need help fixing water temp guage

[84chevyboy]

ok that other day my gauge starting reading crazy, while driving the guage would read super hot but once i start slowin down or speedin up tha needle would starting bouncing all over, so now i change tha gauge to another factory style one that reads C and H it doesn't even move. should i change tha temp sending unit or tha themrostat. and all so that rad. is full of water. so i dont kno why it would read hot

[Prong]

Try this to see if the gauge is good:
Disconnect the wire to the sender. Turn ignition on--it should read cold. Next ground the lead to something and turn the ignition on--it should peg way past "H". If your gauge checks out, try replacing the sending unit.

[82BlazerMan]

What if the gauge always goes to cold?  I've replaced my gauge and both the old and new gauge go to below the C when I turn the ignition on either way, plugged in or grounded.  (I did the test you mentioned above)

[Prong]

It's most likely not getting power to the sender--there's probably a break in the wire somewhere.

[82BlazerMan]

82 Blazer, 305, 2wd, a/c

I don't mean to take over this post, but I tried tracing the wire and noticed it was tied to the a/c.  I don't notice the a/c in any of my wiring diagrams.  Do Blazers have that much different wiring?  Also, the sending unit is on the drivers side of the block right?  The diagram shows it just going all the way to the gauge and that's it.  Am I missing something here?