I have a new correct GM sending unit installed in the console of my 69, but the guage is still reading too hot. When I turn ON the A/C and let it idle to allow the temp to climb (200 at the rad, according to the infared gun), the guage is almost pinned on the hot end. What can be the problem??

thanks for your help guys.

Too much resistance in the temp sender circuit will cause the gauge to read higher than is should.

Clean, check, and tighten all the connections (including in the bulkhead connector).
Could also be the power and ground connections so check them, too.

And when you say 200 at the rad, I think the best spot to check the accuracy of the temp gauge would either be right by the sender or on the thermostat housing.

Is it possible the temp sender was labled incorrectly?

I think idiot light senders are either open or closed whereas gauge senders have a varible resistance depending on water temp. You could check it with a DVM. Connect one lead to the sender tip and the other lead to ground. Just start from a cold engine and see what you get. If it goes from infinite resistance to none suddenly, you have an idiot light sender. If the R value changes in conjunction with water temp increase, you have the right type of sender. You could have the right sender but it could be out of calibration.

alan

thanks for the comments guys!