ok here are the symptoms...
my fan won't come on = fast overheat
my tach jumps around after 5500 at more than half throttle

ps: 94 LS motor, fan plugged in, all fuses are good... im sure its a short somewhere, but where should i start too look?

 

First thing to do is check whether your fan is working by disconnect the fan from the switch then use a small wire to connect the two terminal on the plug together. If the fan running when you turn on the ignition then the fan is in working condition. Let assume that the fan is working, try replace your fan switch; if the problem still persist, then try replace the thermostat. Hope this help

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by zangiefisgod &raquo;</TD></TR><TR><TD CLASS="quote">my tach jumps around after 5500 at more than half throttle
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check the speed senser and the distributer can cause that too .. Plug wires and all

 

probably not related...

the tach is jumping around due to a loose wire on the side of the igniter inside the distributor, unplug it (the blue wire) squeeze the female connector a little tighter then reinstall it.

as for the fan, you should first check the fuse, then try swapping the relays out with another one next to it. If it still doesnt work, check for power and ground a the fan connector. Most likely this will find/fix the problem. if there is both, you need a new fan, if theres not both, find out why... the thermal switch on the thermostat housing provides the ground to the relay, so by jumping it you will only activate the relay.

PM me if this doesnt help you!