Where is the thermostat located on a 1977 cadillac coup deville?

Fat chance. Dealer repair manual is the only place you'll find a color diagram. Since it's a 77, I doubt you'll be able to locate the 6 rather thick manuals. ou might be able to find something online, but it won't be complete or free.

Go on line and buy a manual for the car. That is the only place you will find it, if it exist. GM's diagrams were in black and white up until the 70's, I'm not sure what year they changed it, but I believe they were in color by that year.

GM hid it on the engine at the passenger side front, down below the oil filler cap.

Sounds like you need to drain all the oil out and check for particles in the oil. Some of the new oils have a "cleaner" in them and will loosen debris in an older tranny which will block ports in the tranny. If so... try flushing the tranny with a cleaner availble at most Auto stores. Ask the dealer (preferable a older one who knows about older cars if this will work)!

bank #2 sensor#1 o2 sensor

The cadillac are sensitve to oveheating,once it overheats you basically have a headgasket problem

My sincere apologies for the delayed response.
I am personally going back to answer your unanswered questions.

I do not know if you still need this information, but I am going ahead and answering it anyway. You may want to Print the Diagrams for future use.

CLICK on the following LINK. It has the Belt Diagrams you will need.
If you would: Before Clicking: Let me know if this helped you, or if you have any additional information or questions. Feel Free to contact me at FixYa.com!
Again my apologies...
http://www.2carpros.com/car_repair_information/year/1990/make/cadillac/model/deville/1990_cadillac_deville_drive_belt_routing_diagram.htm

Check to see if your fans are coming on and if not check the fuses and relays.............

IT MEANS THAT A FAULT HAS OCCURED IN THE SYSTEM AND THE SYSTEM HAS BEEN DISABLED, TO FIND THE FAULT YOU MUST HAVE THE DEALER TEST IT FOR THE TROUBLE CODE THAT IS STORED WHEN THE LIGHT STAYS ON.

the problem could be the engine coolant tempature sensor. all1992 through 1994 models equipped with electronic climate control can display trouble codes.replace coolant temp sensor.if problem contines have powertrain control module checked