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Hey I’m new to this and just biult my 89 454, it was a TBI system and I did a carb swap, and a hei Distributor swap too. Was wondering what do I need for the wiring harness to get the truck running and how does the hei get its power?
 

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If it has an automatic transmission it is a 4L80E which is computer controlled so unless you wish to buy a stand alone transmission controller it will need throttle position which will require a TPS kit on the carb and tach signal off the distributor. It may also need a VOS signal off the Manifold Absolute Pressure (MAP) sensor for selecting shift points and modulation. If a manual transmission these are not required.

The speedo is a twisted wire pair it does not go through the computer but must be preserved for the speedo to work.

The fuel system works through the computer and uses pressure of 12 to 20 PSI which is too much for a carb. You will need to rewire that system so it operates the pump relay through the ignition switch or a stand alone switch for fuel. It will need a bypassing regulator set to 5 to 6 PSI on the carb outlet. Since all the TBI plumbing is under the hood already you can just tie the regulator into that. Since you need to establish and monitor that pressure you will need a fuel pressure gauge under the hood as well.

You need a quality wire diagram for the vehicle in order to rewire what the computer does now into an analog system off the key switch or stand alone switch control.

Bogie
 

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If it has an automatic transmission it is a 4L80E which is computer controlled
The 4L80E wasn't released until the 1991 model year. The TH400 was used with the 454 in 1989. You are correct about the TPS however, as unlike prior years, the 1989 trucks used the ECU to control the TH400 kickdown solenoid, based on the TPS and probably MAP sensor. Of course, all you need to do is to get the accelerator-mounted kickdown switch used on carbureted TH400 trucks and use that to control the kickdown solenoid.
 
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