Electrical 88 GT with 91 wiring harnesses, electrical issues

VSK2033

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May 17, 2019
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So I got this 88 GT about a month and a half ago after totaling my 91 notch May and it was more or less a shell with a pile of parts inside so Ive spent the past month swapping everything over using parts from my wrecked car and a 93 parts car and it has been fine until I reached the electrical side and I took out all the 88 harnesses because the plan was to add the 91 steering wheel and airbags but I've hit a wall with it.

I'd say 50% of the electrical stuff works on the car right now, doors, lights mirrors, etc, but nothing else is so mostly dash-related stuff. The other strange thing that I can't seem to pinpoint at all is the front side markers and turn signals along with taillights turn on the moment the battery is hooked up, they stay on even if you remove all fuses and switches so the only way to turn them off is to unplug the headlight harness but that still leaves the taillights on and from all the wiring diagrams I've seen I have no idea how they're getting power before fusebox

If anyone has any advice it would be greatly appreciated!!
 

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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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You are not mixing 88 electronics with 91 stuff right? The wiring colors and stuff changed a lot by 91 plus the addition of the air bag circuitry, be very careful dealing with the air bag stuff, you do not want the bag to deploy when you turn the key on.
The headlight and associated stuff gets constant power from solenoid and has several dedicated grounds.
 

VSK2033

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May 17, 2019
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You are not mixing 88 electronics with 91 stuff right? The wiring colors and stuff changed a lot by 91 plus the addition of the air bag circuitry, be very careful dealing with the air bag stuff, you do not want the bag to deploy when you turn the key on.
The headlight and associated stuff gets constant power from solenoid and has several dedicated grounds.
Yeah no I am not, I looked that up beforehand and saw there was a lot of changes made so it only has 91 wiring harnesses throughout the entire car, but the airbag will also be done at later date, I have the sensors and modules installed inside the car but it's going to take modification to the radiator support to install front sensors so Id rather get all current things finished first, I have the airbag unplugged currently so that's not a concern.

But yeah, I saw that from the manual, I ordered the 91 EVTM and my current problem is not listed in that so I'm very stuck as to why it's getting power. All the grounds seem attached and to clean metal so there has to be something else happening I'm assuming since it's turning taillights on too.
 

evintho

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Nov 12, 2003
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I've posted a lot about factory wiring diagrams....but here it goes again.

1991 Mustang factory wiring diagram

1988 Mustang factory wiring diagram

These are what factory techs used in the dealerships back in the day. They have everything laid out in simple, easy to follow form. They're cheap enough and cover the entire electrical system for each car. I have one for every car I've ever built. In my view, they're invaluable!
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
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Are you absolutely sure all the harnesses used are strictly 1991? I've found that the main harnesses pretty much change every year other than a few minor harnesses which have a larger year span they can be used with.

Here's what I would do. You have the 1991 EVTM right? Go to the page with the parking lights and start right at the beginning of the circuit. I don't have the 91 EVTM in front of me but I beleive it goes through the circuit breaker first and then a black wire which feeds fuse number 4. If you pull fuse 4, the lights should stay off. From there, it goes to the headlight switch. You can unplug the headlight switch there and see if they go out. From there it goes out to the lights. The connections are labeled along with the wire colors and the rear of the EVTM will tell you the approx location of the connector. You'll have to go out and find them and see what is going on there.

I know that's a vague description of what you need to do. I can probably give you more details when i get the 91 evtm in front of me. I admit they are a little bit tricky if you don't know how to read them as the full circuit tends to draw out over a few different pages. In this car it would be charge/power distribution system and then parking/taillights.
 
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VSK2033

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Well, I ended up figuring out the issue, its a mistake I will never make again because I felt so dumb spending the past two weeks figuring it out. Turns out I mistook a bundle of wires with a ring terminal on them coming from the dash harness as a fusible link so I had those grounds connected to the starter solenoid and once I figured that out and put them where it was supposed to be everything magically worked so it was a very humbling lesson for me after spending hours searching and ripping the dash out twice just for it to be a ground in the wrong spot. Thank you for all your help though!!
 
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