2004 Trailblazer will not start. Battery and fuses check out to be good.Tach and speedometer jump around when starter is engaged.

Lots of possible problems. Look here for Trailblazer issues.

http://forums.trailvoy.com/showthread.php?t=72999#/topics/72999?page=1

Check Battery, if you try to charge or jump starter you may hear a click. If you're sure its a good charged battery then consider starter. Try to jump starter by passing switch, cross small wire to larger wire on starter (you may get sparks) (please be careful if you try this) this should make the starter spin & kick in turning the engine. If it only spins then its probably the starter solenoid. If this checks out then check ignition switch & harness connections. That's it..

Before you do any thing check for loose or bad connections. Also check you ground connection to engine. Check Fuses, relay and for a Fusible Link open circuit. Now let me explain what a Fusible Link does. They are short lengths of small gauge wire installed between the Battery and the Ignition Switch, or the Battery and the Fuse Panel. They'll melt and open the circuit before damage occurs in the rest of the circuit. I hope this was helpful you.

do you hear a click if you do its probably your starter cylinoyed if it works when you jump the wire you are bypassing it hope this helps pat

Possible faulty ignition module. Tach output is in ignition module. Module located under coil pack. Remove and have tested at any major parts distributor for free.

On another forum someone said to disconnect the cables from the battery; press on the brakes several times to release any electrical charge still in the system; wait a few minutes and reconnect the battery cables. It worked like a charm--speedometer, tachometer, and display all function now!

check engine to see if it can be turned over by hand it sounds like it's locked up

Check the battery terminals and body grounds. Also check the big post on the starter and the ground cable where it attaches to the engine block.

Hmmmm, from what u say, sounds like everything is ok, but,Bad battery can run lights, radio, but trying to start, will make clicking noise. U jumped it to start, and it did, another sign of bad battery. How are the connections at battery? Tight and clean? No corrosion? Check wire to starter also to make sure it's ok. If all connections are clean and tight, I would recommend removing the battery, take to parts store for free check to see condition of battery (low charge, or if it's shot). If u jump start it, u can also have them check the alternator too (free). U indicate dash gauge @ 14, that should be good. Battery usually has a date 'thingy' on it that u peel off #'s to indicate when battery was purchased, does yours have the 'thingy?'

check on the security system... might need resetting.