My civic is still overheating even after replacing the ect switch and thermostat (the fan never comes on).
Shorting the ect switch by putting a paper clip in the connector causes the fan to come on.
What other troubleshooting steps can I take?
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The fact that the fan comes on when the switch is jumped is very useful since this tests the power source, relays and fan motor.
The fans can be activated by either the switch in the water passage near the thermostat housing or the coolant temperature sensor via the PCM (powertrain control module) located in the cylinder head below the distributor. So testing of both sensor and switch may be needed.
The switch should close and turn on the fans at about 199 deg F. Regardless of what the sensor is telling the PCM about the engine temperature. A simple scan tool would be helpful; it would display engine temperature.
Either the new switch is bad or the coolant around it is not getting to 200 degrees. A low temperature in this area could be caused by poor flow in the system.
Edit / Update:
So that sending unit isn't an on / off switch, it should be a variable resistance device. Here's an overview.
When you read the resistance across the unit, what do you see (in Ohms?) Does that match the chart (in YOUR new service manual) for the current temperature? There are more things than just that sender in the system. When you short it, you are telling the coolant fan control system that the engine coolant is at extremely high temperature, so that really is a lousy test. Its possible the old sender was good, but the control device has problems. (And that would mean, you just wasted good money for nothing.)
Step #1. Buy the manual.
Step #2. Do a methodical test of everything in the system according to the manual.
(I will say, its not clear from your comment. Do you already have the service manual for your car: Make, Model and Year? Do you have a multimeter? If so, apologies from me for assuming that you did not. The service manual will tell you how to accurately verify the correct reading from a temp sensor. Generally you will measure resistance across the terminals when the unit is disconnected from the wiring harness, but follow the instructions in your manual. These all vary a little bit. )