New gauges and harnesses but nothing works
 

So I bought 2 brand new 25' harnesses from the engine to the helm and 2 brand new 36" pigtails to my new gauges (used from eBay). wired everything the same as the ones I removed and as the diagram says with all new marine wires and ring connectors. everything is brand new except the gauges but they sure do look new, came in original boxes and all.

I fire up my engines (twin 7.4mpi) and none of my gauges work.
RPM is at 6k, don't move no matter how high I rev them.
water temp is at 280 on one and 0 on the other
oil pressure is pinned all the way at 80
oil temp is at 0.
Also, when I open the ignition switch on one engine the gauges on the other engine shoot up.

What did I do wrong? All my ground and ignition wires and connections are tight, my senders are hooked up correctly and tight. I was about to replace the water temp sender but its all the gauges. I can't let the engines run for more than a few minutes each because I don't know what the temp is...

Oil pressure switches I replaced: the engine had the mechanical ones so I installed the electric ones but the wire connection was a little rusted so I cut, spliced, and installed a ring connector and connected it with the nut that came with the switch. the connection that was there was the push on type that covers the entire thread. Not sure if this would make it read wrong but that would be only the oil pressure not everything? I was told I wired it wrong so I removed and double checked everything, checked grounds but still nothing. I'm lost here. It all seemed so simple and straight forward.

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Just a guess, but check that your hots and ground sets are connected to matching Port and Std gauge sets. That remark about the key makes me think you might have one or both power leads swapped, so your powering Port gauges when Stb key is on. Running separated battery banks correct?

Power is def not crossed, they each go to their own fuse box. And since I'm in test mode there's only one battery that I switch from engine to engine along with the ears/garden hose so there's not even power to the other engine. I'm gonna switch the ground lead i noticed it's connected to the volt gauge in the old set.

Ok, if that doesn't do it I'd step back and start by powering up the tach only. Verify one works, then bring on the others.

Agree with this advice. Also, you have so many splices and butt connectors and extra wire, down the road if you do get these gauges to work you are going to be chasing issue after issue. Techs will not want to touch it, and you can't blame them. You should be able to unplug the harness for the gauges and lift your entire gauge panel out of the boat to service it. Zip tie everything, run the wire like a factory install and use shrink tube connectors/ends.

Absolutely this^^^^. Any splice in there could be your issue.