The instuments in the dash started going crazy, the tach gauge is stuck, what is the problem?

To many possibilities but check relays first.

Hi, this is a pretty common problem on these Silverado instrument clusters. We can definitely help you out with that. Click here to learn more about these Silverado instrument cluster problems: Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

the gauges you look at are the instrument cluster. (speed, tach, fuel, temp, etc)

Under the carpet, near the gas pedal, there is a ground wire screwed in, and that connection is going to be corroded. Clean up the connection and be sure it is tight. I just went through this with my 97 Seville. Fixed every electrical problem I was having with lights flashing, gauges going crazy, etc...

Sounds like your gauge drive motors are wearing out.  It requires motor replacement inside the instrument cluster.

I fix these instrument clusters and have a 12 year guarantee, check my site at www.DrSpeedometer.com

Or check the video about it:

You-Tube video "eBlE23m0Iio" placeholder [unremark-end]-->(do not delete)

It may have a faulty cluster. Or possibly could have a charging system problem.
Take it to a dealer and have it scanned for codes.
The instument cluster is a module that can store trouble codes. If the cluster is at fault the new one will have to be programmed to the vehicle to work correctly.

this is just a rough guess, but i think it would be the battery or the alternator. Some part store have testers that they will check both for free.

I have this problem too and my new Saab mechanic told me that I would probably need a new instrument cluster. He said a brand new one costs $2500 but that he would order me a used one from a car that had fewer miles than mine as you can set them back but not forward. He both diagnosed and temporarily "fixed" the problem by slamming his hand really hard on the dash over the panel and all the lights and gauges immediately started working fine....for now. I haven't had the work done yet (I have a 1998 Saab 900S with 130K) so if anyone has any other ideas about this I would love to know what they are. This issue with the gauges and dash lights has been happening on and off for over a YEAR and this is the first time someone has had any idea what it is. I already had my ignition switched replaced so we know that's not what it was. THANKS

Speed control sensor,located on the rear axle.If it goes out,it takes instrument panel with it.

Could need new instrument panel cluster but check for corrosion as the instrument panel connector