[mrbios]

2004 TSX automatic with 217K miles original owner radiator fans both work but fail to come unless the AC is activated then cools to normal temp. Car gets regular maintenace and problem started about 2 weeks ago. Today I added coolant. Radiator inside is clean and looks like new.

On the highway the problem does not occur. Only at red lights or otherwise sitting. Turning on AC both fans activate and engine cools right down.

My guess is a temp sensor? Note: the temp gauge on the dashboard works perfectly and there are no check engine lights etc.

Update: I removed and tested the Fan relay and it clicks / tests ok. I checked the Fan Switch aka coolant sensor that is screwed into the side of the radiator near the driver's side and there wasn't any corrosion on the plug and the plug had dielectric grease on it. I couldn't see the pins on the switch itself. Next, I could try shorting the two connections on the plug and see if the fan goes on.

Thanks.

 

[Litespeed08]

Yeah it's very likely that the sensor at the bottom of your radiator is bad since that's what signals the radiator fan to turn on during normal conditions. It's the most common cause of what you are experiencing. Shorting it won't trigger the rad fan if it's bad. If it does trigger the fan then you have an electrical issue, not a sensor issue.

You already know that your fans work, being triggered by the AC as designed. Your car gets air hitting the radiator on the highway so that helps to cool it but relies on the radiator fan when stopped or moving through street traffic.

 

[merlinj79]

Yeah sounds like the temp sensor.

Until you get this resolved, keep your AC on at all times, to make sure you don't overheat the engine unnecessarily... if the needle moves above the "normal" position, that's too hot for an older car unless you also want to replace some other cooling parts like radiator, heater core, etc.

 

[mrbios]

Thanks I'll get a new sensor and update results when I get it installed.