87 shortbed 5.3 vortec not cranking

I m working on my 5.3 swap in my 87 swb chevy. I just got my harness in last week was done by chevy thunder. I hooked up all engine sensors and i am a little unsure about the last connections.Mostly just a good source on this truck for 12v switched igntion. I have the constant 12v from the harness hooked up to the starter and i tried hooking up the 12v switched ignition to an empty space on the factory fuse box under the hood labeled" ignition". I did get it to crank a few times but then i cleaned up some of the connections and re routed some of the wires. Now i cant get the darn thing to crank. There is a wire on the harness labeled"igniton power".I put this to the igniton slot on the factory fuse box. Im pretty sure after i cleaned up my wiring i used the same connection points when the engine was cranking.I just dont understandwhy its not working now. Im sure someone has done this swap on the same style or year model truck. Any tips or help is appreciated?

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Originally Posted by robg2001ls1

I m working on my 5.3 swap in my 87 swb chevy. I just got my harness in last week was done by chevy thunder. I hooked up all engine sensors and i am a little unsure about the last connections.Mostly just a good source on this truck for 12v switched igntion. I have the constant 12v from the harness hooked up to the starter and i tried hooking up the 12v switched ignition to an empty space on the factory fuse box under the hood labeled" ignition". I did get it to crank a few times but then i cleaned up some of the connections and re routed some of the wires. Now i cant get the darn thing to crank. There is a wire on the harness labeled"igniton power".I put this to the igniton slot on the factory fuse box. Im pretty sure after i cleaned up my wiring i used the same connection points when the engine was cranking.I just dont understandwhy its not working now. Im sure someone has done this swap on the same style or year model truck. Any tips or help is appreciated?

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The switched 12v ignition wire sould not be going to the fuse and the constant 12v wire should not be going to the starter. If you left your old trucks wiring harness in tact then hook the starter up exactly how it has hooked to your old starter. The bigger wire for power and the little one for the ignition signal.

The ignition wire needs to go to the acc on signal on your ignition switch. which is probably purple or pink. It will have 12v when the truck should be running and should have under .5v when the key if off and out. This wire has nothing to do with cranking the motor.

The 12v constant wire should be getting exactly that. 12v Constant. Either hook it to your old battery junction block or hook it to anything that will always have 12v to hold the timing memory in the computer.

Did you remove the purple factory solenoid wire from the harness and run something different? I always use the stock stuff to trigger the starter.

Hope you figure it out cause then I can foward this to my brothers best friend and tell him to 5.3 his papa truck!
good luck let us know!

I actually did figure this out. Well sort of. I had to plug back in the stock tbi harness and run the 1st (top left-facing the engine and firewall) red wire constant to the battery. Then i got it cranking. I figure this was about right. Had to get power from the truck somehow. I did get cranking but no fire. So i took the pink wire on the factory tbi harness and also ran it to the battery. (for some reason the truck wouldnt start without it hooked to 12 volts. The "switched igniton wire" i tried running it to the igntion switch but for some reason the computer will not get power unless i hook up that wire to the "IGN" slot on the under dash fuse box. But will stay on constant. I know that aint the right way to hook it up.. also im not getting any power at all to my "fuel pump relay or fuel pump wire on the harness. Any sugguestions?

and also i do have the small purple wire from the trucks factory harness running to the starter.

Niiice lets see pics man!

I will get pics up and/or a video. I got the fuel pump functioning as it should. prime for 2 sec when key is turned and on while running. The last problem i seem to be having trouble chasing down is my cpu has to be hooked directly to a 12 volt source at all times otherwise it will not work. Any sugguestions on this? Ive tried hooking it up to my ignition switch but no luck. it willcrank but no power from the PCM. I tried a couple other switched igniton sources( 12 volts with key turned) and none seem to work.

One wire needs constant 12v and one needs switched. You have relays on the wires correct?

UPDATE! I have the truck running as it should. I ended up wiring the "switched ignition" wire to the "accessory" port on the fuse box as it is the only source i was able to find that was hot with key turned . So now it turns on and off as it should. I was able to wire up my ecm fuel pump wire to the tan and white wire from the original tbi harness and now the fuel pump functions as it should. turn key- on for 2 seconds then on while cranking and when engine is running. I am somewhat having trouble with my throttle cable sticking. Im using a cable from a 98 vortec 350 shortbed. Ive tried changing throttle bodies, same story. either too much slack in cable or not enough.