jshireaz

Hi,
I have a 98 Honda Civic LX. Problem is the radiator fan is not coming on when the engine reaches normal temperature. The temperature continues to rise until it overheats (I turn it off before redline).

For purposes of my rambling explanation, receptacle means female part in the fuze box that a relay plugs into. Pins 3 and 4 of a relay are the ones that are energized to allow current to flow from pin 1 to pin 2.


I jumper the power and the radiator fan runs.
I jumper the power at the relay receptacle and the radiator fan runs.
I measure the voltage at the connector to the fan motor. It never has any.
I pull the radiator fan relay and measure the voltage across receptacles for pins 3 and 4, showing that the engine coolant temperature sensor is sending a 14 volt signal when the car is warmed up.
I see there is voltage available to send through the relay by measuring voltage on relay receptacle ports 1 and 2.
I swap the condensor fan relay with the radiator fan relay and the condensor fan still works and the radiator fan does not.
I insert leads into the relay receptacles for the condensor fan and radiator fan. I touch the relay posts 3 and 4 to the condensor fan relay receptacle leads and I can hear the relay click. I touch the same relay to the leads from the radiator relay receptacle and the relay does not click.
I measure the voltage across the relay receptacles while inserting the relays. The voltage across the radiator fan relay receptacle drops from 14 volts to almost nothing when the relay pins 3 and 4 contact the receptacle.
The voltage across the condensor fan relay receptacle holds at 14 volts. Seems analagous to a weak alkaline battery that reads 1.5 volts but when you put a load on it, it can't supply any amperage.

Anyway. I'm stuck. My gut says replace the coolant temperature sensor but I'm not sure why.

HELP !!!

Thanks in advance,
- J

EE_Chris

If you take out the radiator fan relay and jump pins 1 & 2 (which would connect battery voltage directly to the motor) and it runs, we can assume that part of the wiring is alright.

You said you swapped the condenser fan and radiator fan relay and the condenser fan still continued to work. Sounds like the relay is good.

The relay is energized either through the ECU (pin A27) or the ECT switch located on the thermostat housing. *The ECT sending unit (for the dash gauge) and the ECT sensor (for the ECU) are located at the front of the engine under the distributor.*

Sooo...if you think it's the ECT switch, test it. You can remove it, suspend it in a pot of boiling water and check the resistance across the terminals when it closes. Spec is 196*-203*F for it to close. For the hell of it, remove the A/C diode and test it too (just make sure it reads resistance with the meter leads one way but not reversed).

Worse come to worse, try another ECU.