Wire diagram on starter 86 gmc - 1986 GMC K1500

The purple wire goes to the S terminal on the solenoid from the starter relay . Free wiring diagrams here http://www.bbbind.com/free-tsb Enter vehicle info. year , make , model an engine . Under system click on engine , then under subsystem click on starting . Click the search button ,then the blue link.

What relay ? Starter relay ? Check engine light on ? Do you know how to test power and control circuit at the relay base ? where the relay plugs into ? The PCM - engine computer is suppose to energize that relay , IF it see's a input from the transmission range sensor switch (park or neutral ) or if there are no anti-theft system problems . Pull the relay and look at the bottom (where pins are ) you will see four sets of numbers . 30 & 87 and 85 & 86 .30 should have B+ voltage 87 goes down to the starter solenoid . If you used a piece of wire an jumped those two the starter should crank .(if it's good ) Pin's 86 & 85 - one should have B+ voltage and the other the ECM supply's a ground .
http://www.bbbind.com/free_tsb.html free wiring diagrams . Enter vehicle info. year , make , model and engine size . Under system click on engine then under subsystem click starting . View the starter electrical system diagram .
VEHICLE RELAYS Testing from the relay base
Starter relay works the same

When it won't start testing the electrical systems is the proper way to find a fix . Using a DVOM - digital volt ohm meter and power flow diagrams . Using voltage drop testing !
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Do you know what a wiring diagram is ? Free wiring diagrams here http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model an engine size . Click on engine under system then under subsystem click on starting .Then click on the search button , then on the blue link . This is a wiring diagram of the starter electrical circuit. Do you know what ignition switch is ? the electrical part. , how about the starter relay , transmission range sensor ! There is one thing you can try , move gear selector to neutral an see if it starts then . If that does nothing ,check for B+ voltage going at the starter relay . Jumping pins 30 & 87 at the relay socket , where the relay plugs into . will cause the starter to engage an spin the engine . If you look on the bottom of the relay you will see four sets of numbers 30 & 87 - 30 should have B+ voltage , 87 goes to the S terminal on the starter solenoid . An 85 & 86 are the control side .86 at the relay socket comes from ignition switch through the trans range sensor to the starter relay coil ( electro magnet ) . Watch these videos too .
VEHICLE RELAYS Testing from the relay base

VEHICLE RELAYS Operation Diagnosis

I don't know model year? I looked at 96. You have a starter relay, there is a fuse that supplies voltage to load side of starter relay.
I didn't post diagram because I wasn't sure of model year? I don't know if the wiring diagrams are all the same on this vehicle starter circuit?

Search online for "GMC 8500 wiring diagram"

The problem could be a shorted park netual safety switch . Do you know what a wiring diagram is ? It shows all the componets involved in a electrical circuit ! You IGN.1 fuse 40amp , GM calls it IGN A fuse 40 amp . It feeds the ignition switch and pin 30 at the starter enable relay .When you turn the key to the start position it sends B+ voltage to #8 crank fuse , located in the I/P fuse box ,from there it goes to the park netual safety switch , to pin 86 or 85 of the starter enable relay ! If you pull the relay out of the fuse box an look on the bottom you will see this four set's of numbers ,30,87 & 85 ,86 You can test at the fuse box where the relay plugs in . Here is a video on how to test . The guy presenting the video uses a horn circuit , but it works the same way . It happens a lot , the park netual safety switch shorting out . It may or may not be the problem . VEHICLE RELAYS Operation Diagnosis
Here is a web site for free wiring diagrams
http://www.bbbind.com/free_tsb.html Enter your vehicle info . then under system click on engine , then under subsystem click on starting . Then click on the search button an a link will pop up , click on it .You will see all the componets , connections , wiring colors etc... for the starter circuit for your vehicle .

I found window motor connector for your vehicle at rockauto.com, it was two wires, so that is up and down, correct? Sorry, I don't have wiring diagram for your vehicle year? The closest I could get was 1990 for gmc pickup. If the wire colors match up, it may work?

First make sure the battery is disconnected, if you are simply replacing the starter, hook the terminal to the same place as the old starter, if you are replacing the wiring, follow the same path as the old wiring. If there is no old wiring to follow, you will need a diagram or another similar engine, mabey at a wrecking yard to follow.

he can buy the haynes repair manule at autozone it has the diagram in it near the back cost $15.00  plus helpful for later problems

I can email you a copy of the wiring diagram ... I have an 86 and the manual