I just noticed that when I apply my brakes the front parking lights and both front and rear side marker lights light-up. After checking all wiring schematics I can't find where they are interconnect except maybe using the same main ground. I was hoping if someone would check this on their car to see if it is normal or not on a 88 GT.

[This message has been edited by Imnuts (edited 04-10-2020).]

Just a guess, but I'm guessing that the ground might be bad, or the common wires leading to the ground. If the common ground is bad, the voltage will try to find another ground source, usually by back feeding other components in the circuit like your lights that are coming on when you hit the brakes. I would try checking the wires visually first up to the ground connection and if everything looks good and you have a voltmeter, check to see if you have voltage on the ground wire itself, not the bolt that mounts it. There should be little to no voltage if the ground is good. If you do have voltage at that point, your ground is bad and hopefully can just be disconnected, cleaned up and reconnected.

That's my two cents. Hopefully it gives a starting point anyway... Good luck!

That's what I was suspecting when I first noticed the problem. I did check the ground coming from the brake lights with an ohm meter but your suggestion sounds like a good idea to verify if it's a good ground. The only problem is I have not been able to locate exactly where this main common ground is located. The ground goes through the main large connector located near the negative side of the battery but where after that is not shown in the Fiero repair manual that I could find.

Take all your tail/brake light bulbs out and inspect the sockets for corrosion.
Test all bulbs while you have them out too.

------------------
My World of Wheels Winners (Click on links below)

3.4L Supercharged 87 GT and Super Duty 4 Indy #163

[This message has been edited by fierosound (edited 04-10-2020).]

You don't need to do this to your car.... .....pulling out the wheel well liner will suffice.

[This message has been edited by Gall757 (edited 04-10-2020).]

Thanks Gall757 for the help I will check out that ground.

As for Fierosound's suggestion, not sure how that would have anything to do with my side and parking lights coming on when I apply the brakes. But thanks anyway.

quote Originally posted by Imnuts: As for Fierosound's suggestion, not sure how that would have anything to do with my side and parking lights coming on when I apply the brakes. But thanks anyway.

I think you have 12V from the STOP side of the bulb jumping to the TAIL side when you press the brake.
That feeds the side marker, which is part of the light switch circuit which controls the front lights too.
A tail light bulb with filaments touching will do it.

[This message has been edited by fierosound (edited 04-11-2020).]

84-87
Brake pedal turn on Front lights is something way wrong then a iffy ground.
Because normally Two circuits are not connected to each other.

But

88
Rear lights ground inside of cabin and may have a Sneak Path sim to my Cave page.
88 GT has separate rear brake and turn bulbs so even that should be very hard to do w/ iffy bulbs...
Maybe you have rear lights "mod" and F'd up the wiring or have sockets w/ iffy ground, or wires pinched somewhere in cabin or rear.

Could also be iffy Turn switch or wires to that like Wires pinched, T-switch is F'd, etc.
Make sure Hazard button is release... Is part of T-switch and may do weird things if pressed. If true, Maybe hazard flasher is bad too. The flasher can next to chime unit.

Pull taillights assem out, pulls sockets, push brakes while watches the bulbs.
Should only light 1 filament and only for brake bulbs.

More Need 88 FSM or Alldatadiy.com because external lights power and ground "maps" etc are spend over several pages. You can't use older data for 84-87 Fiero.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

So where is the frame ground located for the brake lights on an 88?

quote Originally posted by Imnuts: ... and thank you for your suggestion.

quote Originally posted by Imnuts: But not noticing it for that long makes me feel like a Fiero fool.

Hey! That title belongs to me. You found it so you aren't any more.