86TpiTransAm

Bare with me as I go through some explanations. If you already know all of this information then great, you're one step closer to helping me out.

On the base model Firebirds and the 91-92 TA/GTA, the tail lights consist of separate turn signal bulbs and brake light bulbs. The turn signal switch is connected to the turn signal bulbs and on the brake light bulbs, which are dual filament, the light switch is connected to one filament and the brake switch is connected to the other filament.

However, on the PRE-91 TA/GTA tail lights there are only 3 bulbs on each side. One bulb is used as just a parking light and the other two bulbs are used as brake lights and turn signals. Unlike the base models and 91-92 TA/GTA, the brake switch is not connected to the brake light bulbs but are instead routed through the turn signal switch. The turn signal switch is connected to one filament of the brake bulbs and the light swtich is connected to the other filament of the bulb.

This is where the "need to know" information starts to come into play... when the turn signal switch is in the normal "off" position and you apply the brakes, the brake switch sends voltage through the turn signal switch to the wires that connect to the brake light bulbs and the brakes light up. When the turn signal switch is in the left position, for example, the circuit from the brake switch becomes open and the circuit from the signal flasher becomes closed and the left brake lights flash. If you apply the brakes at the same time, only the right side brake bulbs get voltage from the brake switch and light up solid.

Now that I'm through all that explanation, here is what I need. I basically need a way to simulate the operation of the PRE-91 TA/GTA turn signal switch circuits on a '91 base model WITHOUT rewiring the entire tail light wiring and/or turn signal switch.

Basically what I have is a wire that receives voltage from the brake switch and a wire that receives voltage from the turn signal switch and they will both be connected to the same filament of the brake bulb but I only want ONE wire providing voltage at any given time. With the turn signal is off, that's easy because only the brake switch would provide voltage to the bulb but with the turn signal on and with the brakes applied I now have two wires both providing voltage to the bulb at the same time. I need a way to control whether or not the brake switch is "allowed" to provide voltage to the brake lights based on whether or not the turn signal is on or off.

I'm extremely unfamiliar with how relays work but I'm hoping that there is a relay that will be able to automatically switch between the wires based on whether or not voltage is being applied to the turn signal wire.

I've attached a crappy little diagram that I drew up quickly in paint to show an idea of what I'm looking to do.

Any help with this would be extremely appreciated.

Fullsizewagon

Here's a solution I think might work for you. You'll need two c/o relays.
Notice that your blinker relay might not like the loss of a filament, so as to slow or stop blinking while you apply the brakes. The remedy for that would be to obtain a blinker relay that's insensitive to load. Of course then you get no warning about any burnt-out blinker filaments.

86TpiTransAm

That idea that you're showing had crossed my mind. One problem I see with it though is if I have the blinker on, when I come to a stop and apply the brakes, the stop light switch will activate the relays and the lights would no longer blink and I need them to blink even at a dead stop so the person behind me won't get pissed off - or pull me over if it's a cop.

One thing I had considered was putting just the stop light switch on the relay and connecting the turn signal switch directly to the filament but then my problem is how to get the turn signal switch to activate the relay and make it go "open" so the lights only blink and get no voltage from the stop light switch while blinking. I can't splice off the blinker wire and connect it to the coil because I believe that wire only provides intermittent voltage from the flasher while the blinker is on which would cause the relay to switch on and off with the blinker and when the blinker should be off, the relay would be switched off and the voltage from the stop light switch would keep the light on and this would essentially just cause a solid light instead of a flashing one.

QUESTION: How exactly does a diode work? I understand that it's a one way path for voltage but would using one help with this situation if it were hooked up in some way. Just throwing out ideas.

One last thing I've considered is pulling the steering column apart and checking for an unused pin on the turn signal switch, labelled pin P in some wiring diagrams. The wiring for the '90 TA/GTA turn switch and the '91 Base Model turn switch are identical except that on the '90 TA/GTA, there is a Pin P which the stop light switch is connected to. In this setup, the stop light switch circuit is in the closed position during normal operation with the turn signal in the normal "off" position and when the turn signal switch is activated, the stop light switch circuit to that particular side becomes open and the brake lights flash. If my '91 turn signal switch is the same (and GM claims it is), then I should be able to connect the stop light switch wire to Pin P and my problem would be solved.

Taking that steering column apart isn't something I look forward to though.

Fullsizewagon

Ok, so the blinker has to have priority over the brake light.. Well, it figures I guess, though it isn't something we have to consider over here where it's all hacked up and rewired, using extra orange lamps unless there are reverse lights etc. that can be refitted with orange bulbs..
Taking the column apart is a task that should be left to the i..t who designed it, so I came up with another idea for you. It involves two diodes and capacitors. I don't like using such devices loose in a car but it should work and is the simplest solution I can think of right now.
The concept is that the diode charges the capacitor with each flash as soon as you turn on the blinker and retains it long enough to hold the relay for a while after each blink ends. Ideally the capacitor should be big enough so that the relay manages to hold between each blink, but even if it doesn't you just get the effect of a shorter off-time of the lamps but it will still blink.
The exact value of the capacitor will differ with the properties of the relay, the speed of the blinker, and to some degree the charging voltage, so you will just have to try it out. I don't know of anyone ever having tested such a circuit in a car.
For starters you can try at least 4700 microfarads (and 16 Volts rating).
The diode can be 1N4001 through 1N4007.
Be sure to secure the capacitors with tape/ties etc. as its legs could eventually break from vibrations if just left dangling.

