Engine harness
 

Putting my truck back together, I have a 1977 under dash gauge/harness and I’m attempting use a 1976 idiot/light engine harness. What is going to be the outcome , anybody know ??
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Re: Engine harness
 

I think you'll be okay with that combination.

Looking at the wiring diagrams for both years and comparing the pinouts of the firewall bulkhead connectors, it looks like all of the major circuits in the engine harness kept the same terminal locations.

The warning light style engine harness will have a dark blue wire for an oil pressure light switch. Assuming you're using the '77 gauge cluster w/mechanical oil pressure gauge, you can simply leave that wire unused.

Both gauge & warning light style engine harnesses have a dark green wire for the temp gauge or temp light. Just make sure you have the correct sending unit (for the temp gauge, not a switch for a temp light) installed in the engine.

Re: Engine harness
 

The beautiful thing about the 73-76 lamp harness is you already have an oil pressure sensor wire to the cluster so you can use the more plentiful 78-80 tach clusters.