Help With Electrical Problem

[MikeN]

I installed a used instrument cluster with gauges in my 64 C10 several years ago to replace the warning light cluster. I used reproduction wiring harnesses from M&H. Everything worked great, although the used cluster showed it's age cosmetically.

Recently I bought another used gauge cluster, refurbished it cosmetically, and installed a tach with one of rideblue's custom brackets. Everything went fine, except on the test ride the temp gauge was non-functional. Everything else was perfect.

I pulled the cluster back out and replaced the faulty temp gauge with the known good one from the first cluster. Now I have a weird electrical problem and I simply cannot figure it out. There are two plugs on the gauge harness. If I plug only the smaller of the two plugs into the main harness, the engine will crank over and run, although the gauges do not work. When I plug in the second one, it shorts something, and the starter won't even click or turn over when I turn the key.

I took pictures of the old cluster and harnesses before disassembly, and I'm sure I installed the second temp gauge with terminals and wiring correctly. I simply cannot figure this out. Any opinions or advice are welcome. Thanks.