal/brake circuits. Two methods can accomplish this. One method will put to use the signal switch assembly from a steering column and splice its wires accordingly. Mount it in such a way as to hide the bulk of it in/under the dash with a homebrew signal lever and hazzard switch. The second, a better method, uses a relay system. The advantage it gives you is less passenger compartment intrusion. A relay can be controlled easily with a small switch, while the relays can be located anywhere. The signal activating switch can be located just about anywhere along the dash with minimum real estate taken up behind the dash. Ditto for the hazzard switch, although it does take up a bit more behind the dash space.
One additional note, you can't accomplish the above with just a switch.

The basics of the combination brake/signal/hazzard system is to switch in the flasher to the appropriate lamp, and provide a continuous brake light for the other lamp. If using hazzards, brake activation overrides the hazzard function to provide continous brake illumination. A less than ideal system for some situations.

Three wires run to the rear of the vehicle for the lighting; left signal/brake, right signal/brake, and l/r tail.

Basic GM Column Signal/Hazzard Switch
If you elect to use a salvaged steering column assembly (or your replacing the original column with a later unit) the GM harness has the following connections; Signal/hazzard/brake schematic based on General Motors assembly..

Utilizing Relays
The add-on relay schematic follows the GM schematic, It uses two relays for coordinating the flashers and brake switch. Refer to the relay schematic for wiring details.
The parts used;

If you are adding front signal lamps two additional relays (SPDT) will be required. If one simply adds the front signal lamps to the circuit above, your front signal lamps will also convey braking information.
-The front lamps are fed to a relay.
-Each relay coil is connected to its respective side of the signal switch (S2).
-The normally open (NO) contact of each relay is connected to the signal flasher line.

Marker/Tail Lamps

So far I've been throwing in marker and tail lamps without any further mention of them in the circuit or any representation in the schematics. These lamps are also refered to as "parking lights" by some, and serve as general outside illumination. Tail lamps refering to the rear of the vehicle, marker being the small side lamps as well as the front general illumination. These lights do not tie into the signalling or brake system in anyway, they only share the same lamp by way of dual filaments.

The 1157 is by far the most common dual filament lamp used, it contains a 3cp and a 32 cp filament. The 3cp filament serves as the marker/tail lamp, the 32cp filament as signaling and braking.

If your light switch has no provisions for marker lighting, the dashboard instrument light circuit can be tapped to drive a relay (SPDT) which in turn can switch a high(er) current source (battery) to drive the marker/tail lamps.

The Argument for Separate Signal & Brake Lights

The most obvious reason to separate the brake and signalling lamp is to provide a more accurate picture to the driver behind what your vehicles status is. When the lamps are separate, braking is obvious through two constantly glowing brake lamps, and signalling is obvious from the lamp which is flashing next to the operating brake lamp. When these two functions are combined into one lamp, a bulb malfunction will not convey all the required info to the following vehicle, now, you have a burnt out lamp, and a flashing lamp on the other side. Is this vehicle just signalling? Is this vehicle slowly slowling? Sure, observing the vehicles characteristics as it goes down the road will allow you to figure out whats going on, however, real world situations are usually a little too close and too sudden to rely on simple observation. We've also been conditioned by the bright glowing red light to slow/prepare to slow down. When lamps burn out, much of that expected information is not there for people to process. If anyones followed a fullsize pickup with a burnt out bulb, you know what I'm driving at. Of course, none of that does any good if you don't check your bulbs! How often have you had to wonder about the mentality of the driver infront of you who has no side brake lamps, and one lonly bulb glowing in the third high-mount brake light?

Another drawback to combined signal/brake lamps is during hazzard operation. Ever have to hold a vehicle on a slope with the brakes and want to warn other drivers you're not going anywhere? That does not work with a combined function bulb. Brakes take precedence, and if you need to warn others, AND keep from rolling your in a bit of a quandry.

Slight modifications to the wiring will be necessary. Two additional wires will be required. Wiring will be as follows; 1 wire for left signal, 1 wire for left brake, 1 wire for right signal, 1 wire for right brake, 1 wire for tail (marker). +1 wire if reverse lights are utilized.

Bulbs should be setup as a dual filament tail/brake lamp. Single filament signal lamp (If you are using red lenses for your signal lamps, a dual filament bulb can be used to incorporate a tail lamp in the signal housing).

Additionally, separate brake/signal lamps provide a less meshed together signal/hazzard/brake switch arrangement. Giving an overall easier to wire up system.


-Dual filament bulbs only fit the socket one way (retaining pins are on different planes). Before commiting any wiring to permanence, check the lamp socket to determine which lead is for the tail filament (low brightness) and which is the brake/signal filament (high brightness). Grounding of the lamp is through a third wire or sometimes through the socket. In any case attach ground leads accordingly.

-If salvaging a later model column don't just hack away the wires coming out of it. Get as much of the wiring harness as you can. If possible trace out the wires to the hazzard and signal flashers (older GM's have the signal flasher mounted near the radio of all places, Hazzard on the fuse box) getting this precious bit of wiring will save you time when you're mocking up a system. And don't forget the brake switch.

-Reverse Lights. Wether you do or don't, run a wire to the rear of the vehicle at the same time you run wires for the tail/stop/signal wiring. Falls into, pay a little bit now, or a lot later.

-Liscense Plate Illumination Gets tied in with tail (marker) lighting.

-1157 dual filament bulb (3cp, 32cp) for tail/signal, tail/signal/brake. -1156 single filament (32cp)



Remember to employ all safety precautions when working on any vehicle.
This information is only intended as an overview and may not include all the necessary information, data, or facts to complete the swap.
Remember, every vehicle is unique, and research for your particular vehicle is recomended.


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This Page Created By Alexander M. Bilan (gmguy89@netscape.net)
All Rights Reserved
August 14, 1997
Revision 1.2 September 28, 1997