Just converted my 88 coupe to have AC.
It's a 1988 2.5L 5 speed.
The interior harness was swapped out along with the HVAC unit, front compartment harness.

all fuses are good.
third brake light lights up when brake pedal is pushed in.

thoughts?

IIRC the brake lights generally route thru something and the third brake light is direct from switch.
So I would find diagram and see where it goes from there,on alot of vehicles it can route thru the turn signal switch.

Problem at the C500 connector?


Was the new interior harness from an 88?
Do the front park/side marker lights work?
Did the replacement harness include the C500 connector?

My 88 schematic shows the ground for all the rear lights (except 3rd brake light, uses C201) runs through the C500 connector pin F10 (black wire), going
inside the car to the ground terminals by the pcm & behind the dash.

The 86 tail light harness has its own ground in the right rear trunk area.
F10 (F0) terminal on an 86 harness is a cooling fan wire.
Looks like more wires might be different between the 88 and 86 C500.

That explains everything...
interior harness is from an 86 coupe.
good news though. Im putting a different tail light harness in to ad Cruise control...which happens to be from an 86.

the interior grounds are good.
just for kicks, wich pin on the 88 tail harness has the ground? Id like to jump it to the fram to make sure thats the only issue

There's a picture of the 88 and 86 C500 in this thread: C500

The 1988 "F10" black wire is the same position as the 1986 "F0" dark green/white stripe wire
In 86 the "F0" (F10) wire turns on the v6 trunk mounted blower fan (not used on 4cyl) and the radiator fan when grounded.

If you ground it now (86 interior/88 rear harnesses) with the key on all the lights should work & the cooling fan should turn on, key off just the brake lights should work.

There are some differences in the two pictures (G7,G8,J9) but when I traced them through the schematics for a 4 cyl. they looked like they will work, the differences are for a GT.

[This message has been edited by AL68 (edited 11-17-2013).]

Gotcha.
Ill try it out.

One thing to be aware of is that the dash lights and marker lights are on the same circuit.

The switch is old school and gets corroded after a while.

It can be easily disassembled and cleaned. good idea It can develop resistance and you'll find the marker lights and dash lights get dim. Cleaning the switch cures this

Arn

I didn't know the inside wiring was so different - have to remember that when I start swapping parts between an 86GT & Formula I have.

There's a 88 4cyl/5spd/ac T-Top car in one of the U-Pull-It's near me, probably a little too far away from you.
Somebody in the Mall should have one.

quote Originally posted by AL68: I didn't know the inside wiring was so different - have to remember that when I start swapping parts between an 86GT & Formula I have. There's a 88 4cyl/5spd/ac T-Top car in one of the U-Pull-It's near me, probably a little too far away from you. Somebody in the Mall should have one.

yeah a bit too far

87-88 have difference colors wires and some different connectors on the interior harness. tail light and front compartment HVAC harnesses are the only direct swap parts with wires. oh and the power windows/locks/mirrors (with the correct power block above the fuse box.

the only year I know inside and out electronically is 1988. but I'm learning all the differences now...yup, took me 2 days and throwing my neck completely out to do it but I am smarter now. and knowledge is priceless