Will not start after swapping the wiring harness

BRAD KILLEN

MEMBER
1996 GMC SIERRA
V8
2WD
AUTOMATIC
290,000 MILES

I replaced the motor on my 1999 model GMC with a 1996' motor. I was using the original harness. I was having problems all over the place, so I decided to put the 1996' harness in it! The wire plug on the passenger side under the hood that comes into the inside of truck was cut its 8 or so wires under the hood and on the inside, it drops down to a 4/6 wire count. Would that wire cluster have any effect on the truck not starting? All it will do when I turn the key is the brake light and one other light in my dash panel will light up will not turn over. The fuel pump is kicking on, but truck will not turn over at all. Can you give me some helpful info on my situation anything at this point will help me?

6 Replies

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

Hang in there. We'll get it figured out. I need more information. Are you referring to the original wiring harness that came with the truck or the one from the 1996? Also, I need to know the engine size.

Also, since the vehicle is a 1999, the wiring is going to be a good bit different. I'm not sure if you are able to connect the PCM and other important connectors.

What color are the wires in the cut harness? Could you take a few pics and upload them for me to see? Also, do you still have the original harness? If the engine is the same size, it should have been useable. You may have had to change a few sensors, but it should have worked. Let me also know if you are using the original PCM.

I will watch for your reply.

Take care,

Joe

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Monday, January 31st, 2022 AT 8:14 PM

BRAD KILLEN

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4 POSTS

5.7 vortex and I got a new PCM computer that was programed to the VIN number of the 96' truck! It's not telling truck to crank whenever I turn the key to crank it and I night have some of the connectors in the wrong place but it's getting fire to the started. I got sparks at the starter.

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Tuesday, February 1st, 2022 AT 6:01 AM

BRAD KILLEN

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4 POSTS

It's was giving me three codes po300, a po161 and a 1406 before.

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Tuesday, February 1st, 2022 AT 6:03 AM

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Brad, is the 1406 a P-code (P1406)? Also, what do you mean by sparking to the starter? Is something shorting?

At the starter, there should be a heavy gauge wire from the battery, 12v at all times. The smaller wire is the trigger wire that should only get 12v with the key in the start position. Is that happening? Also, if you get sparks when you try starting it, are you certain everything is tight and not damaged?

I attached two different schematics below. The first two are for the 96 start circuit and the last two are for the 99. They are very similar. Take a look through them and let me know if that is how it's wired.

Joe

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Tuesday, February 1st, 2022 AT 3:32 PM

BRAD KILLEN

MEMBER
4 POSTS

Got new PCM that was programed to the vin number of the 96 truck but it's all in the 98 truck and the starter is hot but when I turn the key it does nothing.

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