73-87 Chevy _ GMC Trucks > Engine/Drivetrain

90 Tbi swap into 85 K10 wiring help needed

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Hi all,
I知 currently swapping a 90 tbi engine into my 85 K10 and I知 down to the wiring part. I have the complete donor truck and I have pulled the complete wiring harness and ecm to use in the 85. I have the engine completely installed. I知 waiting on a new fuel tank, pump and sending unit. I値l complete the fuel system when I get those parts. I知 not that good with wiring but I can follow instructions and wiring diagrams. I知 pretty sure I have the wiring figured out for power and grounds. The ecm is a 1228747 with a AKWA eprom. Nothing else is readable on the ecm. I知 pretty lost on how to connect the new harness to the existing gauges. Any help will be greatly appreciated.

Put the wiring diagrams for the 1985 and 1990 side-by-side.  Compare the I/P cluster pinouts between the two and repin accordingly.  The basic color codes between those years should be nearly identical, but there are important differences, such as the vehicle speed buffer and VSS from the speedometer head on the TBI-equipped vehicle.  The ECM requires a VSS input either from the speedometer or a VSS generator tapped off the speedometer cable. 

Thanks for the help. I hadn’t thought about comparing the 2. I guess I was overthinking the whole thing.
Will the vss signal from the 85 work with the ecm from the 90 or do I need to get something else to make it work?

Dash Cluster Pin Diagrams is another reference that may help.  Our Technical Forum has a comprehensive selection of OEM Service and Wiring Manuals.

The 1985 model trucks had no VSS; there was no need.  Either pick up a later model (1987-1990 R/V) cluster that has the cable-driven speedometer with integrated VSS and external buffer or fit a VSS generator onto the transmission end of the existing 1985 speedometer cable.  Jags that Run used to supply VSS generators that retrofit nicely.

I thought that the 85 with cruise control had to have a vss signal to control the speed. That痴 what I was thinking about using for the ecm input. Am I wrong?


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