I sure could use some advise on a faulty temp gauge in my 95 accord.
With the sending unit hooked up,
as soon as I turn the key to on, (not start)
my gauge immediately runs up to full hot.
This is before the engine is even cranked.
What would cause this?

 

well on mine someone screwed with the cluster before i bought it and they broke it and my temp and fuel gauge just shot all the way up. i replaced the cluster with one from a junkyard and it works now. so it might just be the cluster thats not working right.

 

Apparently, the sending unit is not hooked up or has a bad ground connection to the block. Both of those would cause your symptom.

 

If the temp gauge is reading full hot, then the gauge is working. The only way for it to read that hot is for the engine to actually be that hot, or for the sending unit to be shorted. Unplug the sending unit and check the resistance with an ohmeter with one lead on the connector side and the other on the thread side of the unit. If the resistance is very low, like say 1 ohm, replace the sending unit. If you unplug the sending unit, then turn on your ignition, the gauge should read full cold since the gauge "sees" an infinite resistance at the sending unit since it is unplugged. I would replace the sender anyway since it is pretty cheap. PM me if you need any help. Remember to only use a little bit of thread seal tape when installing the new unit. too much tape and you may not ground the unit properly with the threads.