Anyone have the wiring diagrams for the tachometer, the gauge in the middle of the picture, and the combo gauge, in the top of the picture?

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Did you check Jeff B's website?

Don't know about it. Do you have the link handy?

Jeff has a great site. save this link...

http://bowlsby.net/914/Classic/

Likely you already know this but that is 911 stuff, are you trying to figure out wiring for a /6 conversion?

Combo gauge requires wire run from console harness plug to dash board. You will need two jumpers for the red wire for 12v and a brown jumper for ground. Electric speedometer requires a bunch of stuff.

Trying to help with some of the leg work for the mechanic, and the LS3 conversion. Everything takes time.

Tach is wired correctly in the top picture. The mechanic will need to intercept the black wire with purple stripe (wire on the middle terminal) back at the relay board and reroute that wire to your LS3 ECM's tach output pin. The ECM should have 3 connectors that are colored, one of the connectors will have the pin that you are seeking.

Also,

Some LS ECM's will have such a weak weak signal, he may need to install a pull up resistor to make the tach work.

The combo gauge is pretty straight forward, seems that this mechanic is more mechanical and less electrical. Retrofitting a LS into a 914 is serious business and will take some knowledge and thinking outside the box. I would hate to see your converted car have some electrical fire because this mechanic is not up to snuff with wiring. Not to be taken the wrong way, just my .02 cents