I am going to take the questions out of order again.
The 4 cylinder tachometer issues is only with the 96 Impala SS's. The 96 Caprice's PCM tach output is a standard 8 cylinder output. If the issue was dues to that the tachometer would read double of the actual speed. I would double check to make sure you ran the wire to the correct position. It could also be a problem with the cluster itself. - Peter(pcordes1979)
So if you have a stock 96 Caprice PCM Peter says it is plug and play.I found that Gary Melier says the same and he made many.
Voltmeter from a cold start could go up to 15V. Running warm the gauge is about the same as my car.
Gauges sticking? You are going to have to be very descriptive.
They will drop to below the range of the gauge, and seem to have trouble popping up when the car is started. I had to take off the clear lense and manually move the temp gauge and make sure it was "popped" on.
I can only say what I know about most gauges. The Caprice cluster does not Drop to Zero when the key turns off. It usually stays close to the position it was last at. When you do a "cold start" and just turn the key to run the oil pressure should return to zero (engine not running) and temp should go cold (Cold start right!). Volts will read and Fuel should read.
If the temp/oil/fuel gauge is unplugged from it's sensor it will "pin" high and off scale.
If a gauge has it's ground removed it will "pin" below zero. This should not be possible if the gauge reads a value when running. As a guess you have a grounding problem. I think the gauges are getting a "feedback ground through another circuit" (I do not have time to explain this sorry). It would explain why the gauges zero. It may explain the tach as well.
First thing would be to pop the old cluster in and see if the gauges work normally.
I bet they will if they worked before.
If the old works, the next move is to get the factory book out and track down A6 in the right cluster plug(C2). Also B4(C2) and B3(C1 left plug)
The A6C2 is the suspect one and you will have to sort out for yourself if it is getting to the right place on the Z28 cluster.
See pics, thanks to the person that posted these! (I have not checked them for correctness).
I do not want to get between a buyer and seller!