WHEN I RUN MY 92 f150 5.o. with Windows down the temp gauge runs at the m in normal. But with the ac on it runs at letter L or past. What is normal temperature settings for this gauge. And what may be problem for overheating with ac on? Thanks.

 

As you know that condenser in front of the radiator sends a lot of heat to the cooling system. If the air passages through the radiator are clean, good.
A water pump that is on it's way out could do this and so could the viscous fan clutch.
The gauge needle usually stayed in the center , normally. It will rise a little with increased engine load , but should not get too much to the right of the middle.
If no other explanation can be found it might pay to have the actual temp read with another gauge.

 

Ok but when I turn the ac off because of the overheating the needle on gauge goes back to the middle. The r or m. It cools down. All this while driving. Does that rule out fan clutch. Thanks for feed back. Well check temp with another gauge asap.

 

It depends on what temp the fan clutch senses.
Something else you can check,
if the radiator is original and looking inside at the cooling tubes, you may need to lower the coolant level to see this, if there is a lot of what is called solder bloom it could be restricting flow through the radiator causing it to be borderline and inadequate to handle increased load at which point replacement is necessary. There were times when radiators could be cleaned (cooked) and reinstalled. That was a looong time ago. If you tried that with today's radiators you would end up with a wonderful shower nozzle !

 

Ha shower full of nozzle is right. I'm run through it this weekend and see what I come up with.. thanks for help.