86TpiTransAm

Great idea! I had considered the capacitor idea but wasn't sure how it would work. One question I have with this setup is does the capacitor dissipate the voltage on it's own? How long after the turn signal switch is shut off would the capacitor dissipate? Basically how long after I turn off the signal switch would the relay shut down and swith back making the brake light circuit closed once again?

Fullsizewagon

The capacitor discharges exponentially through the resistance of the relay coil. The formula is simply T=R*C where T is the time in seconds for the capacitor to discharge 63% of its initial voltage leaving 37%, R is the load resistance in Ohms, and C is the capacitance in Farads. 4700 microfarads = 0.0047 Farads.
The properties of the relay comes into play here because they all have different fall-out voltages. The fall-out voltage could make the timing vary by a factor of two or three I guess. There's also large variations in the resistance of different relays (2-3), giving a worst case variation of a factor of 9. So in order to keep the capacitor small you'd want a relay with high resistance and low fall-out voltage.
An ordinary car relay could have a resistance of 160 Ohms which with 4700 microfarads would give a T of 0.752 seconds. The actual time would be modified if the fall-out voltage of the relay is not 4.8 Volts (37% of 13V).
You loose around 0.8V of the charging voltage (13.8V) in the diode.
You may want to increase the capacitance until the relay just barely does not drop out with each blink, but its not absolutely neccessary.

86TpiTransAm

So basically what you're saying is that a capacitor keeps 37% of the voltage and that I need to find a relay that will shut down and switch back at 38% or higher but at the same time I don't want it to shut down too quickly because I want to be able to keep the relay on in between pulses of voltage from the turn signal switch? So therefore, I should try to find a relay with a shut down voltage as close to that 37% as I possibly can and play with the number of microfarads of a capacitor to get the capacitor to hold enough voltage long enough to keep the relay on during blinks.
(Luckily math is a strong point of mine so I should be able to figure this out fairly easy if I can figure out some of the unknowns such as delay between blinks)

So after I get this figured out, we're basically talking about after I make the turn and the turn signal switches off there's going to be less than 1 second before the relay will shut down and give control of the lights back to the brake switch. That's a very acceptable number.

Fullsizewagon

Almost. After the time T the cap's voltage is down to 37% of the initial 13V it was charged up to by the end of the blink. After 5T it's down to 2%. You don't need to look for a relay with a matching dropout voltage, but the lower it is the more actual time delay you'll get for a certain capacitance. (Higher coil resistance is just as important and gives a longer T, but that's in the formula already.) You'd need a variable power supply to find their dropout voltages, but it's not completely neccessary. Basically, just grab some relays, measure their resistance, use the type with the highest resistance, and try it out (with different capacitors if neccessary). If the cap is just as big as it needs to be then you won't be able to discern the recovery time from the blinking itself. If the blinker gives about 80 blinks per minute and about half of that is dark-time, then that gives you an off-time of 0.375 seconds. If the capacitance is a little too small to hold the relay fully between each blink then all that will be noticed is that the rear lamps will stay on a bit more of the time when you are braking at the same time.

86TpiTransAm

Ok, a couple more questions. So what happens if a wire charges a capacitor with 13v and the capacitor drops off to 4.8v (for example) and then the wire hits the capacitor with another 13v? Can a capacitor be overcharged?

One more thing. This is just something that I'm considering and I'm not even sure if I'll do this or not but since you seem to be extremely experienced with electronics I figure I'd ask. There are 3 brake lights on each side and my intentions are to make all 3 flash when I turn on the blinker. What would be needed to create sequential blinking of the lights? So the inside light flashes first and then a split second later the next light flashes and then a split second after that the last light flashes. I know I would need to figure out a way to delay the voltage for a split second before it makes it to the next bulb but I'm not sure how that would be done. Maybe use resistors before each bulb?

Fullsizewagon

The wiring (and a diode) in a car has a very low resistance so the cap will be charged to 13V in some tens of microseconds, and it will not exeed that voltage. A cap has a voltage rating that must not be exeeded so if you use a 16V cap or higher in a car it'll be ok. An electrolytic cap like this, is also polarized so it's important to connect it the right way, or else it might overheat and explode. It also has a so-called "ripple current rating" but that's nothing to worry about in a slow circuit such as this. It can be recharged unlimited times and will have a life span much longer than the car.

I guess sequential blinkers with 3 bulbs could be made with 2 relays, 2 resistors, and 2 capacitors (each side). Just replace the diode with a resistor and you get a delayed turn-on and turn-off relay. Connect the first bulb direct and use that to control the first relay which then turns on the second bulb and also controls the second relay which in turn turns on the third bulb.
The resistor must be of a low enough value so as not to prevent the relay from turning on at all. If you use a resistor value of around 20-25% of the relay resistance I guess you'll be ok. Try it out without a cap first to make sure it works reliably. Use the resistor value then in the equation above to calculate the neccessary cap value from the delay you want.

86TpiTransAm

Thanks for all the help! I'm sure I'll post back with the results when I finally get everything setup. (or I'll post with more questions if I run into any issues along the way)

Thanks again!

Fullsizewagon

You're welcome, hope you succeed, and please post back results, it's very interesting!
Btw., if you go as far as the sequential setup you'll need supplemental 12V back there, to supply the delayed lamps with current (since it's in-between the original blinker blinks).. Even that could perhaps be had from the blinker supply using big diodes and caps, but maybe it would be stretching it too far, I don't know..