1999 OBW  How are the radiator cooling fans activated. What sensor is sending a signal to turn them on/off?

It's the larger temp sensor on the water manifold under the intake.  look down on the back passenger side of the intake and you will see the two sensors.

One is for the Dash, the other goes to the ECU.  

The relays in the engine fuse box will turn them on and off.

 

Why, what's up?

I bought this car with severely over heated ej25d . I swapped in an ej22, and sold car to a friend. A few weeks later his son was driving the car and it started overheating. I bought the car back from my friend. 

 

I just started the diagnosis of the overheating issue . No hydrocarbons in the coolant. Did a cooling system pressure test and found a coolant hose leaking on the crossover water  manifold. No more overheating so far. However I haven't seen the radiator cooling fans come on.

 

The fans turn on when ac is turned on so I know they work. I haven't tested the fan relay yet. I thought the fan relays were under the hood. 

 

I realized that I really didn't really understand what the process was that operated the activation of the cooling fans. So the cooling temp sensor sends a signal to the ECU and then the ECU sends a signal to the fan relays? 

 

I guess my next diagnostic test will be to watch the live data for cooling temperature on my scan tool. If i see 200 degrees on the scan tool  and no fans coming on I will jumper the relay plug and if that turns on the fan I will test the relays.

I think the fan turn on temp is 204°F.

 

The ECU grounds the fan relays when it wants the fans to turn on.

 

If its cold outside it will take a while for it to get warm enough. Keep the hood closed to keep heat in the engine bay. You can try running it at about 2000 rpm for a few minutes to help heat things up.

is it 'really' getting hot? Does the gauge even slow-down near the 'normal' reading or does it just peg-out? might double-check the alternator's output.  has someone removed the thermostat or used an aftermarket type?

 

 

The ECU grounds the fan relay at 204 degree. Got it.  

 

I've taken the car on my normal drive cycle route. The temp. gauge never goes over half way between cold and hot. The drive cycle is about half an hour with mix of highway and local driving. That drive cycle usually gets other Subaru's fans to turn on this time of year. 

 

I'm not sure which thermostat is in there. I guess if I have no fans on at 204 degrees I will jumper the fan relay socket and if the fans come on I'll check to see if relays are closing

 

I got engine to 204 degree while looking at live data on my scan tool and the fans turned on. Problem solved . Thank all.

Edited January 29, 201511 yr by steve56

maybe he was complaining about a hot 'smell'. Sometimes the inner boot splits and throws grease on the exhaust or some other fluid drips onto the exh.

 

??????