Hello,
I have a 1989 Honda CRX 1.5L that has two fans on the Radiator. The car seems to want to overheat and bubble out into the overflow bottle. I have replaced the cap and tested the fan motors. I noticed that the radiator seems to boil over before the fans ever kick on. I have been told that it may have something to do with a "Thermal Switch" that tells the fans to turn on and off, but the gauge in the car never get to "hot" before it bubbles over.
I guess my first question would be... Where is this "Thermal Switch"(or what ever it is called on the Honda) and is there a way to test it to see if it is functional?
Second, Could the radiator be causing these symptoms? The person I got the car from had it sitting for over a year and the radiator seemed to have only rusty water in it, not anti-freeze at all. I plan to replace the Radiator, thermostat, and hoses, but if the fans will not even come on I'm sure I will have to track down that problem as well.
Also, when I replace the radiator is there any special things I can do to clean out the old water from inside the engine before I put the new radiator and thermostat in? I would rather not let all the rusty water run right into a new radiator.

Thank you in advance for any help or a point in the right direction.

Drain the radiator and flush the complete system, including heater core. Be sure and remove the two drain plugs on the block. Refill with 50/50 mixture of coolant and distilled water.

First of all I would change the thermostat, it's probably frozen closed, that's why it gets hot before the fans come on. Also flush the block and radiator. Check your fans by hot wiring them to the battery, one fan is for the AC and the other (the one on the engine side) is for cooling. If the fans are OK the thermal switch is on the engine side of the radiator at the bottom. The one on my CRX has 2 prongs on it, unplug them and with the switch on but the engine not running jump the plug and the fan should come on, means your relay is OK, I am going through the same thing, I think my thermal switch is bad, good luck.

I had the same problem and it was the thermal switch, very simple to test. The switch itself is by the intake manifold on d-series engines (see image). Just unplug the connector and jump it with a wire, if the radiator fan comes on when you turn the key then you found the problem, if not try replacing the relay in the fuse box under the hood.
http://z.about.com/d/autorepair/1/0/k/C/57459313.gif